Note: Descriptions are shown in the official language in which they were submitted.
- ~ 329733
6lo5l-2o24D
This invention relates to optical thin film ~lakes and
coatings and inks incorporating the same and a method for making
the same and more particularly to optically varlable thin film
flakes and inks incorporating the same used in anti-counterfeiting
applications.
In the past attempts have been made to make lamellar
pigment materials in the manner disclosed in Patent No. 4,168,986
with the desire to obtain improved specular reflectivity. In
United States Patent No. 4,434,010 there is disclosed an article
and method for forming thin film flakes and coatings. There is,
however, no disclosure as to how optically variable thin film
flake~ for incorporation into paints and inks can be produced
which incorporate the use of subtractive colorants to block out or
minimize undesired colors. There is therefore a need for new and
improved optically variable thln film flakes, paints and inks
incorporating the same and methods for producing the same.
According to a broad aspect of the invention there is
provided, in a method for producing an optically variable printing
ink, providing a flexible web, depositing a release coat upon the
flexible web, depositing an optically variable multilayer thin
film coating on the release coat, passing the web containing the
release coat and the optically variable multilayer thin film
coating thereon into a solvent to dissolve the release coat,
removing the optically variable multilayer thin film coating from
~he web and causing the same to break into optically variable
flakes having first and second planar surfaces, drying the
optically variable flakes, sizing the optically variable flakes so
that ~hey have a physical thickness which is measured in a
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direction perpendicular to the layers of the coating and having an
aspect ratio of at least 2 to 1 respectively for the first and
second surfaces parallel to the planes of the layers of the
multilayer thin film coating and surfaces perpendicular to the
planes of the layers and so that ~hey have a maximum dimension
ranging from approximately two ~o twenty microns and introducing
the sized flakes into a liquid ink vehicle so that they are
dispersed therein to provide a printing ink giving a color shift
between two distinct colors at two different angles of incident
light.
According to another broad aspect of the invention there
is provided, in an article adapted to be utiliæed in the
production of an optically variable device, a flexible web, a
release layer formed on the web, an optically variable multilayer
coating disposed on the release coat, said coating having first
and second surfaces facing the release coat, subtractive colorant
means disposed on the first surface of the coating and serving to
provide in combination with the multilayer coating two distinct
colors at two different angles of incident light and substantially
no color at another angle of incident llght.
According to another broad aspect of the invention there
is provided, in a method for formin~ an optically variable device
by the use of a flexible web of material, depositing a release
coat upon the flexible web of material, forming a multilayer thin
film optical coating on said release layer, forming subtractive
colorant means on the surface of said multilayer coating,
separating said flexible web of materlal from said release coat
and securing said multilayer thin film coating with said
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subtractive colorant means thereon to another surface so that the
multilayer interference coating can be viewed by reflection
through the subtractive colorant means.
Figure 1 is a flow chart showing the optically variable
ink process.
Figure 2 is a flow chart showing the optically variable
ink manufacturing process.
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Figure 3 is a graph showlng th~ reflec-ta~ce of a
: magenta-to-grQen shi~ter a~ lO lncidence.
Figure 4 is a graph showiny the reflectance for a
gold-togreen shi~ter at 10 incidenc~.
Figure 5 i~ a graph ~howing the reflectance of a gold-
to-green ~hi~er with and without blu~ ght bl~cklng
pi~ment.
proc~ for m~ing an optic~lly variable ink ~OVI)
is shown in ~igure 1. As ~hown there~n in a c~nverting
. 10 ~tep 2 01, thQ fl~xible web is coated with a ~olvent
. soluble polymer. The web is formed of a suitable
~, insolu ble flexible ~ateria l u~ ing
poly~thylen~terphthalate tPET), or alternatiYely~ using
polymers such as polycarbonates and Kapton (trademark).
