CA2203645A1

CA2203645A1 - Optical security articles and methods for making same

Info

Publication number: CA2203645A1
Application number: CA002203645A
Authority: CA
Inventors: Shih-Lai Lu
Original assignee: Shih-Lai Lu; The Minnesota Mining & Manufacturing Company; Minnesota Mining And Manufacturing Company
Current assignee: 3M Co
Priority date: 1994-11-02
Filing date: 1995-09-21
Publication date: 1996-05-17
Also published as: AU689327B2; CN1162289A; DE69508607D1; EP0790896A1; JPH10508549A; US5591527A; US5743981A; KR970706972A; DE69508607T2; CN1058665C; EP0790896B1; WO1996014215A1; AU3637395A; BR9509569A

Abstract

Optical security articles (1) and methods of making same are described comprising a first layer (14) substantially transparent to visible light and having a first refractive index, the first layer having a relief pattern on a first surface (18) thereof and a substantially smooth second surface (16), and an adhesive layer (24) coterminating with the first layer. The adhesive layer substantially completely fills and makes contact with a first portion of the plurality of geometric concavities, the adhesive layer having a second refractive index which is substantially similar to the first refractive index of the first layer. A second portion of the plurality of geometric concavities are precluded from contact with the adhesive layer by a corresponding plurality of separation layers (34), each separation layer having a separation layer refractive index which is different from the refractive index of the first layer.

Description

-W O96tl4215 PCTnUSg5/12005 OPTICAL SECURllY ARTICLES AND MET~ODS FOR MAKING SAME

Background of the Invention 1. Field of the Invention The invention COI~CclllS novel optical secu-ily articles which can be used as tl~s}Jd~enl OVCIla~i~ to protect do~ e--ls from l~lllJCIing. The invention also conce~ -s mptho~s for making such artic!es.

2. ~bte~ Art Docv~.. ç-~ls often have adherent ll~lsparellt ovella~ls to provide proteclion against dirt, moisture, and general wear and tear. A typical protective ,a.~,..l overlay has a plastic film bearing an aggressive adhesive layer by which it can be pc~ n ,.~ly adhered to the face of a docum.~nt T.~ s~,alc.lt uvc-l~s are known which do not obscure underlying il~....alion, such as those having legend-co..l-q;~ g 15 sheet ms~eriolc which have rct-U.cllective legend and rctlu.cIlective bac~ground areas that are ~I,s~ ly i~ ble under Ol.]i.-dl ~ diffuse light viewing con-lition~
but are dearly d~ gu.!~l.^ble under retlorcllective viewing conditions. Attempts to tamper with h~.-..alion over which the overlay has been applied result in readily ~etectsb'e disruption ofthe overlay.
Other l-~r.;.~.a e~l overlays can be imaged with a pattern or legend, such as a holographic pattern or image that is readily noticeable only when viewed ret.(,r~,nectively, and can be adhesively bonded to a doc~ .l without obscu.ing the face ofthe docunlent Both refl~ctin~ and llarl~pa~cllt hologram Ovcllày~ are known.
Holographic ovc~la~, however, suffer from the problem that the hologram surface relief pattem is typically produced using a pattem of light intc~rcrence fringes formed by the i It~.~e~.lion oftwo cohclenl light beams, typically laser beams, which Ic luhcs e~.~cl~ivc lasers and other e4~ n~ ll It would be advont~eo--~ if a l~ spafe~l overlay could be developed which would avoid the use of holograrns.
Shaped plastic articles, inr1uding s~ s~ ;qlly totally intemal reflective films, have been made by re~liG~qtin~ a microstructure-bearing surface with an ultraviolet ræ.~liqtiQn-curable organic oligomeric composition; however, their use as prolc.;livc WO 96/142is PCT/US95/12005 and/or security overlays for il"pO~ t docun-Pntc and other articles is not known or su~ested Summary of the Invention In accordance with the present invention, optical security articles are plevenled which can be used, for example, as protective l,~,spa,.l,l ovellayv for ~lthpntic~ted vubsllales such as doc~ n1c~ idP-ntifir,~tiQn cards, and the like. The inventive articles can include an image, such as a design or logo, to enh~nce the ~Jlh~ ;r.ity ofthe s~s~ e and also provide high tamper-rçc;ct~nre Typically and preferably, the image is viewable only across a narrow range of viewing angles and is viewable in ~mh:e~t (diffuse) light, thus afrolding a readily appare,lt verific~tion ofthe .lh. r,1;.,;ly ofthe s~sl-~le. The invention also provides methotlc for making such optical se~,u,ily articles.
One aspect of the invention is an optical security article comprising:
a) a first layer s~lbs~ ly l~s~ ;nt to visible light and having a first refractive index, the first layer having a structured first surface and a ~ubv~ l1y smooth second surface, the structured first surface conl~ul;s;llg a plurality of geo..,~t.;c concavities and col~t;s~ol ding peaks; and b) an adhesive layer cote, ~ g with the first layer, the adhesive layer s Ibs~A~ lly completely filling and making adhesive contact with a first portion of the plurality of geGl"et~ic concavities, the adhesive layer having a second refractive index which is s~lbvl~ lly similar to the first refractive index ofthe first layer, thus folll~,ng at least one l,~lv~,a,enl area, ~he~. a second portion of the plurality of ~eol.~e~ . ;c concavities are precluded ~om contact with the adhesive layer by a separation layer disposed between the second portion ofthe plurality of geol"~ l,ic concavities and the adhesive layer. Fle~lably~ the separation layer contacts only the peaks of the structured first surface and has a separation layer refractive index which is diLrere"l from the first refractive index, ~lth~mp;h these cond;l;ons are not necescl.y to produce the desired optical reflection back lL,oll~ll the first layer. Pl~îel~bly~ there are a plurality of areas having non-ovt;~lapp;l~g sepa,~lion layers (i.e. all s~lbslA~ lly in the same plane appro~ t~ly parallel to the ;~5llale to be protected), each of the separation layers having the same or very similar réL~ re index, and thus a plurality of ll~lSp~elll areas. The second portion ofthe plurality of geolllellic concavities and the separation layer thus define at least one reflective area.
As used herein the phrase "geoll.~,tlic concavity" means a concavity 5 defined by shaped protlus;ons which have at least two planar facets, such as prisms, pyramidal p~ us;ons, cube-comer protrusions, and the like. The phrase does not include concavities defined by protrusions which do not include planar facets, such as protrusions present in holographic films. The term "ll~l~enl" means l~An!~ ;ng at least 90% of in~Y~nt light in the visible ~,ccllulll (about 400-700 n~l-o...~l~rs wave length), as dctel~ ed by a standard spe~;lrophotometer. In prerelled embo~limpnt~ the plurality of geometric concavities are defined by a series of parallel linear prisms having planar facets (the facets ofthe linear prisms are either ~ llcllical or a~lll,llëtlical), such as in a ;~sl~ l;Ally totally internal reflecting film. "S~ iAlly totally internal reflecting" means that the film has a T-Test Value of 5% or less, wherein the T-Test is further e-p~ ed herein. Another plertlled embo~impnt is that wLereill the plurality of geolll~tlic concavities ofthe first layer ofthe article coll.plises a diffraction grating.
Another plerellèd embodiment is that whe.eill the plurality of geometric concavities of the first layer are defined by cube-corner structures, thus making the article r~lolenective when viewed under retroleIlecli~re viewing conditions.
~ The first layer IJI értl ~bly Col~ iSeS an organic polymeric material selc~1ed from the group consisting of free radical addition-poly,-,~,i~d polymers and lhellllopla~lic materials, particularly plerelubly a free radical addition-polymerized polymer.
In plertl I ed articles in accordallce with the invention, the adhesive layer 25 colll~lises a hot melt (lh~loplaslic) adhesive or a plei,su-e-sensitive adhesive. A
surface ofthe adhesive layer opposile the plurality of geGII.~l-ic concavities ofthe first layer may have a second layer removably adhered thereto, such as a release liner.
The plurality of separation layers preferably co...p.ise an im~ng material which may be colored or colorless. One pl~;rêlled im~ng m~t~ l iS an acrylic ink.
Another aspect of the invention colll~"ises a tamper-proof article colllpl;~lg the inventive optical seculily article secured by the adhes*e layer to a ;~uS~llale to protect the ;,uS~llale from l~llpelillg and/or to çnh~nce the appearance of n~thPnticity ofthe s~b.,~ale. The substrate may be e~s~ ;Dlly any ;,~sl-ale such as metal, wood, paper, plastic, ceramic, but is typically a photograph of an ID card or drivers license, a bank note, a stock certificate, or the like.
If the s~p& alion layer is an imD~ material, such as an ink imprint of a S logo, the logo is plO...: .e.~1 when the tamper-proof article is viewed at the correct oripnt~tionl coming to the foregro,md over the underlying sul~sl,ale. When the tamper-proof article is lt;G-ie ded~ the logo disdppe~ and the underlying substrate is clearly visible. If an attempt is made to remove the optical security article, the continuous adhesive layer col ~ ;~ the v~lid~tir~ logo is at least partially desl-oyed.
Methods of making the optical security articles and tamper-proof articles of the invention are also described herein.
A first method for making an optical se~;u.ily article ofthe invention co...yl;ses the steps of:
a) plt;pa.ing a coatable composition co,-ll,lising a curable organic resin;
b) providing a production tool Colll~liSing a first plurality- of geometric concavities and cG..~ onding peaks;
c) coating the contn~'e colllpGs;lion onto the production tool, thus sllb~ ly filling the first plurality of concavities;
d) contncti~ a first plastic film with the co~tnble composition on the 20 prod~c.tion tool, at least one of the production tool and the first plastic film being flexible;
e) eAI,oshlg the composition to conditions s~fficient to cure the resin of the composition to yield a first interrnP~ tecGnl~lis;ng the first plastic film and a cured composition, the cured composition having a first refractive index;
f) removing the first inttl.. eJ;~,le from the production tool, the first int~-. .ç~ e having a structured first surface and a sllbst~nti~lly smooth second surface, the structured first surface CGIll~ g a second plurality of geolll~l-ic concavities and cGll~,i,ponding peaks which are the reverse image of the first plurality of geolll~-ic conc~vilies and cGll.,s~,onding peaks;
g) providing a second plastic film having first and second surfaces;
h) appl~ g a separation material on a sPlected area of the first surface of the second plastic film to form a second inltl...e ~ e; and CA 0220364~ 1997-04-24 W O g6/142is PCTrUS95/12005 i) adhering the second inte,..~ediq~e to the first intel...edi~te with an adhesive layer, the adhesive layer having a second refractive index which is ~.ibsl;...l;Ally similar to the first cfi--clive index ofthe cured co""~osi~ion, such that the sepqrntiQn material is dis~,osed h ~ ~n a first portion of the second plurality of &eo...~ tl ic 5 con~ avities and a first portion of the adhesive layer to form a reflective area, and ~I.c~ill a second portion of the adhesive layer adhesively cont..-t~ and fills a second portion ofthe second plurality of concavities to form a l~s~,arenl area.
A method of making a tamper-proof substrate co""),ises the further steps of:
j) re~o~ g the second plastic film from the optical se~;u~i~y article, thereby cApoiing the adhesive layer; and k) adhering the optical se~ u,ily article to a substrate employing the adhesive layer.
Alternatively, the first inventive methods just described may be modified lS in that the coatq-~le co""~osilion may be applied first to the first plastic film, and then the coated film contqcted with the production tool, this modification being the second inventive method A third inventive method for making an optical se~ u~ily article of the i"~elltion method co",p,ises:
a) prepa.ing a coatable co""~osilion CG~ JliS;ng a curable organic resin;
b) providing a production tool co...p.ising a first plurality of geo".ctlic concavities and co"~ ,onding peaks;
c) coating the cost~hle composition onto the production tool, thus s~t;--.l;Ally filling the first plurality of geo",~ic concavities;
d) cont~cti~ a first plastic film with the co~At~le composition on the production tool, at least one of the production tool and the first plastic film being flexible;
e) c,.po~ing the composition to conditions sufficient to cure the resin of the co",po~;lion to yield a first in~e,...e~ le ofthe first plastic film and a cured 30 collli)Gs.tioll, the cured composition having a first refractive index;
f) removing the first inte,..~e~ e from the production tool, the first il.lt;,...e.~ e having a structured first surface and a substAntiA~Ily smooth second surface, WO g6/14215 PCTtUS95tl2005 the structured first surface co.,.~.is;"g a second plurality of geomd-ic concavities and cGl,~,~ol~d;ng peaks which are the reverse image of the first plurality of geometric collca~ilies and col,~,..yonding peaks;
g) providing an adhesive layer, the adhesive layer having a second 5 refractive index which is s.,l,st~ y similar to the first refractive index ofthe cured CO...i GS l;on, the adhesive layer having first and second surfaces;
h) applying a separation material on a sFIe~ted area ofthe first surface of the adhesive layer to form a second intc;,...e.1;~1e, and i) adhering the second intt,...e~i~le to the first inte,~e ~ e such that the I0 separation material is disposed bel~ee.~ a first portion ofthe second plurality of peol. ~ct~ ic concavities and a first portion of the adhesive layer to form a reflective area, and ~hcre;n a second portion ofthe adhesive layer adhesively co~tactc and fills a second portion ofthe plurality of gP,O...(~,ic concavities to form a l-~r.~arenl area.
The third inventive method may be modified in the same manner as the 15 first inventive mpthod~ thus produçi~ the fourth inventive method The inventive optical security articles provided herein thus comprise a first l-~nspalenl optical layer having a first refractive index RII and having a smooth surface and a structured surface which bears a microstructure having a plurality of optically utilitarian geometric concavities, the smooth surface being ~d~pted to receive 20- inr;dPnt light and having a light~ g area and a light-reflecting area. A second e.~l layer, having a second refractive index RI2, iS secured to the structured surface ofthe first layer and contacts and bonds to the flicco..~ ;l;es in the light-e area. The second refractive index RI2 iS sllb;~l~nl;~lly similar (pr~lably itlentic~l) to the first l~acli~e index Rll. A separation layer is disposed bc;lween the 25 first l~ art,lt optical layer and the second l,~ spale..l Iayer in the light-reflecting area such that the second l-~ spare--l Iayer does not contact the geo,..ct~ic concavities, thus ~efinin~ the light-,tiflecl;..g area due to air gaps b~t~neel~ the separation layer and the second l,~,s~ e,ll layer.
Furtha underst~ntli~ of the invention will become appa~h~l from 30 reviewing the brief desc,i~Jtion of the dlawing figures and description of prefe"ed embo~;... -~1s which follow.

