CA1329733C - Optical thin film flakes, replicated optical coatings and coatings and inks incorporating the same and method - Google Patents

Abstract

Abstract of the Disclosure Articles and methods for forming optically variable thin film flakes and replicated coatings having prese-lected optical properties. The articles generally comprise a flexible web of material and an optically variable thin film coating formed on one surface of the web. The optically variable coating is separated from the web to form optically variable thin film flakes. The flakes are disposed in ink and paint vehicles to provide optically variable inks, paints and the like.

Description

- ~ 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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~3~733 . -2-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.
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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 , : . ~

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.
:, 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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g 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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~ 329~33 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 :-~s .
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~32~ 3~3 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.
' 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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132~733 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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~',: ', , ' ~ ' ,, ' : ' ~32~7~3 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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: . , -~329~33 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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~3~9733 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 ~ 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 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.
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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.

, `' 132~33 The optically variable ink is also advantagPous in that it can be utilized with existlng printing processes without alteration.

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Claims (9)

1. 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 the 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 they have a physical thickness which is measured in a 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 they have a maximum dimension ranging from approximately two to 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.

2. A method as in Claim 1 wherein the variable flakes are sized by the use of an air impact pulverizer.

3. In an article adapted to be utilized 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 light.

4. An article as in Claim 3 wherein said another angle of incident light is between first and second angles of incident light.

5. An article as in Claim 3 wherein said another angle is at a higher angle than the first and second angle of incident light.

6. An article as in Claim 3 wherein said multilayer coating is symmetrical when viewed in reflection from either of said first and second surfaces.

7. In a method for forming 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 material from said release coat and securing said multilayer thin film coating with said subtractive colorant means thereon to another surface so that the multilayer interference coating can be viewed by reflection through the subtractive colorant means.

8. A method as in Claim 7 wherein said multilayer thin film coating is formed in an inverted manner on said web.

9. A method as in Claim 7 wherein said multilayer thin film coating is formed symmetrically on said web and wherein said subtractive colorant means is provided on both sides of said symmetrical multilayer thin film coating.