By way of example, a 142 gauqe web ~n be utilized. The
web is coated with an acrylic based polymer. One
acrylio ba~ed polymer found to be ~ati~factory i~ ~ne
deslgnated as 517-1 and is marlufac~ured and sold by
Thermark ~lvi~ion o~ Av~r Inte.rnational locat~d at
Schererville, Indiana. Th~ acrylic based polymer is
applied to the web in a suitable manner such as by
gravure co~ting and dried in force air dyers. The
polymer coa~ applied to the web is soluble in at least
j one solvent. Exa~ples o~ suitable solvents are acetone
ànd meth~thyl~etone. It should be appreciat~d that
~`~ other than acrylic polymers, oth~r material~ can be
utilized ~or a relea~e layer. For example, instead of
using a suitable hardcoa~ as provided by the acrylic
polymer, it is possible to evaporatQ a thin film coating
on~o thQ web would be solu~la in certain liquids~ Such
a thin layer ~ould ~e odium ~luoride or sodium chlorids
which co~ld be di~olved with water. Also it
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132~733
should be appreciated that other release layers which
have very low adhesion could be utilized which would
permit mechanical removal of the optically varlable
thin film either by the use of a vacuum or by the use
of air jets.
After the converting step 201 has been carried out,
the flexible web can be placed in a vacuum coating
chamber for performing the vacuum coating step
consisting of depositing an optically variable device
(OVD) or optlcal thin fllm onto the web as shown by
step 202. The optical variable device can be an
optical multilayer structure of the type hereinbefore
descibed. Alternatively, it can be of the type
described in United States Letters Patent Nos.
4,705,300 and 4,705,356. Optical variable devices of
this character can be deposited onto the web in a
conventional manner in a vacuum chamber such as by
the use of electron beam and resistive heating
sources as well as by sputtering.
. . .
After the multilayer coating has been deposited on
the flexlble web, in the vacuum coating process, the
soluble polymer layer and the adhering thin film
which forms the optically variable device is stripped
îrom the carriPr web. This can be accomplished batch
wise or in a continuous fashion as shown by step 203
by passing the web through a bath of a suitable
solvent, such as acetone. As the soluble polymer
layex is dissolved by the acetone, the thin film is
separated from the web mechanlcally. As the thin
fllm is being removed, it breaks lnto optical flakes
which are of a slze on the order of 50 to 200
microns. If a continuous pxocess is being used, the
web as it emsrges îrom the solvent, can be engaged by
a metal doctor blade to
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mechanically separate any remaining thin film from the
web.
The optical flakes, after they have be~n removed from
the carrier web either in a batch process or a
continuous process are then reduced in size as
hereinafter described and formulated into an ink as
shown by step 203. Thereafter, the ink can be utilized
in various printing processes as shown by step 204.
A more detailed manufacturing process for making
optically variable ink from an optical variable device
manufactured in the manner hereinbefore described as
shown in Figure 2. As shown therein, the soluble
polymer coated web or substrate i5 prepared in step 206
as hereinafter described. The coated web is then
supplied to a vacuum roll coater in roll form as shown
in the step 207. In the vacuum roll coater, a thin film
multilayer coating can be applied over a given width
using a single evaporakion source with appropriate
; masking or can be applied to almost the full width of
the vacuum roll coater using multiple evaporative
~; sources and appropriate mask:ing. After coating by
J vacuum evaporation, the web is removed from the roll
~3 25 coater and is slit in step 208 to remove any defects or
I unwanted trim (edge non-uniformities).
During the ~diting process in steps 209, the spectral
properties of the thin film coating can be ascertained
and supplied to a computer to provide a running color
; average of the coating. This makes it possible to
modify the color at a later step as hereinafter
described in the event that the color is slightly off
the desired color for a particular roll. This makes it
possible to custom blend to obtain an exact color
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by ~ither adding a lower or a higher color. By having
available a color profile extending along the width
and length of the web, it is possible to ascertain the
average color of each given roll. By way of example,
if average dominant wave length of a roll is, for
example, 495 microns and the desired wavelength is 490
microns, this desired wavelength can bP obtained by
adding some lower wavelength material having a
wavelength of 4~5 microns to achieve the desired 490
microns.