W Og6/142i5 PCTnUS95112005 Brief Description of the Drawing FIGs. 1-3 are cross section~l views through three embodimp~nt~ of optical se~u~ artides ofthe i..~ ion secured to a ;..lb~l,ate, FIGs. 4 and 5 are scl~ ic process flow diagrams of the methods of 5 making the inventive articles; and FIGs. 6-7 are s~h~ ;c diagrams of two usefiul methods of making the i..~,~,.lti~e artides.
These figures, which are ide~li7e~l are not to scale and are int~n-led to be merely illustrative and nol~l;...it;.~g Description of Preferred Embodiments L Optical Security Articles A. Reflecting Layer Refe..i.lg to FIG. 1, an authentir,~ted article embodiment 1 is illustrated, 15 having an optical se~,u-ily article 10 secured to a sub~l.ale 12 such as a bank note, credit card, drivers license, pe. ~onal idçnfific~tion card, stock certificate, and the like, article 10 filnctiQr~ to protect s~llate 12 from tan~p~;.ing and pc.l.aps provide a more ~,~"l~ liC appuh~ce.
Article 10 comprises a first Iransl)arelll, prere.ably continuous layer 14 20 which is typically and preferably a thin, flexible polymeric film and comprises a smooth surface 16 and a structured surface 18, structured surface 18 bearing a plurality of optically utilitarian geo..-- ~l ic concavities and peaks, the geo-.~ ic conca.~ilics and peaks ~,r~rc.~bly defined by parallel prisms having planar faces or facets.
Article 10 is typically and preferably ~cllureIlective in the sense that inrid~nt light striking smooth surface 16, within certain angular ranges, will be ~s~ ly totally internally reflected at structured surface 18 in at least some areas of article 10. When structured surface 18 is a plurality of parallel linear prisms and the angle bclween facets is 90, light i~ d~ ~1 upon smooth surface 16 will be totally internally reflected when the in~ident light is refracted by smooth surface 16 and then strikes the facets of structured surface 18 at an angle, with respect to the normal ofthe facet, which is greater than a critical angle. The critical angle, in air, is defined as the arc sine of the reciprocal of the index of refraction of the material. In a~dition a si~ific~nt portion of the inrid~nt light WO 96/142i5 PCT/US95/12005 striking smooth surface 16 which produces refracted light which strikes structured surface 18 at angles less than the critical angle will be ~ c~ ed through first layer 14 and the fe ~ nder ofthe incid~nt light will be reflected by smooth surface 16.
I,.~,s~,e~ e ofthe ."ltl~ce angle ofthe in~id~Pnt light relativeto smooth surface 16, 5 there is n~P.gli~ble abso,~lion of light by first layer 14.
In the pi~fe.,ed first embo~limpnt 1 illustrated in FIG. 1, first layer 14 co~ -- ;~,s a ~lbs~ y totally intemal reflecting film (sG~ es rerf .~ed to as anoptical lighting film) of the type ~ osed in U.S. Pat. No. 4,906,070 (Cobb, Jr.), the dicclos~.re of which is incorporated herein by r~relence. 'ls~lbst~ntiDlly totally intemal 10 rPflecting" p~;,lains to the optical quality ofthe film. The optical quality oftotally intemal rPflecti~ films ("TIRF") can be evaluated as follows. The optical quality of a TIRF can be evaluated with appa~ s int ll~din~ a laser (Spectra-Physics Inc. Model 117A) with a spatial filter, a beam eypander~ and a collimator. Two diaphragms or irises are placed 18 and 38 cm from the laser and aligned lhf .~;wilh, and an armular sample holder with an opcnil~ 6.35 cm in f~ f~tlFl iS placed 84 cm from the laser. Directly behind the sample holder is an integ,aling sphere (with a 3 cm ~iA~-f,tÇ~ apt;,lu,e) and a LABSPHERE ML-400 r~1iorneter. Using the diaph,agms or irises, the laser is focused through the aperture to obtain a clean circle of light of about 3 rnm tliDnl~ter on a black surface mounted on the sample holder. A source inlf .IS;ly measurement of 100% is taken with no sample in place. The TIRF to be tested is then mollnted on the sample holder with its flat surface facing the laser and its grooves ~ g vertically. Unless otherwise lepGIlcd, T-Test Values are ",eas-lr~d at ambient tc~"p~alu~e. Readings are then made at from 12 to 15 di~erenl points on the T~F within a 5 cm ~ eter area while making sure that none of the light strikes the frarne of the sarnple holder. The f~a~;~c are averaged and mllltiplied by 100 to give percent ~ n~ icsiol~ which is the T-Test Value of the TD~F sample. T-Test Value is a criterion of the fidelity of rep!ic~tion of the TIRF. Smaller T-Test Value p~ ges intlic2te better fidelity of replic~tion than larger pelce-.l~f.~ and a T-Test Value of 5% or less in~;cntes that the film is ~lb~ ly totally internal reflecting In U.S. Pat. No. 5,183,597, plastic articles having microstructured surfaces are dic~ sse~ such as ~or~llective cube-comer ci.eel;ng Fresnel lenses,totally intemal r~flecfl~ films, il~"l,alion Call~ring discs, and the like. A dictin~ion is WO 96114215 PCI/US95tl2005 g drawn be~.~n microstructured surfaces having a fine microstructure (diffractio glalings~ video master discs) and those having relatively large or "coarse" microstructure (~.h~ the microstructured surface co.~ .c a plurality of optically utilitarian di~co~ g l~1ging in depth from at least 0.025 mm to as great as about O.S mm).
5 For the ~ oses of the present invention, the optical quslities of both fine and large i,clo~tl~ctured cube-corner and plisn~lic films are useful.
When using a two tlimPnQ;on~l array of parallel linear prisms, prism grove depth and pitch (~ nee b~ . een peaks) may be ndjl~cted to achieve the desired balance bet~n reso~-ltion ofthe image (preferably a logo or other indicia formed preferably via 10 an acrylic ink) and the allowance ofthe adhesive to ndequ~tçly fill the desired portion of grooves to produce ll~l~l,arenl areas. The lower limit in prism size (groove depth) and pitch is dtle~ ned by diffraction (a pitch of about 0.5 ll.icrc,llleler results in a diffraction grating); however, smaller prism depth and pitch hlcrease resolution of the logo. For ~,alest logo resolutiQn a depth of 2 lll,c-ollleters or less is prerellèd. The upper limit 15 of prism groove depth and pitch is delellllined by the ability ofthe adhesive to n1eq~l~tely flow into the grooves. When a two fl;~ ncion~l array of parallel prisms is used, individual prisms may have either the same or dirrerell~ facet sizes, pitch and angles bc l~.een facets. Preferably, the size of each prism is the same, and the facets all make an angle with the normal to smooth surface 16 of appro~ Ply 45 and angle betv~ce 20 facets of 90. By choosing the prism shape (i.e. by sPle.~ P the facet sizes), the verification angle can be ndj~lsted Ref~,.ling again to FIG. 1, structured surface 18 of first layer 14 pterGIablycC;lllplise,s a linear array of a plurality of min;~t~lre, s~b~tA~ ly right angled icoscçles prisms ~l ~1ged side-by-side in parallel relationship to form a plurality of peaks 25 20 and grooves 22. The ~lb~ lly pel~.çndicul~r~ planar facets ofthe prisms in this ~mbof~ nt pr~l ably make an angle of appro~ hlla~ely 45 with smooth surface 16 when smooth surface 16 is ...~ A;ned in a planar position, and 90 bct~e~::n facets.
First layer 14 is secured to sub~lla~e 12 through a second ~ s~,~en adhesive layer 24 which is secured to structured surface 18 of first layer 14 and 0 adhesively contacts and fills a first portion of the geo.,l~ ic concavities in a plurality of areas 30, second layer 24 having a rer,acli~e index RI2 which is ~ib~ ly similar (preferably idçntic~l) to the refractive index RI~ of first layer 14. ~l~;rt;-~bly the WO 96/14215 PCr/lJS95/12005 ratio of RII:RI2 preferably ranges from about 0.9: 1.0 to about 1.1: 1.0, more pl èrel ubly from about 0.97:1.0 to about 1.03:1.0, and is particularly prefel~bly 1.0:1Ø For example, if poly~l,onale were used as first layer 14 (RII=1.58) then adhes;~e layer 24 would preferably be an ethylene vinyl acetate adhesive (RI2=l.54). T.~ en~ areas 30 5 are dF-~ntcd as such because in~idPnt light striking front surface 16 in these areas is substLq~ y completely lli~n~ led through the inventive optical se~,ulily article to underlying ~lb;~lale 12. The l~e-..q;~ ~P. area or areas ofthe inventive article are denoted as reflective areas 32, reflective areas 32 being des~ilibed in further detail below.
In reflective area or areas 32 of article 10, a plurality of separation layers 10 34 are disposed bel..~en a second portion of geollletlic conca~ ies (in the case of PmhodirnPnt 1, grooves) of structured surface 18 and second lluh~lJalenl adhesive layer 24. Since second ll~sparelll adhesive layer 24 does not contact the ~).eo~ ic concdvil;es in reflective areas 32 because of separation layers 34, pockets of air 36 are formed b~ ,en the second portion of concavities ai~d the sep~alion layers 34, rc,llll.ng 15 reflective areas 32. Thus, an air interface is provided along the surfaces ofthe geGIlletlic conca~ilies in reflective areas 32. It should be unde.~lood that the terrns "air", "air pockets" and "air interface" as used herein are general terms and reflect plerellèd embo~imPntc only. The en~ironlllent in which the optical se~iulily articles ofthe invention are produced will detel Illine generally whether pockets of air, nitrogen, or 20 some other fluid (gas or liquid) are formed.
As alluded to previously, there is at least one and typically a plurality of the reflective areas 32 in the inventive articles. These areas are denoted as being reflective because incidçnt light striking front surface 16 of article 10 in reflective areas 32 at an appropliale angle of in~idence is sllb~ lly letlulenected as des.;lil,ed above.
25 Air pockets 36 along the surfaces of the geolll~ llic concavities are needed for ~èt~olenection to occur in reflective areas 32. Air in air pockets 36 provides for a ~i~.;r.c~ change in refractive index along the surfaces ofthe second portion of geo. - -Ftl ic concavities in reflective areas 32. The change in refractive index should be at least 0.1 Rl units, more preferably at least 0.7 RI units. Conversely, as described above, 30 in ll~ls~al~,lt areas 30, there is su~s~ y no change in refractive index along the surfaces of the first portion of geo..~- 1. ic concavities. Thus, in ll ~rls~ ,.ll areas 30, light is t~ ed through first layer 14 and thrc gh second layer 24 to underly-ing;..ll,..t.ale 12.
Rere,li"g to FIG. 2, tamper-proof article e",bod .~ 2 is illustrated in ClOS3SeCtiOn F-mbodimpnt 2 of FIG. 2 is similar to embo~lirnPnt 1 of FIG.l except that 5 optical security article lOa is formed via a difre~ t process and thus has a slightly di~,enl construction (the p,occsses of prochlction are des_,il,ed herein below).Sepa~aliol layers 34 extend 041y a portion ofthe ~l:c~ ce b~t~cen structured surface 18 and s~,dte 12. This does not change the filnction of sepa~alion layers 34, however, in ple~enling flow of adhesive 26 into a second portion of Peo,..etlic conca~ilies to form 10 air po~l~Ptc 36, thus prod~c;ne a plurality of reflective areas 32 and a plurality of ll~uljyarc.lt areas 30.
First layer 14 preferably co,..p,iscs a polymeric material selected from the group cQI.~ c~ of addition poly,..elized resins (actinic or particle radiation-cured), and embossed ll.~,.",oplasl;c materials. The structured surface of first layer 14 is most 15 preferably formed using an addition poly-merizable resin, more prerelably an ultraviolet or visible M~li?tion-curable resin, due to the ability to opl,nh~e curing co~rlitiol~ and photoi~ o,:; for these resins used in their production. First layer 14 prere,~lyCG~ liSeS a co""~cs;te plastic article characle,ized by a flexible or rigid plastic support layer, preferably a plastic film, one surface of which bears ",icrosl".~cture dçfining 20 structured surface 18, such as parallel prisms or cube-corners. The microstructure is formed with the use of a coatable cû~pGs;lion cGlllylis;ng an addition poly",~ le and crosslinkable resin and a production tool or master, as described herein below.
Useful and prerelled materials for plastic films onto which may be applied the l....;,u~ cture include polyester, such as polyethylene terel)hlh~ e~
25 polyamide; polyca,l,onale; poly(vinylchloride); poly(vinyli~neçhloride); c~ ose acetate buly~dte, celll~lose acetate p,op.onale, poly(ether sulfone), poly(r"~,ll,y' : ' - ,rylate), polyur~thalle, and the like.
Polycall.onàle is one prc;relled plastic film material because it is econo...;c~l optically dear, and has good tensile sl,e, glh. When its thickness is about 30 0.1 to about 1.2 mm, its has cllffi~ nt ~t~englh and fl~oYihility to permit it to move a bead ofthe coatable co",pos;l;on across a rigid master negative moltli~ surface, as des_,il,ed inU.S. Pat. No. 5,183,597, incorporated by lerere, ce herein.

