The present invention relates to thermal transfer printing wherein images
are formed on a receiving substrate by heating extremely precise areas of a
print ribbon with thin film resistors. This heating of the localized area causes
transfer of ink or other sensible material from the ribbon to the receiving
substrate. Sensible material is typically a pigment or dye which can be detected
optically or magnetically.
More particularly, the present invention is directed to coating
formulations (thermal transfer ink formulations) and thermal transfer media
(ribbons) obtained therefrom which produce printed images with photochromic
properties.
Thermal transfer printing has displaced impact printing in many
applications due to advances such as the relatively low noise levels which are
attained during the printing operation. Thermal transfer printing is widely used
in special applications such as in the printing of machine readable bar codes and
magnetic alpha-numeric characters. The thermal transfer process provides
great flexibility in generating images and allows for broad variations in style, size
and color of the printed image.
Photochromic compounds have been employed in various articles such as
window glasses, sunglasses and films. It would be advantageous to provide
images having photochromic properties (photochromism) which are generated
by thermal transfer printing.
Photochromism means such characteristics of a material that the material
develops a color under irradiation with excitation rays such as ultraviolet rays
and returns to the initial uncolored state when allowing the material to stand.
That is, photochromism means that the material is reversibly colored and
discolored repeatedly.
It is an object of the present invention to provide images on articles by
thermal transfer printing, wherein the images are hidden or invisible to the
naked eye for purposes of security and identification but detectable under
special conditions.
According to the invention a coating formulation for a thermal transfer
layer which transfers an image basis to a receiving substrate when exposed to
heat, characterized in that said coating formulation comprises wax, binder resin,
solvent and a sensible material which comprises a photochromic pigment, a
mixture of photochromic pigments, a photochromic dye, a mixture of
photochromic dyes or a combination of one or more photochromic pigments
and one or more photochromic dyes in an amount sufficient to change the color
of the image basis upon subsequent exposure to U.V. light.
Also according to the invention a thermal transfer medium comprising a
flexible substrate and a thermal transfer layer positioned thereon, said layer
having a softening point in the range of 40°C to 250°C, said layer comprises a
coating formulation according to any preceding claim.
Further to the invention an article carrying an image basis, said image
basis changing colour when subsequently exposed to U.V. light so as to
produce an image, characterized in that said image basis is produced from a
thermal transfer medium.
The invention will now be described by way of example only with
reference to the accompanying drawings in which:-
Fig. 1 illustrates a thermal transfer medium of the present invention in a
printing operation prior to thermal transfer; Fig. 2 illustrates a thermal transfer medium of the present invention in a
printing operation after thermal transfer; Fig. 3 is a representation of a transparent image of the present invention
on a substrate following exposure to U.V. light; and Fig. 4 is a representation of another transparent image of the present
invention on a substrate following exposure to U.V. light.
Thermal transfer ribbon 20, as illustrated in Figs. 1 and 2, comprises
substrate 22 of a flexible material which is preferably a thin smooth paper or
plastic-like material. Tissue type paper materials such as 30-40 gauge capacitor
tissue, manufactured by Glatz and polyester-type plastic materials such as 14-35
gauge polyester film manufactured by Dupont under the trademark Mylar®
are suitable. Polyethylene naphthalate films, polyamide films such as nylon,
polyolefin films such as polypropylene film, cellulose films such as triacetate film
and polycarbonate films are also suitable. The substrates should have high
tensile strength to provide ease in handling and coating and preferably provide
these properties at minimum thickness and low heat resistance to prolong the
life of heating elements within thermal print heads. The thickness is preferably 3
to 10 µm. The substrate or base film may be provided with a backcoating (not
shown) on the surface opposite the thermal transfer layer. Positioned on
substrate 22 is thermal transfer layer 24, also referred to as a functional layer.
The thermal sensitivity of thermal transfer layer 24 is determined by the
softening point of the wax and binder resin therein. This thermal transfer layer
has a softening point below 250°C, preferably below 200°C and most
preferably from 50°C to 125°C. Softening temperatures within this range
enable the thermal transfer medium to be used in conventional thermal transfer
printers, which typically have print heads which operate at temperatures in the
range of 50°C to 300°C, more typically, temperatures in the range of 60°C to
125°C. The thermal transfer layer 24 contains a wax and binder resin which
are preferably compatible so that exposure to heat from print head 30 uniformly
transfers thermal transfer layer 24 from substrate 22 to synthetic resin receiving
substrate 28 and forms image 32.