In the next step 211, the thin film is stripped from
the web. By way of example, this can be accomplished
by taXing the rolls and placing the rolls on an unwind
roller and having the web pass through a solvent bath
and then being taken up by a wind-up roller. The web
as it passes through the eolvent bath can pass through
a series of rollers which are positioned below the
level of the solvent bath. Ii any of the thin film
coating still remains on the web as it emerges from
the rollers in the bath, this remaining thin film can
be removed by a metal doctor blade which scrapes the
remaining thin film from the web. The doctor blade
, typically is positioned on the outside of the roll on
the wind-up side so that any adhering flake will fall
back into the solvent bath. As explained previously,
the flak~s in this operation have a tendency to drop
off in sizes of approximately 50 to 200 microns.
The flakes as they fall from the web will fall to the
bottom of the tank containing the ~olvent because they
have a much higher specific gravity as, for example,
approximately 3 whereas the solvent has a specific
gravity of approximately 1. After the settling has
occurred, the clear solvent liquid above the flakes
can be drained from the upper part of the tank
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1329~3
containing the ~olvent. ~he flake~ can then be re~oved
from the tan~ and used as hereinafter de~cr~bed.
: Alterna~ively, the flakes with the remaining solvent can
then be ~iltered and pulled dry as ~hown by step 212 by
5 the use of ~ vacuum filter of a conventional type.
Thereafter, fresh solvent is sprayed ove~ the optically
variable flakes forming the ~ilter cake remaining in ~he
er to rem~ve any last ~races of the ~o~uble polymer
~' or o~her extraneous material ~rom the fl~kes. The
10 ~ilter cake is then removed from the filter and ~roken
~: up and then laid out to dry in an a~r ove~ at
~tmospheric pressure at a suit~ble temperature as, for
example, 100 for a p~riod of time ranging from
approxima~ely ~ to lo hour~ ~ al~o shown by step 212.
~5 After the ~lake~ h~ve be~n dried, they are placed in
a suitable solvent s~lution, ~uch a~ acetone or methanol
and ultrasonically agitated using a conventional
ultrasonic agitator as, for exampl~, a Branson o
;J (t~a~emark) sonic ~ismembrator for a suitable period as,
20 for example, approximately 1 hour to reduce the particle
~ize to approximAtely 2-20 mlcrons. Thereafter, the
~lakes are ~ain ~iltered to remove ~he solv~nt and are
3 air-dryed in an atmospheric ov~n at a 3uitable
temperature, as ~or example 75 overnlgh~ of until they
2S are dry.
In order to reduc~ the flakes to a still smaller size,
.~ a~ for exampl~, a size ranging from 2 to 5 microns, the
dryed 1akes are sub~ected to an air grind in a suitable
impact pulverizer such a~ on~ manufactured by Garlock
30 Pla~tomer Product5, a division of Colt Indus~rieS on
Fxlends Lane, New~on, Yennsylv~n~ 18940. By way o~
example, a TX laboratory model of the air impact
pulveriz2r has ~een u~ilized to grind alumina
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up to a rate of 8 pounds an hour using a 10 mesh feed
to produce particle sizes down to sub micron size, as
for example, 0.65 microns. It has been found by using
this air impact pulverizer, 2 to 5 micron size can be
readily achieved without destroying the color
characteristics of the flakes. It should be
appreciated that other grinding techniques can be
utilized. However, care must be taken so that the
grinding will not destroy the color characteristics of
the flakes.
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A particularly attractive feature of th~ air impact
process for producing the small size optically variable
thin film flakes is that an aspect ratio of at least 2
to 1 can be achieved, and a fairly narrow particle size
distributlon can be obtained. The aspect ratio is
`~ ascertained by taking the largest dimension of a
surface of the flake parallel to the planes of the
layers of the thin film and a surface (the thickness)
perpendicular to the planes of the layers. In
addition, the air impact process eliminates the need
for additional solvent dispersal and solvent removal
steps.