CA 0220364~ 1997-04-24 WO 96/14215 PCTrUS95/12005 Another p-ere-led plastic film onto which the microstructure may be applied is the polyethylene terephth~l~te film known under the trade de~ Al;on DSCT-PET, from Douglas ~ncon Co., having a thickness of 0.127 mm. The plastic film thickness used in first layer 14 may range from about 0.1 mm up to about lO mm.
The visc~sily of the coatable compositions useful in producing the structured surface 18 ofthe articles ofthe invention is an hllpol~ con~;dPration, as alluded to briefly previously. If the ViSCGS;Iy of the coat~ble composition is too high, the composition will not flow ~e~ ely into the gP,o.~ tlic concavities where and when desired. If the viscosity is too low, filling ofthe conca~ilies is ~cco..~rli~hP,d more 10 efficiently~ but the resins tend to shrink more upon curing.
The viscos;ly of the coatable composition prefélably ranges from about 1000 to about 5000 c~ ;poi~e (CpS) (measured using a Brookfield viscometer, # 3 spindle, at 30 rpm, at 25C). Above that range, air bubbles may be cnl,apped in the co...l os l;on, and the comrocitiQn may not c~ ~letely fill the concavities ofthe master.
15 If an attempt were made to obtain a ViSCGS;ly below that range, the overall equivalent weight (weight per number of reactive groups) ofthe ?~(lition poly."e,i~ble resin ofthe co~t~ble co-"posilion would be so low that the resin would experience shrinkage upon curing to such an extent that the cured resin would not f~ithfi~lly replicate the master ...ol li~ ~g surface. Pl~fel ably the viscosity of the resin ranges from 2,000 to 3,000 cps.
20 Within that prefel.ed range, the coatable co...pos;lion CGlllpl;S;l-g the addition polymerizable resin should completely fill the concavities without the need to apply more than hand ple,~ le. However, when the concavities are llmlcl~slly deep and/or narrow, it may be desirable to reduce the ViSCGS;Iy below about 2,000 CpS, because some shrinkage is pleÇelldd over any failure to fill the concavities co,..pl~:~ly.
In order to achieve the desired co~t~ble composition viscGs;ly, it is plefélled to employ a reactive or non-reactive (inen) diluent in the co?t?ble co."posilion. EthylP!nF ~ ly unsaturated monomers such as the alkyl acrylates, or others tlicrlosed in U.S. Pat. No. 4,576,850 (Martens), which is incorporated byler~rence herein, are suitable reactive rlihlent~ As used herein the term "diluent"
comlolFs a low rnr'ec~ r weight (less than 500) organic material that decreases the viscosity of the composition to which they are added WO 96/14215 PcrluS95/12005 Low molecul~r weight acrylates are one prert. . ed t-ype of reactive diluent. Acrylate reactive dih~pnts prt;re,-ed for use in the invention typically have a mcle~ r weight ranging from about 100 to about 500, and include ethylene glycol d.acl~ldle, ethylene glycol rlimpt~scrylate~ h~.~nediQI diacrylate, triethylene glycol S diacrylate, ~ tll~ylolplopane triacrylate, glycerol l-;u,l~ld~e, pentaerthyitol triacrylate, penta~rlh~ilol l.i...ell.acrylate, pentaery-thritol tetraacrylate and pentze,yll--ilol t~t-~..~lLacrylate. Methyl ...~ Ac~ylate and ethyl mP,t~--rylate may also be used.
Other useful reactive ~ nt~ include monosllyl, polyallyl, and poly....lhallyl esters and amides of c~bù~ylic acids (such as diallyl phthslste, diallyl ~s.~iipate~ and N,N-diallyladiparnide); tris(2-acryloyl-u~yc~ yl)iso~;y~urate~ 1,3,5-tri(2-methacryloxyethyl)-s-triazine, acrylarnide, methylacrylamide, N--..eth~lacrylamide, N,N-dimethylacrylamide, N-vinylpyrrolidone, and N-~i..ylpipe.idone itiQn poly.--e.i~able resins useful in the practice ofthe invention are those resins capable of being initi~ted by exposure to radiation, a photoinitiator, a 1~ thermal i- ilialor, or cG---binalion ofthese. Non-particle radiation in~ludes ultraviolet rz~istiQn~ inr.~ed radiation, and visible light, while the most cGI.... only used and pr~re,led particle radiation used is elc~,lron beam irradiation A cG... Ibillalion of particle and non-particle radiation curable resins may be used, but resins which may be i~ ;ated by W or visible light are prwe,.lly prere..~d.
20 ~ ~ Addition poly...~ able resins poly...t.i~e via a free radical .~echan~ or an ionic .~.erlU~ Free radicals or ions may be produced by addition of photo .i1 ;r~GI ~ or thermal initiators to- the resins When a pholoil.;l;a~or alone is used, or when it is c.~osed to non-particle radiation such as ultraviolet radiation or visible light, the pholoi~-;l ;~l or gel~lales a free radical or an ion This free radical or ion initistes the 2~ pol~l.le,i~lion ofthe resin.
Examples oftypical and plert..ed addition poly~ne~ ~le resins for use in the coatable cG...po~;lions useful in the invention include: polymers, oligomers, and ... OI~O.. ~ which are ethylenically unsaturated, such as styrene, divinyll,el zene, vinyl tO~I ~n~, and a.l,nlopl~l resins having pe--d~ll unsalulaled C~hbGnyl groups, and the lilce, 30 ~inc~ ~ those having at least 1.1 penda..l alpha, beta unsalul~led CalllGIlyl group per mole~l~le or oligomer as des_-il,ed in U.S. Pat No. 4,903,440, which is hereby ih~cOl~Glaled by rererel-ce); acrylated resins such as isocya,.~lrate resins having at least W 096/14215 ~ 9~/12005 one pe~ n1 acrylate group (such as the triacrylate of tris(l,yd~ o~yethyl) iso~i~allul ale)~
acrylated u~ hane resins, acrylated epoxy resins, and isocyanate derivatives having at least one pend~lt acrylate group.
A particularly ".t;L.-~d compGs;lion is a one-part, solvent-free, r~ tion 5 ~litiQn_polymerizable~ crosslinLable, organic oligo... ic cG...pos;lion having hard se~ c and soft se~ plus a pholo ~ or present at from 0.1 to 0.5 % by weight ofthe resin, ~LC,~till the hard se~ are preferably polyurethane and the soft Se~J..f~ are piefelably polyester, as ~i~closed in U.S. Pat. No. 4,576,850 (Martens), previously i.lco.~,o.~led by ..,fe.ence herein. A particularly plefe .ed composition 10 form~ ted in accordance with Martens is given in Fyamr'e 1, below.
It is to be understood that Illi~lules of the above resins could also be employed. The term "acrylated" is meant to include monoacrylated, .~ono~ ylated~ multi-acrylated, and multi-meth~crylated monomers~ oligomers and polymers.
It is noteworthy to mention that monomers which are solids at room tenlpc.~lule may be used if dissolved in a suitable solvent. This is the case with the triacrylate oftris(hydl.~y~thyl) iso~;ya,.u.ale ("TATHEIC"), one particularly pi~ru.ed resin, which is a solid at room t~.llphalure. When this monomer is used, the "polymerizable resin" for which viscosily reduction is a~ ed inchldes the solvent, 20 which may or may not be reactive with the monomer, but plc;rt;-ably is reactive with the monomer (and is ll.erefore considered another monomer). One p-ere"ed solvent forroom t~ 'lpelalllre solid acrylated monomers is l.hl.etLylol p~opane triacrylate("TMPTA"); however, solvents such as these are more collti-;tly referred to as reactive Pnt~ when the poly",e.~le resin is already liquid at room twll~lalure (i.e., about 25 25C). When TATHEIC is used, the colllbillalion of TATHEIC/TMPTA is considered as the poly~le. ble resin. The weight ratio of TAl~IC/TMPTA may range from about 1:2 to about 2:1, more preferably from about 1:1.7 to about 1.7:1, most p.~r~.~ly 1:1.
Acrylated iso~;yd~lulale oligomer resins are the pres_..lly prere,.ed 30 addition polylll~. ble resins. IsO~ya"u~a~e resins useful in the invention include those having at least one penda,ll acrylate group, which are desc,ibcd in U.S. Pat. No.