The coating formulation of this invention comprises the components of
conventional coating formulations such as one or more waxes, binder resins
and solvents. However, the sensible material (pigment or dye) employed is a
photochromic dye, a mixture of photochromic dyes, a photochromic pigment, a
mixture of photochromic pigments or a combination of one or more
photochromic dyes and one or more photochromic pigments.
Photochromic compounds suitable for use in this invention are those
classified as organic photochromic compounds. Many of such compounds are
known to be homogeneously mixed with organic high molecular weight
compounds in the preparation of photochromic films and laminates . Suitable
photochromic compounds include the spiro compounds of formula V disclosed
by Takahashi et al. in U.S. Patent No. 5,266,447. These include spiroxazine
compounds, spiropyran compounds and thiopyran compounds of the formulae
in columns 5-6 of U.S. Patent No. 5,266,447.
Other examples of suitable photochromic compounds include the
benzopyran compounds disclosed by Kumar in U.S. Patent No. 5,429,774, the
benzothioxanthone oxides disclosed by Fischer et al. in U.S. Patent No.
5,177,218, the dinitrated spiropyrans disclosed by Hibino et al. in U.S. Patent
No. 5,155,230, the naphthacenequinones disclosed by Fischer et al. in U.S.
Patent No. 5,206,395 and U.S. Patent No. 5,407,885, the naphthopyran
compounds disclosed by Knowles in U.S. Patent No. 5,384,077, the
spiro(indoline) naphthoxazine compounds disclosed by VanGemert in U.S.
Patent No. 5,405,958, the ring compounds disclosed by Tanaka et al. in U.S.
Patent No. 5,106,988 and the spiro-benzoxazine compounds disclosed by
Rickwood et al. in U.S. Patent No. 5,446,151. Mixtures of such compounds
are preferred and are available commercially from sources such as Color Change
Corp. of Illinois and Xytronyx Inc. of San Diego, California. Mixtures are
typically used to provide variations in color.
The photochromic pigments/dyes are preferably added to the formulation
in manner consistent with conventional methods for introducing conventional
pigments or dyes. However, alternative (non-conventional) methods for
preparing the coating formulations of this invention may also suitable. The
photochromic dye/pigment is employed in an amount sufficient to change the
color of the thermal transfer layer formed therefrom when exposed U.V. light.
The photochromic dye/pigment is typically employed in an amount in the range
of about .01 to 50 wt.%, preferably 0.1-25 wt.% based on the of dry
components. More preferably the amount employed ranges from about 1 to 10
wt. % and most preferably about 1 wt. % based on dry components.
The coating formulation of the present invention can be prepared in
conventional equipment. The preferred method is to mix the solvent, wax
components and binder resin at an elevated temperature, preferably about
65°C. When thoroughly mixed, the photochromic pigment/dye is added and the
resulting mixture mixed at an elevated temperature, preferably from about 60°C
to 65°C. The pigments are typically ground in an attritor.
The coating formulation comprises wax as a main dry component.
Suitable waxes provide temperature sensitivity and flexibility. Examples include
natural waxes such as carnauba wax, rice bran wax, bees wax, lanolin,
candelilla wax, motan wax and ceresine wax; petroleum waxes such as paraffin
wax and microcrystalline waxes; synthetic hydrocarbon waxes such as low
molecular weight polyethylene and Fisher-Tropsch wax; higher fatty acids such
as lauric acid, myristic acid, palmitic acid, stearic acid and behenic acid; higher
aliphatic alcohol such as stearyl alcohol and esters such as sucrose fatty acid
esters, sorbitane fatty acid esters and amides. The wax-like substances have a
melting point less than 200°C and preferably from 40°C to 130°C. The amount
of wax in the coating formulation is preferably above 25 wt.% and most
preferably ranges from 25 to 85 percent by weight, based on the weight of dry
ingredients.
The coating formulation of this invention also comprises a binder resin.
Suitable binder resins are those conventionally used in coating formulations.
These include thermoplastic resins and reactive resins such as epoxy resins.
Suitable thermoplastic binder resins include those described in U.S.