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After the flakes have been sizPd, they can be blended
with other flakes to achieve the exact color desired by
adding flakes having a higher or lower wavelength to
achieve the desired result. This sizing and blending
process is represented by step 213 in Figure 2.
The sized and blended flakes are then introduced into
an ink polymer vehicle which consists of a main vehicle
with various additives in step 214.
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It should be appreciated that various types of ink
vehicle systems can be utilized. For example,
ultraviolet cured solvent systems, oxidative systems
and catalytic systems can be utilized. One type of ink
system which has been found to be satisfactory for use
with flakes is a catalytic system supplied by Dal Val
(trademark) Ink and Color, Inc. at 3101 Taylor's Lane,
Riverton, New Jersey, 08077 under the designations of
5-X-2575, a low temperature curing catalyzed varnish,
10 and 5-X-2605. Another one found to be suitable is an
epoxy based gravure ink supplied by Gotham Ink and
Color Inc. of Long Island City, New York under Nos.
66908 and 66909.
In connection with optically variable inks, it may be
desirable to add transparen~ dyes or pigments to the
ink formulation to operate in a subtractive mode to
modify the colors and/or to block unwanted colors. For
example, in the case of a gold-to-green shifter, the
addition of yellow dyes or yellow transparent piyments
,20 blocks the blue reflected light at large viewing
angles. Blocking pigments can be added as a separate
overprint ink layer or can be mixed directly into the
optically variable ink, as shown by step 215. By way
of example, if yellow is the color to be utilized,
`25 various transparent yellow blocking pigments are
available. For example, cromophtal yellow 3G (C.I.
pigment yellow 93) can be obtained from the Pigments
Department of Ciba-Geigy of Ardsley, New York 10502.
~'Sunset Gold HR (trademark) Transparent 1281 (C.I.
pigment yellow 84\3) can be obtained from Harshaw
(trademark) Company and Diarylide (trademark) Yellow
Toner AAOA-Transparent 1275 also can be obtained from
Harshaw. 11-1405 Novoperm yellow HR Extra Transparent
(C.I. pigment yellow 83) can be obtained from American
Hoechst (trademark) of Coventry, Rhode Island, as well
as 11-1424 Novoperm yellow RH-02 and 11-1400 Novoperm
yellow HR.
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The yellow pigments are typically supplied in a yellow powder
of a sub-~icron size and are introduced into the ink as it
is being mixed and milled to a percentage ranging from 2
to 30% by weight of the resulting optically variable ink.
However, in order to achieve a brighter color it is desirable
to utilize a lower percentage by weight of color, as for
example lS%. The mixing and milling operating shown by step
214 is carried out to obtain good dispersion of the flakes
which have been added to the paint vehicle. The mixed paint
can then be packaged into desirable containers and shipped
to the user as shown by step 21~.
The optically variable ink produced in accordance with
the present invention can be utilized with various
conventional printing presses without modification of the
presses. For example, the optically variable ink can be
utilized in various printing processes, such as lithographic
printing, letterpress printing, intaglio printing, gravure
printing, screen printing, ink jet printing, and by
electrostatic printing. Since the optically variable
printing ink can be utilized with printing processes
providing high resolution such as Intaglio, lithographic
and relief printing, it c2n be utilized for producing
security~type documents. As it is well known to
those skilled in the art, the film
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thickness after it i6 applied as a wet film on full
solid coated paper can have the following thicknesses:
Process Microns (approximate)
Sheetfed Litho 5.0
Sheetfed Letterpress 7.5
Web Offset 7.5
Web Letterpress 10
Gravure (Intaglio) 30 (variable)
Screen 25-125
fr~m 'What The Printers Should Xnow About Ink' by T.
Scarlett and N. Eldred, Graphic Arts Technical
Foundation, Pittsburgh, Pa. 15213, 1984, p. 2.
From the above it can be seen that the gravure or the
~creen ink ~ilm thickness~s are greater and thus gives
greater color saturation than with the thinner ink
films.