WO 961142i5 PCT/US95tl2005 4,652,274, incG.I,olaled herein by rerelcnce. As mentioned previously, one particularly p~r~ .ed iso~;ya~ lale material is TAl~IC dissolved in TMPTA.
Ac~yldled urelhane oligomer resins . re pleÇe-ably acrylate esters of L~nhuAy-termit ste~l isû~alc e-l~nded polyester or polyether polyols esterified with 5 low -'ec~ls~ weight (less than about 500) acrylates (such as 2-k~droA~_lhyl acrylate).
The ..I"lbcr avel~f- molecular weight of p,~re"ed ae,ylaled ur~th~e oligo",er resins ranges from about 300 to about 10,000, more preferably from about 400 to about 7,000 F.~ S of conll"~rc;ally available acrylated ur~hane oligomer resins are those marketed under the trade design~stiQnc "UVITHANE 782" (available from Morton Thiokol Chem:-s-l) and "CMD 6600", "CMD 8400", and "CMD 8805" (available from Radcure Sperigltips).
Acrylated epoxy oli~omPr resins are acrylate esters of epoxy resins, such as the diacrylate esters of bi~phf nol-A epoxy resin. F.Ysmr'~s of con....e~cially available acrylated epoxy oligQ~ ,r resins include those known under the trade dec;g,- ~l;onc "CMD 3500", "CMD 3600", and "CMD 3700", also available from Radcure Specialties Non-radiation curable urelllane resins, epoxy resins, and polymeric isocyanates may also serve as the poly,ll~,i~ble resin in coat~hle co"ll)os;Lions useful in the invention. U~ es useful in the invention include those disclosed in U. S. Pat. No.
4, 933,373, hlCGIIJtJIaled by ,~ference herein, which are the r~etion product of shon-20 chain, active hydrogen fimrtiQnql mol~o~ such as l,i.llelhylolplopane mono~llyl ether,ethanol amine, and the like; long-chain, active hydrogen fimr,tionql diene prepolymer, such as the Ly~lluAy-te~ ed polybut~liPne co.. e~ .;ally available from .Atochem Inc. under the trade dec;~n~l;on "Polybd R~5HT"; a polyisocyanate, and a clocclil~L ;.-g initiator. S~it-ble cros ~ L;ng ;nll;alol~ are organic peroxides, such as benzoyl 25 peroxide, and the like. Ur~ ai-e catalysts may be used, ~Ithollgh not r,ss~..l;Al such as those m~ntioned in U.S. Pat. No. 4,202,957.
Epoxy resins have an oxirane (epoxide) ring and are polyl.ltl~ed by ring openl,lg. Epoxy resins which lack ethylenically unsaturated bonds require the use of phûto; ~ o~ ~. These resins can vary greatly in the nature of their backbones and 30 ~ll,st;luent groups. For ~ ~ ..plç, the backbone may be of any type normally ~cso~ ed with epoxy resins and subs~ ;~u~ -.l groups thereon can be any group free of an active Lyd~u~,ell atom that is react*e (or capable of being made reactive) with an oxirane ring W O96/14215 PCTrUS9S/12005 at room t~.llpclalllre. Reyle3enla~ e e,~, ples of acceptable s~lbstitll~nt groups include hsiQ~Pns, ester groups, ether groups, c~lfon~te groups, sil~y~ne groups, nitro groups and phosp~te groups. F.Y~mp'es of prefel.ed epoxy resins iacking ethylenically uns~lu,aled groups indude 2,2-bis[4-(2,3-epo~,,upu~y)phenyl] y~u~ e (diglycidyl 5 ether of b-~kF1~ol A) and cûl,u,,erc;dlly available materials under the trade ~Ç~ ;Ol~
"Epon 828", "Epon 1004" and "Epon 1001F" available from Shell Chemical Co., "DER-331", "DER-332" and "DER-334" a~.~ilable from the Dow Ch~om:~i Co. Other suitable epo~y resins lacking ethylenically unsaturated groups include glycidyl ethers of phenol formaldehyde novolak resins (e.g., "DEN-431 " and "DEN-438" available from the Dow 10 Chemical Co.).
Addition poly",e.i~ble resins require an initiator, as previously nF,d Examples of useful iLIia~ûr~ that g~en~.ale a free radical upon exposure tor~isti~n or heat include organic peroxides, azo cûn~pounds, q~inonF c~ benzophenûnes, nitroso co,,lyu~lnds~ acryl halides, hydrozones""e.caplo cû",yûunds, y~lyliulu 15 cû~yOullds~ triacrylim:~7.oles, bicim;d~7oles~ chloroall yll.i~inFs, benzoin ethers, ben_il ketals, thioY~nthonF c, and acelophF-none derivatives, and mixtures thereo E~"ple~ of pholoi~ ol~ that when e,.yosed to visible radiation generate a free radical are des_lil,ed in U.S. Pat. No. 4,735,632, incolyû,aled herein by r~;rt:lence. Useful co,.. ~ ~i;ally available photoinhi~tors include those known under the trade decignqtion IRGACURE, from Ciba-Geigy Corp., Ardsley, NY. In particular, the photoin;l;Ato,~known under the trade dçci~stiQnc IRGACURE 651 (2,2-~imethoxy-l-2-diphe"
eth~-ol-e), lRGACURE 369 (2-benzyl-2-N,N-dimethylamino-1-(4-,,,olyholinoph~nyl) 1-1,.,~, n~ne)~ and IRGACURE 184 (1-l,ydloAy.;yclohexyl acelophenone) are useful in fc.,llu,,g the structured surface 18 of the first layer of the inventive articles. The pholo~ lor known under the trade dçcignstiQn DAROCUR 1173 (2-l,ydro~y-2-methyl plopioph~none), from EM Industries, is also a useful photo;~ 0r.
The photoin;l;~or w-"pou-.d is used in only in the amount n~ce;,s~.y to pe,rullll its intPnded filnction Generally, the amount ofthe photoinitiator prt;r~,ably is about 0.1 to about 1.0, more yl ef~;~ ably no more than 0.5% by weight of the resin cûlnyûs;liûn~
Cationic photoiniti~tors generate an acid source to initiate poly",~.i~lion of addition poly"~.; i~ble resins. Cationic photoLliators can include a salt having an WO g6/1421S -17- PCTnUS95/1200S