Patent Nos. 5,240,781 and U.S. 5,348,348 which have a melting point of less
than 300°C, preferably from 40°C to 225°C. Examples of suitable
thermoplastic resins include polyvinyl chloride, polyvinyl acetate, vinyl chloride-vinyl
acetate copolymers, polyethylene, polypropylene, polyacetal, ethylene-vinyl
acetate copolymers, ethylene alkyl (meth)acrylate copolymers, ethylene-ethyl
acetate copolymers, polystyrene, styrene copolymers, polyamide,
ethylcellulose, epoxy resin, xylene resin, ketone resin, petroleum resin, terpene
resin, polyurethane resin, polyvinyl butyryl, styrene-butadiene rubber, saturated
polyesters, styrene-alkyl (meth)acrylate copolymer, ethylene alkyl (meth)acrylate
copolymers. Suitable saturated polyesters are further described in U.S. Patent
No. 4,983,446. Thermoplastic resins are preferably used in an amount of from
2 to 35 wt.% based on the total dry ingredients of the coating formulation.
Suitable reactive binder components include epoxy resins and a
polymerization initiator (crosslinker). Suitable epoxy resins include those that
have at least two oxirane groups such as epoxy novolak resins obtained by
reacting epichlorohydrin with phenol/formaldehyde condensates or cresol/formaldehyde
condensates. Another preferred epoxy resin is polyglycidyl ether
polymers obtained by reaction of epichlorohydrin with a polyhydroxy monomer
such as 1,4 butanediol. A specific example of suitable epoxy novolak resin is
Epon 164 available from Shell Chemical Company. A specific example of the
polyglycidyl ether is available from Ciba-Geigy Corporation under the trade
name Araldite® GT 7013. The epoxy resins are preferably employed with a
crosslinker which activates upon exposure to the heat from a thermal print
head. Preferred crosslinkers include polyamines with at least two primary or
secondary amine groups. Examples being Epi-cure P101 and Ancamine
2014FG available from Shell Chemical Company and Air Products, respectively.
Accelerators such as triglycidylisocyanurate can be used with the crosslinker to
accelerate the reaction. When used, the epoxy resins typically comprise more
than 25 wt.% of the coating formulation based on dry components in view of
their low viscosity. Waxes are typically not necessary when reactive epoxy
resins form the binder.
The solvents employed in coating formulations of this invention can vary
widely and are dependent on the solubility of the binder resin. A preferred
solvent is mineral spirits. Other suitable solvents include esters, ketones,
ethers, alcohols, aliphatics and aromatics. The solids content of the coating
formulation is typically within the range of 15 to 100 wt.% (hot melt),
depending on the viscosity of the dry components therein.
Although not preferred, the coating formulation may also contain another
sensible material or pigment in addition to the photochromic pigments/dye
discussed above. These are preferably colorless pigments used as filler or light
in color so as not to interfere with the photochromic effect. The photochromic
pigments may be used to change the color of light colored pigments. The
additional sensible material is typically a coloring agent, such as a dye or
pigment or magnetic particles; however any coloring agent used in conventional
ink ribbons is suitable, including carbon black and a variety of organic and
inorganic coloring pigments and dyes, examples of which include
phthalocyanine dyes, fluorescent naphthalimide dyes and others such as
cadmium, primrose, chrome yellow, ultra marine blue, titanium dioxide, zinc
oxide, iron oxide, cobalt oxide, nickel oxide, etc. Examples of sensible materials
include those described in U.S. 3,663,278 and U.S. 4,923,749. Reactive dyes
such as leuco dyes are also suitable. In the case of magnetic thermal printing,
the thermal transfer layer includes a magnetic pigment or particles for use in
imaging to enable optical human or machine reading of the characters. Use of
magnetic pigment particles is expected conflict with the objects of the present
invention in most cases, but the use of such particles is not excluded from this
invention. The additional sensible material or pigment is typically used in an
amount of from 0 to 40 parts by weight based on the total dry ingredients of
the coating formulation.
The coating formulations may contain conventional additives such as
plasticizers, viscosity modifiers, tackifiers, etc.
A preferred formulation is that containing a mixture of rice bran wax in
an amount ranging from 60 to 95 wt.% based on the total dry ingredients, an
ethyl vinyl acetate copolymer binder resin and the photochromic pigment.