The aspect ratio is important in that it helps to
ensure that the flakes will land either on their top
and bottom sides and not on their ends. It can be
appreciated if the flakes fall on their ends, that
there would be no color 6hift from the flake. It is
important that the optical variable device be symmet-
rical so that no matter which side the flake lands on,
it still will give a color shift. In other words, the
color will be maintained. Thus it certainly is desir-
able not to have a one-to-one aspect ratio but rather
be at least two-to-one or three-to-one. Since the
total thickness of the optically variable thin film is
approximately .9 microns, the 2 micr~n dimension is
approximately the smallest dimension desired for the
flakes. By utilizing an aspect ratio of at least 2 to
1 and greater, preferably 5-10, to 1, gives assurance
that a major proportion of the flakes will land in the
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ink vehicle with an orientation such that the surfaces
providing the color of the flakes will be facing
upwardly since the thin film coating is symmetrical
and those surfaces have the larger dimensions.
It should be a~preciated that with the different wet
film thicknesses it is easier to print with thicker
layers of material and in addition, this makes it
possible to utilize a smaller percentage of optically
variable device flakes in the printing media. Thus
with gravure it is possible to utilize only 25% by
weight of optical variable device flakes whereas with
letter press printing and other thinner coatings it is
necessary to increase the percentage of optical vari-
able device ~lakes to 45 to 50% by weight.
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If a color shift between two colors with change of
viewing angle such as a typical gold-to-gree~ design
is desired for an optically variable ink for anti-
counterfeiting applications, a five layer symmetrical
design of the type MDMDM where M is a metal layer and ,
D is a dielectric layer. The materials used for M and
D can be chosen from a wide variety of substances.
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It has been possible to achieve very good color
control with optically variable inks. To ascertain
this optical variable inks were air brushed onto three
different surfaces outlined below.
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, 1. Hi-Gloss Paper
2. Bond Paper
3. Bond Paper With Water Base Base Coat
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Delta E color measurements based on the CIE Lab color
coordinate system were taken. Three samples were
chosen to be standards against which all other samples
were compared. The Delta E values are charted below:
Standard #
High Gloss Bond no Bond with
Base Coat ~ase Coat
Sample # lA 3A 5A
High lA -- 6.18 3.39
Gloss 2A 1.62 5.36 2.56
Paper 7B 0.58 5.86 3.04
8B 0.31 5.93 3.15
Bond 3A 6 14 -- 2.95
Paper 4A 4 61 2.06 2.20
15No B.C.9a 4.59 2074 2.60
lOB 4.71 2.67 2.62
Bond 5A 3.34 2.87 --
Paper 6A 3.53 2.59 0.56
WB B.C.llB 2.96 3.3B 0.78
12B 1.74 4.41 1.80
The ahove chart taking the -- in Column lA as the
standard can be seen that the change in color from the
standard in terms of Delta E units is only 1.62, .058
and 0.31 which shows that the difference in color from
one sample to the next i5 minimal. For currency type
paper, the color change is also very excellent ranging
from 0.56 to 0. 7a and l.B0. The change in color is so
~mall that for these samples it is undetectabla by the
; human eye. I~ has been found that when the optical
variable ink is applied to paper which does not have a
base coat, then the values of delta E are slightly
higher because the optically variable device flakes
are not lying in a completely flat plane. ~y
utilizing a base coat a more planar surface is
provided which provides a surface that gives a higher
color purity when the optically variable ink is
; applied to the same.
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In using the yellow pigment blocker in the optically
variable ink, the yellow pigment has a tendency to
settle above the optically variable pigment since it
has a specific gravity of approximately 1 with respect
to the optical variable device flaXes which have a
specific gravity of approximately 3. In certain
applications, however, the best approach in blocking
out the blue reflected light at high viewing angles is
to print a top coat vehicle containing the yellow
pigment layer over the optically variable ink layer.