onium cation and a halogen CG~ g complex anion of a metal or metalloid. Other useful ç~tionic photoinitiAtors include salts of organometAllic c~ pleY cations and halogen-cG~ g c~ leY anions of a metal or metAIIoitl which are further des_-il,ed in U.S. Pat. No 4,751,138, illcûl~Jo-a~ed herein by lerélence. Still other useful r?tion c 5 phot~ GI~ are orgd~G ~11;C salts and onium salts, des_-iled inU.S. Pat. No.
4,985,340, and European Patent Applications 306,161 and 306,162, both published March 8,1989, all inco.~o,~led herein by r erèrence. Yet other useful cationic phot~ include ionic salts of an O~ o.~.e~s~llic cGI.~pl~ - in which the metal is s~lecled from the rll~m~ of Periodic Group IVB, VB, VIB, VIIB and VIIIB, such salts being described in European Patent Application 109,581 (published May 30,1984 c~llJOlated herein by lerel~nce.
~ dition polyll,c~able resins require an initiator such as a photoi~ or and/or radiation energy ~erélably photoinilialû,~ and radiation energy are used simultAnP~oll~ly. Indeed, addition polyl,.c~i~lion rates generally increase with15 telllp~l~lule, so that these resins may be cimllltAneollcly exposed to a heat source The total amount of energy required is primarily dependent upon the resin chpmistry and seconda,ily on the ~ L~GSS and optical density ofthe coated composition and polymeric film or production tool For therrnal energy, the oven tt---pe, alul e will typically range from about 50C to about 250C for about 15 mimltes to about 16 hours 20 For free radical ndditiQn poly...c~i~alion in the absence of heating while cA~,Gs.ng solely to W or visible radiation through a plastic film or plastic production tool, in order to fully poly...e i~e all ethylenically unsaturated monomer, the W or visible energy level should be at least about 100 milliJoules/cm2, more prefé.ably ranging from about 100 to about 700 milliJoules/cm2, using W MtliAtion of about 350-380 nanûllle~
Ultraviolet radiation refers to elecl,u.. A~Ptic radiation having a wavelength within the range of about 200 to about 400 nAno....,lPrs, prerélably within the range of about 250 to 400 n~no~elers Visible rA.~ tion refers to ele.;l,u, ,AgnPtic radiation having a wavelen~;lh within the range of about 400 to about 800 nanometers, and preferably in the range of about 400 to about 550 nAnometPrs Electron beam irradiation, a form of ionizing ra~iiAtiQll~ can be used at an energy level of about 0.1 to about 10 Mrad, and preferably at an energy level of about 1 WO g6/14215 1 ~ 9sll2oo5 to about 10 Mrad, at accelerating po~ ial ranging from about 150 to about 300 kilode~llun volts.
The master or production tooling found suitable for use in the present invention may be p,isl,.a~ic, cube-corner, or the like. For prismatic masters, the pitch may range from about 4 micrometers up to about 0.5 millim~ter, more preferably from about 8 micro,.lct~.~ to about 20 rnicro",~e,s, and the groove depth prefe,ably ranges from about 1 to 20 mic,u",.,t~ , preferably 5 to 12 ll,icr~,lllcter~. For cube-corner geog,~hies, each minute cube corner ele "-~.1 has the shape of a trihedral prism with three e,.l.osed planar faces, sl~b.~ 3lly ~c.~,~n~ lq~ to one another, with the apex of the prism vertically aligned with the center ofthe base. The angle betw~ien the faces is prer~lably the same for each cube-corner c~ -l in the array, and will be about 90 degrees. The cube corner e1P~ ~e-~lS are pler~labl~ all ofthe same .1;~ ;ons and are aligned in an array or pattern of rows and colvmnc~ the bases being in the same plane, and ~ nt ek .,.~ being conti~loll~ at the edges of their bases such that there is no spacing or flat areas bel~ en ndjæcçnt el~ nl~ Generally, each cube-corner ~l~ment has a side edge ~ ,çnr on up to 0.025 inch (0.635 mm), preferably less than 0.010 inch (0.254 mm). The body portion ofthe first l,an:,~are,ll layer 14 (that is, the portion of layer 14 e~ g from the base ofthe prism or trihedron to smooth surface 16) will range in thicl~nçss from about 0.002 inch to about 0.030 inch (0.05 to 0.075 mm), preferably 0.003 to 0.010 inch (0.075 to 0.25 mm).