Mineral spirits are a preferred solvent. This preferred formulation is made by
mixing the solution of mineral spirits, rice bran wax and ethyl vinyl acetate
copolymer binder resin for about 15 minutes at a temperature of about 65°C,
after which the photochromic dye is added at about 60°C to 70°C for about
two hours.
The thermal transfer ribbon of the present invention comprises a
substrate as described above, preferably polyethylene terephthalate, and a
thermal transfer layer comprised of wax, binder resin, sometimes residual
solvent and a photochromic pigment. The thermal transfer layer is preferably
obtained from the coating formulation of the present invention. Suitable waxes,
binder resins and photochromic pigments are as described above. The thermal
transfer layer (functional layer) preferably has a softening point within the range
of about 50°C to 250°C which enables transfer at normal print head energies
which range from about 100°C to 250°C and more typically from about 100°C
to 150°C. The thermal transfer ribbon of the present invention can be prepared
from formulations of the present invention in the form of either a solution,
dispersion or emulsion. Once applied to the substrate, a portion of the solvent
can remain in the coating. The ribbons can be prepared by conventional
techniques and equipment such as a Meyer Rod or like wire round doctor bar
set up on a conventional coating machine to provide the coating weights
described above. The coating weight of the thermal transfer layer typically
ranges from 1.9 to 4.3 g/m2. A temperature of about 65°C is maintained during
the entire coating process. After the coating formulation is applied, it is
optionally passed through a dryer at an elevated temperature to ensure drying
and adherence of the functional layer to the substrate. The thermal transfer
layer can be fully transferred onto a receiving substrate such as paper or
synthetic resin at a temperature in the range of 75°C to 200°C.
The thermal transfer ribbon of the present invention provides the
advantages of thermal printing. When the thermal transfer ribbon is exposed to
the heating elements of the thermal print head, the thermal transfer layer
softens and transfers from the ribbon to the receiving substrate with some of
the silicone resin backcoating therein.
The images of this invention are preferably derived from the thermal
transfer ribbons of this invention and comprise a single layer of the wax, binder
resin and photochromic pigments/dyes, as described above, transferred from
the thermal transfer layer onto a substrate. The images of this invention are
preferably transparent until exposed to U.V. light which is achieved by
excluding colored pigments.
The images can be patterned in fine detail as shown in Fig. 3, which is
an image 100 of this invention in the pattern of a bar code. When transparent,
this image enables the identification of goods or authentication of articles
without disrupting the appearance/packaging of the goods or articles. The
image 100 can also be a decorative pattern for novelty items such as cards
shirts, etc. as shown in Fig. 4.
EXAMPLES
Coating Formulation
A coating formulation of the present invention is prepared by mixing
mineral spirits, wax and binder resin in the proportions indicated in Table 1 and
heating the mixture to 60°C for 15 minutes. A mixture of photochromic dyes
available from Xytronyx Inc. in the amount indicated in Table 1 is added to the
resultant mixture at a temperature of from about 140°F to 150°F for about 2
hours.
Material | Wt.% Dry | Wt.% Dry - Range | Grams Dry | Grams Wet |
Rice Bran Wax | 78.0 | 40 - 85% | 93.6 | 93.6 |
Ethyl Vinyl Acetate Copolymer Resin | 7.0 | 2 - 30% | 8.4 | 8.4 |
Photochromic Mixture | 15.0 | 1 - 30% | 18 | 18 |
Mineral Spirits | -- | -- | -- | 480 |
Total | 100.0 | | 120.0 | 600 |
Thermal Transfer Medium
A thermal transfer medium of the present invention is prepared by
coating a formulation as defined above onto a 4.5 µm Polyester Mylar Film by
E. I. Dupont de Nemours & Co., Incorporated at a coat weight of from 1.9 to
4.3 g/m2. The solution is coated onto the mylar film at 70°C using a doctor bar
and subsequently dried.
The preceding examples can be repeated with similar success by
substituting the generically or specifically described reactants and/or operating
conditions of this invention for those used in the preceding example.
From the foregoing description, one skilled in the art can easily ascertain
the essential characteristics of this invention, and without departing from the
spirit and scope thereof, can make various changes and modifications of the
invention to adapt it to various usages and conditions.