In order to achieve excellent color purity, the op-
tically variable ink must have a good aspect ratio as,
~or example, at least two-to-one, preferably 5-10 to
one, as pointed out above. The optical variable
device flakes should not be agglomerated but should be
thoroughly dispersed throughout the ink. There should
be good overlap of the flakes. The ink should have
good flow characteristics. If the paper on which the
printing is to occur has a rough surface, a subbing
layer may be required for currlency type applications
where high color purity is desired. Alternatively,
calendared currency paper may be very desirable to
decrease surface roughness.
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As also pointed out previously to obtain good
optically variable ink durability, the vehicle itself
must be durable and must meet press requirsments. It
must be able to post cure, i.e., it must be cross-
linked after the print step. As also pointed out
previously, air oxidization, catalyst and UV
vehicles are available which cross-link after
printing. The optically variable device flakes or
particles which are provided as a part of the
optically variable ink must be lnert or alternatively,
the flakes must be made oleophobic and hydrophobic or,
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in other words, they must be encapsula~ed so they will
not react with chemicals such as bases sr aclds.
For a good quality gravure or Intaglio ink, the flakes
or particle ~ize should be in the range o~ 5-15 microns.
This particle size will allow the desired color purity
while still allowing for fine line printing. If fine
line printing is not de~ired, then larger s~ze particles
may ~a u~ed, up to 100 micron~ or so. For cover~ge the
~lake or particle loading or ~lake~ ~hould r~nge 30 to
lo 50~ by weight for lett~rpress and offset and lo to 30
for grAvure and Intaglio.
In the graph i~ Figur~ 3 ther~ is ~hown the
re1ectanc~ which can b~ obtain~d with a magenta-to-
green ~hi~ter of the pre~ent inv~ntion. The curve 221
~hows the spectrum of the foil and represents the
reflectance of the coatiny on the polyester wab. The
curYe~ ~22 and 223 are of in~ mad~ ~rom optiaally
variable flakes m~dQ in accordan~e with the present
invention from the foil which is repr~sented by the
Z0 curve 221. The spectra of t:he ink were m~de from
~amples p~epared ~rom 20% ~y welght of optically
variable pigment in Gotham 6690~ re~in catalyzed with
2.5% by weight Gotham 6~909, cured at 200 ~or ~our
~inut~s (Go~ham In~ and Color~ Co., Inc., ~ong I~lan~
~5 ~i~y, New York 11101). Curv~ zZ was obtained from ink
prepared ~rom ~lake~ without any grinding (as removed
~ ~rom the web by ~olvent dis~olving ~he hardcoat/release
: layer) whereas t~e other curve 223 was obtained from in~
prepared from ~laXes that had been ~round in methanol
~ing ultrasonic dismembermen~ or 1 hou~ (Sonifier
(trademark) Cell Disruptor manufactured by Branson Sonic
P~wer Co. set at a power setting o~ 9). ~ha optical
~i variable f lakes in thi~ grinding proce~
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were reduced to the size of approximately 5 to 20
microns. As can be seen from the curve 223, this
grinding process had a very small deleterious effect
on the reflectivity of the optically variable flakes.
; 5 In addition, it can be seen that there is also very
little degradation in the quality, including
reflection, compared to the reflection received from
the foil itself, before it is removed from the web.
In Figure 4, there is shown another graph giving the
reflectance of a gold-to-green shifter of the present
invention without the use of a blue-light blocker. As
in the graph in Figure 3, the graph in Figure 4 has
three curves 231, 232, and 233 in which the curve 231
represPnts the reflection from the foil on the web,
curve 232 represents the reflection from ink utilizing
optical variable flakes obtained by removing the
optically variable coating from the web by the use of
a solvent but without any grinding and the curve 233
represents the reflection obtained from an ink using
optical variable flakes which have been ground down to
a particle size ranging from 5-20 microns. The inks
were prepared in the same way as described in
connection with the graph in Figure 3. Here again it
can be seen that the reflectance from the inks is
still very good and corresponds very closely to that
o~ the foil itself in that there is little degradation
by the grinding of the optically variable flakes to
the 10-20 micron size. Note that the peak positions
in wavelength for the optically variable ink
correspond almost exactly to those for the optically
variable coating as prepared on th~ vacuum roll coated
web.