B. Adhesi~re Layer In all optical sec~ P-~ho~ P~ of the invention, second layer 24 prere,ubly COIll~Jl;SeS an adhesive layer sele~,led from hot melt thermoplastic adhesives and pres;,."~-selisili.~e adhesives, although it is within the invention that layer 24 could comp,ise a l~ e of two or more layers in which the layers contacting structured surface 18 and s~sl,ale 12 are adhesive in nature, l-q-millqtes being less p,~lled due to cost. Adhesive or second layer 24 functions both to adhere first layer 14 to substrate 12, and in conjunction with the sel,aldtion layer(s) 34 form a logo or other indicia.
In ple~lled embodiment 1 of FIG. 1, layer 24 is produced as follows. A
tL~,...opl~l;c film, such as polypropylene film, polyethylene film, polyester film, and the like, has applied thereto a separation layer of the desired logo or other indicia. The -CA 0220364~ 1997-04-24 indicia-bearing f~lm is then heat l~min~ted to structured surface 18. Biaxially oriented polypropylene film is one prefel. ed film useful in this method, in which the film is printed with an acrylic ink, such as the acrylic ink known under the trade designation RAGE-800, available from Advance Process Supply of Chicago, IL. A useful printing process is the screen printing process, such as described in U.S. Pat. No. 5,011,707, incorporated herein by reference. In the heat l~min~tion process, an adhesive having a melting te~l~pe.dt.lre lower than the irlk is used to l~min~te the logo-printed film to the structured surface, using, for example, a l~min~tor heated to about 140C. The composite is then cooled to allow the film to be removed, after which the composite is reheated to melt the adhesive and l~min~te the security article to a substrate.
In embodiments 2 and 3 of FlGs. 2 and 3, respectively, adhesive layer 24 is either a pressure-sensitive or hot melt thermoplastic adhesive, which has been applied as a continuous layer to a plastic film such as those mentioned above. In these embodiments, the separation layer is printed directly onto the adhesive layer. If a hot melt adhesive is employed, it is simply extrusion coated onto a film such as by the method described in U.S. Pat. No. 4,977,003, incorporated by reference herein. The image-bearing hot melt adhesive layer is then heat l~min~ted to the structured surface as in embodiment 1. If a pressure-sensitive adhesive (PSA) is employed, it is preferably an acrylic PSA, typically coated from solution onto the plastic film. In either case, the film is removed a~er adhering to the structured surface, expos'n~the adhesive layer, which is then l~min~ted or pressed onto the substrate to be overh~et.
A hot melt adhesive is preftll~d where a perrnanent bond is required.
The process is also probably more easily made into a continuous production process when a hot melt adhesive is employed. A disadvantage of the use of hot melt adhesives is that heat lamination equipment is required. PSAs afford the processor the opportunity to select the peel adhesion desired, from temporary, as with adhesives known under the trade designation "POST-IT", or permanent. Also, heat l~min~tion equipment is not required when using PSAs. However, the coating of a PSA may evolve solvent vapors.
Suitable hot melt adhesives for use in the invention include polyolefin and alkylene copolymer-based hot melt adhesives, described in Chapter 23, and polyester and polyamide high performance hot melt adhesives, described in Chapter 28, bothchapters appearing in the Handbook of Adhesives, Third Edition, Van Nostrand ~MENDED S~IEE~

W O96/14215 PCTnUS95/12005 R~inhold (1990), which are hlco"~ul~led herein by rerele.-ce. The useful polyolefin hot melt adhesives are primarily ethylene-vinyl acetate (EVA) copolymers ranging from about 18 to about 40 mole percent vinyl acetate, and may be produced in a wide range of c~ ros;~;~ n~ and melt indices. A lower melt index intlic~stes an increase in mcle~lt~r 5 weight and generally improves heat seal ~Llellglll, fl~ .il~;l;~y, hot tack and cohesive lh A high melt index improves low t~llpclal~re pc,ro",lal ce and decreases melt visco~ly. Other than EVA polymers, atactic pol~"uyylene (APP), low density polyethylene ~LDPE), and low mole ~uls~ weight polyethylene (LMWPE) may be used,~hhol-gh APP is tacky as produced and is not easily formolsted Polyolefin 10 homopolymers, such as LDPE, are de~ le if they have high molec~ r weight.
Polyamide polymers useful in prepa,~lion of hot melt adhesives include - those produced using the following IllOIIGlll~ dibasic acids such as dimer acid (d",~ ed fatty acids), dodec~noic acid, sebacic acid, azelaic acid, and adipic acid;
amino acids, such as 11~ o~J~ eç~noic acid; lactams such as caprolact~m and 15 ~odeçql~t~m and ~ es such as ethylene li9mine~ hP~ ylene ~
diethylene ~ ";--e, triethylene disminç~ pip~,~zme, dipiperidyl propal e, and POIYOAYI" ol,ylene di~ r From these ",ono".~ . ~, two general classes of poly ~es useful in hot melt adhesives may be chara~ ,ized as: polyamides from long chain,~ b'e oil-derived acids and short chain ~ ;nf5, and polyamide copolymers and 20 terpolymers of short chain ...-.no....,. :i. Of the latter, hot melt polyamide films are obt~ f d by extruding terpolymers such as nylon 6, 6-6, and 12 or nylon 6, 6-9, and 6-12 on release paper. Manufacturers of polyamides include Rilsan Corp., Bostik F~ctf~n Systems Group, and Emser-Werke.
Polyester polymers useful in p~epa~alion of hot melt adhesives are 25 generally polymers of more than one acid or ester and one or more glycols. ~u---ble acids include those mentioned above in the production of polyamide polymers, plus t~.~k~ and isopk~hiliG acids; dialkyl esters such as dh~clL~l telcph1l~Al~tç, dh~
adipatye, di",eth~l azelate, and the like; and suitable glycols include ethylene glycol, 1,4-iol, 1,6-hf AI~ 4_cycl~h~Ys~ne d;~ h~n~l In form~ ti~ hot melt adhesives useful in the invention, the composition typically co...fl. ;~s three cGlllpontll~s: about 20 to 80 weight percent of a polymer;

WO g6/14215 P~-1'/U~3~/12005 about 20 to 60 percent of a t~qçL ifiPr, and about 10 to 30 percent of a wax, with minor o~ ~ls of other ingredients, such as W stabilizers and ~ntioXi~qvrtc Tackifiers are co~ o~ly added in hot melt adhes1ve formulvtionc to il~crease the r ih^~;on ofthe polymer to various ;,~l~ates. This is due to the increased ~ g of the ;~.tl ~le due to lower ViSCGSity of the composition Many t?^~ifiers are co!.. F,- ~;ally available and useful in the invention, but they may generally be calego-~ed as h~Loc~l,on resins (particularly the C5 aliphatic resins colllylised largely of olefins, ~li.^l^'inc, and isOpl~. e monomers)~ rosin esters and polyterpenes.
The rosin-based tn-~ifiers are useful because they tend to be cor..l~?~ihle with a wide range of adhesive colllyolle~lls. Natural rosins have as their main conctitu~pnt abietic acid, which co.~l-qillc a conj~g,q-ted double bond, thus making them subject to degradation by W, oxygen, and heat. The natural rosins must be modified by hydrogr~ ;Qn, disployo-lionation, or di-ll~ ion followed by esterification with glycerol or pentzc.ytLIilol.
Terpene-based tnc~ifiers may be derived from telyl .lline sulfate and Cn.^nP, Examples include alpha-pinene, beta-pinene, and dipelllene. The alpha-pinenes are more co...~ ;hle than the other tel ~elles with EVA copolymers and are thus prefe..ed tn^l~ifiers with EVA copolymers. Tackifiers which are optically clear are pl~--ed for use in the present invention, such as styrenefisobutylene resins cG.~1~in;'~g a salulaled polymer bacl~one, available from Lawter Interrqtion~ql~ and Amoco's alpha-I,elL;lsly.~ne and poly~ es - Waxes are preferably in~ de~d in useful hot melt adhesives at a weight pe.~enl~&e r~ g from about 20-30 percent to lower cost and reduce v;scos;ly, ~Ith~ugh the trend is for reduced amount of wax in these adhesives. This is due to the lower amount of microcrystalline waxes produced by crude oil refiners, and by the efforts of polymer mqn~lfqc~lrers to modify their products to afford the polymer with plope.lies normally supplied by the wax. Wax content primarily increase the soll~ g point and cohesive sl.englll ofthe hot melt formulation. Microcrystalline waxes (m.p.
- about 90 C), s~ ,lic waxes (m.p. about 100-120 C), and higher melting pal~ln waxes (m.p. about 65-70 C) are often used, the former two for their high telllp~alure yrope~ lies and greater cohesive ~ nglh, the paraffin waxes for their barrier and heat seal plOpw lies.

WO 96/14215 PCT~US95/12005 As a typical eAa."ple, one useful hot melt adhesive colllpGa;lion CGIlllJl ises 30 percent of the EVA copolymer known under the trade dç~i~Ation ELVAX 220 (du Pont), 50 percent of a poly~el~J~ne resin, 20 percent of a mi~..oc",alalline wax, and a small arnount of an ~ntioy~ nt all perCf ~Age5 based on 5 weight.
Hot melt adhesives may be applied to plastic films using e~ ... e known in the industry, such as slot die coaters, roll coaters, pattern coaters, spray applicalo,a, and foam applicalola A wide variety of PSAs may be used in articles ofthe present invention.
PSAs having 180 peel adhesion ranging from about 170 to about 1000 gm/cm, more p-~;Çt;lably ranging from about 390 to about 560 gm/cm, are all useful PSAs, the 180 peel ~h~ n ll.easuled using a alandard test procedure such as follows. A standard glass plate (10.2 cm x 30.5 cm) is cleaned using one wash of iiAcetorl~ alcohol followed by three washes of n-heptAne. With very light tension, a sample (2.5 cm x 40 cm)having a PSA-b~ ci7e coating is then applied along the center of the standard glass plate, PSA side down. The sample is then rolled once with a 2.04 Kg hand roller. The a~nda,d glass plate is then secured to a ho,izonl~ platen in a standard peel Atih~cion tester known under the trade name "IMASS." One end of the sample is then attAçhed to a hook which was a part ofthe peel adhesion tester. The sample is peeled from the ~lalldar~l glass plate at a 180 angle (i.e., one end ofthe sample was pulled toward the other end) by moving the platen ho,~onlally at a speed of 228.6 cnJmin (90 inlmin), and the force r~u,r~ recorded, in gm/cm of sample width, for various dwell times.
The intern~ cohesive all ellglll (shear slle~ ) is a l-,eaa.~ of the cohe.,;~ ess or internal strength of an adhesive. It is based upon the amount of force re.~ui,ed to pull an adhesive strip from a alandard flat surface in a direction parallel to the surface to which it has been affixed with a definite pl esa.lre. It is measured in terms of time (in mim~tçs) re.lui,~d to pull a standard area of adhesne coated sheet material from a st~irl~ss steel test panel under stress of a conslanl, standard load. Internal cohesive all~.lglh (shear strength) of useful PSAs for the present invention can range from about 1 minute to over 10,000 ~ lçs Shear alren~lh tests are typically con~ucted on adhesive coated strips applied to a stainless steel panel such that a 12.5 mm by 12.5 mm portion of each strip is -CA 0220364~ 1997-04-24 in firm contact with the panel with one end portion of the tape being free. The panel with coated strip ~tt~rh~d is held in a rack such that the panel forms an angle of 178 with the extended tape free end which is then tensioned by application of a force of one kilogram applied as a h~nging weight from the free end of the coated strip. The 2 less S than 180 is used to negate any peel forces thus insuring a more accurate determination of the holding power of the tape being tested. The time elapsed for each tape exarnple to separate from the test panel is recorded as the shear strength.
PSAs useful in the structures of the present invention are known in the art and are compositions which may include one or more of latex crepe, rosin, 10 isobutylene polymers, cu-l,arone resins, acrylic-based copolymers, vinyl ethers, alkyd adhesives, rubber adhesives based on rubbers such as natural rubber, synthetic rubbers, and chlGlinated rubbers, polyisobutylene, polyvinyl isopropylene, polybutylacrylate, polyvinyl n-butyl ether, and polyacrylate esters and mixtures thereof. The prese.llly p-trelled PSAs, because oftheir ~nded shelflife and recict~nce to det~c~ifying under 15 ~tmospheric conditions, are acrylic-based copolyrner adhesives as dicrlosed in U.S. Pat.
No Re 24,906. One ~Y~mple of such an acrylic-based copolymer is a 9S.5:4.5 (measured in parts by weight of each) isooctylacrylate/acrylic acid copolymer. Other pref~ d adhesives are a 68:26:6 terpolymer of ethyl acrylate, butyl acrylate, and acrylic acid; a 96:4 copolymer of isooctylacrylate and acrylamide; and a 56:40:4 terpolyrner of 20 isooctylacrylate, vinyl~cet~tç, and acrylic acid. Such acrylic ~scan be coated on the back side of sheet-like se~,.,c ,ls of coated abrasive out of a solution of heptane:
isopropanol solvent and the heptane: is~propanol solvent subsequently evaporated, leaving a p.~s;.."e-scns;live adhesive coating C. Separation Layer The separation layer(s) must be comprised of a material which can retain its integrity while the inventive optical colllpos;le is being produced, and also must perform its function in the finished coln~osile of scpar~liog the second portion of the concavities of structured surface 18 from the second layer 24 in the reflective area or areas 32 ofthe inventive optical security articles. In p-ef~;lled embodimentc~ the separation tayer comprises an imaging material. In addition, the imaging material is preferably selectively applied so that some or all of the information can be in the forrn of WO 96/14215 PcrtuS95/120os so-called "flip-flop" images that are viewable only across a narrow range of angles and change color across that range of angles. Such images can be viewable in ambient light, thus ~rording a readily app~rvnl verific~tion of the allthenticity of the doc~m~nt A
"flip-flop" image can afford a second level of authP~-lic;ly~ by beco~ P. bright when 5 viewed r~vt.o~vnec~ ely, thus making it espe~iqlly difficlllt either to tamper with the documPnt or to counterfeit the novel overlay. When optical se~u,ily articles ofthe invention are used as l,~.",&renl protective overlays for nuth~ntic~ted substrates such as idçntific~tion cards, the im~ng material is preferably colorless so that the im~n,~
material does not totally mask the underlying ~ul~sl~ate in the reflective areas 32 ofthe 10 article.
In le;.~,onse to air-pollution re~ tion~ pleÇel,ed im~in materials are those which are form~ ted to harden (dry) upon exposure to radiant energy: W, elevlron-beam~ and illLared M~i~tion Lithographic and letterpress inks are now available for web-fed and sheet-fed printing presses that will cure-dry ins~ ly when 15 eAposed to ladialol~ ofthe required intensity.
P~-vÇellvd imaging materials for use in the present invention as separation layers are acrylic inks, which coll.l,lise a reactive (Illonolllviic or oligomeric) acrylate vehicle, and a photoinhi~tor. One pl-vr-vlled acrylic ink is that known under the trade de,;enAIion RAGE-800, available from Advance Process Supply, Chicago, IL, which 20 hardens to a colorless image.
~ Ithough the optical security articles ofthe first and second emb~im~nt~
illusllaled in FIGs. 1 and 2 can be effectively used as ll~1s~are.,l protective overlays, separation layers 34 typically subsl~rl;ally mask areas ofthe substrate 12 underlying the sepa~alion layer. This tends to s.ll,s~ ly prevent viewing ofthe s~s~,ale from angles 25 which are s.lb~ lly perpendic~ r to smooth surface 16 of first layer 14. Depen~
upon the nature of ~sl~ale 12, this m~Cl~in~ may be undesirable.
In a third p-er~vlled embodiment 3 (FIG. 3), an "off-axis" article 10b is provided. Article embodiment 10b is similar to the optical security articles 10 and 10a desv,il.ed above except the linear prisms of structured surface 18 are asy"""etlical. The 30 optical sevu,ily article 10b of embodiment 3 differs from the optical seculily articles of emho-lim~nts 1 and 2, 1 esl)ecli~/ely, in terms of pt,ro""ance by shifting the angle at which ~ r~ >, of 5~llale 12 occurs. Typically, m~QI~in~ of ~ul~ le 12 when using optical se~,ulily article 10b occurs at angles which are not sllbst~nt~ y pell~en~1iclll~r to smooth surface 16 of first layer 14. The angle at which m~inE occurs depends on the relative ,~;~.. rn_:one of facets A and B of the prisms, as depirted in FIG. 3, which in turn are det~....... ;~-ed by the pitch and groove depth s~le~:tecl D. Other Embodiments Other e."bod;~ c of optical security articles of the invention are possible. A fourth embodiment is wllelc;~ the Srst layer 14 co~"p,ises a plastic film having a diffraction grating as structured surface 18. A fifth embo~limpnt is v~/Lelein 10 layer 14 co",p,ises a s~l-ksl~ lly totally internal reflectinE film in some portion, and second portion is a diffraction grating. If a diffraction grating is employed, an optical se~,u,ily article is formed having holographic pro~;,lies without re~luiliilg e~L~ip".~,.,l for producing holographs. The diffraction glaling~ provide decorative aspects, in that ;~1. df 1~l white light is separated into seven co",~ol1enl wavf lf ~hs, thus producing a 15 "lahlbOw" effect.