In Figure 5 there is shown still another graph which
, shows the reflectance of a gold-to-green ink shifter
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~32~33
-16-
~ made in accordance with the present lnvention with and
: without the blue-light bloc~er. This ink was deposited
onto a polyester clear film sub6trate. curve 241 shows
the reflectance from the PET ~ide with an optical
variable ink (ovp) utilizing optically variable flakes
: therein serving to provide a gold-to-green shifter
withou~ the blue light blocker. Curve 242 ~hows the
r~flectance ~rom th~ ink ~id~ of the ~ame ~tructure for
which the re~lectance i8 6hown in curve Z41. Cu~va 2~3
~hows the reflectance from the PE~ ~ide havin~ a gold-
to green shifter utilizing ~ blua-lisht blocker. Curve
244 shows the reflectance from the ink ~ide of th~ gold-
to-green shifter ~hown in curve 243 uslny a blue-light
blocker in yellow pigment. The ink utilized f or the
curve~ shown ~n Pigure 5 was prepared with three qram~
of gold^to-gree~ optical varia~le flake~ which had been
- ultransonically ground to 5-20 mic~on particle 3ize.
The optical variable ~la~es were then mixed w~th 7 grams
, o~ Del V~l ~trademark) Th~rmose~ ~arn~h 5-X-2575
: 20 catalyz~d with 10~ ~hermoset cataly6t 5-X-2605 and cured
at room t~mperature (Del Val Ink and color, Inc., 13~1
Taylor' Lane, ~1verton~ New Jersey 08077). The top two
aurves 241 and 242 show the re~lectance a~ a function
of wavslength w~en the lnk is prepared and cast on~o a
~5 polyester film and then v~ewed directly at tha coating
and also through he polyester film. The lower ~wo
curves are ~imilar to those ourvQs de6cribed above bu~
are for inks p~epared with 9.1% (by total weight) of the
; bl~s--light blockar (Cromophthal yellow pigment),
optically variable flakes and the polymer vehicle. The
Cromophthal yellow i~ manufactured by Cib~-Ge~gy, Glen~
Falls, New YorX. Th2 curves 243 and 24q clearly show
~' how the blue-liyht blocker in ~h~ form o~ the yellow
,' pigment effectively blocks the blue light at 400 nano~eters.
.
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~3297~33
-17-
From the foregoing it can be seen that there has been
provided an optically variable ink which can serve as
a printing ink which can bs applied to papers of
various types including currency paper. This
optically variable ink will exhibit two distinct
colors, one color when it is viewed straight on or in
a directlon normal to the surface of the article on
which the optically variable ink appears and another
color when viewed at a substantial angle, as for
example, 45 Thus the paper which has an optically
variable ink printed thereon can be readily examined
by the human eye to ascertain whether or not an
optically variable ink is present by merely
ascertaining the color shift by change in viewing
angle. Transparent pigments and dyes can be used to
block out undesired colors in the spectrum between the
two desired colors. They also can be used to block
out undesired high angle colors. Further, these
additives can be used to modify the colors wanted at
the varlous viewing angles. ~:n addition, the use of
the optically variable ink makes it impossible to
duplicate an article with the same colors on color
copiers because only one color can be copied or
because the optically variable ink reproduces as black
rather than as a color or because the color (at normal
incidence) i5 not faithfully reproducad.
:
Therefore it can be seen that opticall~ variable inks
made in accordance with the present invention have
numerous applications. They can be utilized for
! 30 various decorative purposes. They also can be
i utilized for anti-counterfeiting purposes in currency
type papers, as well as ~ecurity papers.
,
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132~33
--18--
The optically variable ink is also advantagPous in
that it can be utilized with existlng printing
processes without alteration.
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