II. Methods of Making the Inventive Optical Security Article~e Methodc of making articles of the invention are now desc, ibed.
Many mf thode may be employed to produce a transparent plastic layer 20 having plural geo..lf t~ic concavities, inrlvdi~ use of ruling engines (particularly for produri~ diffiraction ~;lalul~,s); batch cast and cure methods such as des..,il,ed in ~C`;33J~&~,'S U.S. Pat. No. 5,183,597; continllolle cast and cure processes; and embossing methods such as those desc~ibed in ac~ ee~s U.S. Pat. No. 4,895,428, all illcOl~olaled by lefc~nce herein. Of these, the most pl e~ell ed are the batch cast and cure methotie 25 when first layer 14 is relatively thin and high production speeds are not a concf "l, and co~timlQus cast and cure processes where production speed is desired.
The four inventive metho-lc diecussed previously may be reples~nled sc.h-~ ;c~lly in Figs. 4 and 5, where Fig. 4 illustrates a process flow diagram for inventive methodc 1 and 2, and Fig. 5 illustrates a process flow diagram for inventive 30 methods 3 and 4. One first obtains a master, as desc,ibed in the Martens patent. An ess~ ;q~ step for all ofthe inventive processes is to prepare the co~t~le CO---pGS lion.
The co~ta~le composition is made by co..-ki~-: lg together by any suitable mixing technique the sPlected free radical addition-poly.,.~ able resin, the photoinilialor system and the optional additives, inclu~ing any dill~ents F.Y~mples of mixing techniques include low shear and high shear mixing, with high shear miY~ing being pldre~l~d. Ultrasonic energy may also be utilized in cor..l.: .s~ion with the mL~ing step to 5 lower the cG...pos;lion viscosity. The amount of air bubbles in the slurry can be ...;l~;.~.;7ed by pulling a vacuum during the miYing and/or coating steps (or btl~.~en the miYing and coating steps). In some i~.c~-c~s it is prèrt..~d to heat, generally in the range of 30 to 70C, the composition to lower the viscosity. It is i...po- l~-l the cG-..posi~ion have rheological pfOpC- Iies that allow it to coat well.
The composition is coated on either the front surface of a bac~in~ or onto a production tool (master). This coating can be accomplished by any convention~
technique such as roll co~ting~ gravure coating~ knife cQ~ting spray co~ting~ hsco~ti~ vacuum die coati~ die coating and the like.
After the composition is coated onto the b~r~ing or master, the 15 co-.-t)os;lion is sandwiched belween the master and the bar~ine and the composition is exposed to a radiation source producing radiation in at least some portion of the W
and/or visible s~,e-;l-u-.., and other optional energy sources, depending on the resins and photo;l ilialo.~ used, to initiate poly..,c.i~alion, cure the binder precu,~o.~ and form the shaped plastic article. The res.llting article is peeled from the master and is generally 20 ready for use.
The production tool or master colllains a plurality of concavities. These concàvilies are essenti~lly the inverse shape ofthe structured surface 18 and are res~,ons;ble for genela~ g the shape ofthe structured surface. The dimensions ofthe concavities are selP~cted to provide the desired shape and rlimpn~ions of the structured 25 surface 18. If the shape or dimensions of the master concavities are not p.ope ly fabricated, the res.llting master will not provide the desired dimensions for the geo-..et-ic conca~ilies of structured surface 18.
The concavilies can be present in a dot-like pattern with spaces b~;lween adjacent concavilies or the concavities can butt up against one another. It is prt;re" ed 30 that the concavilies butt up against one another. ~d~itiQnqlly~ the shape of the COl ca~/ities is preft~ly sele~ed such that the cross-sectional area ofthe structured surface decl eases away from the baçL ing CA 0220364~ 1997-04-24 The production tool can be a belt, a sheet, a continuous sheet or web, a coating roll such as a rotogravure roll,. a sleeve mounted on a coating roll, or die. The production tool can be composed of metal, (e.g., nickel), metal alloys, or plastic. The metal production tool can be fabricated by any conventional technique such as engraving, hobbing, ele~ L~ofol""ng, diamond turning, and the like. One prefe"edtechnique for a metal production tool is diarnond turning.
A thermoplastic production tool in sheet form can be replicated of f a metal master tool. The master tool will have the inverse pattern desired for thethermoplastic production tool. The master tool is preferably made out of metal, e.g., copper which is ele~.lroplated onto a plastic master, the latter produced by diamond turning. The thermoplastic sheet material can be heated and optionally along with the master tool such that the thermoplastic material is e nbossed with the master tool pattern by pressing the two together. The thermoplastic material can also be extruded or cast onto the master tool. In both cases, the thermoplastic material is cooled below its melt flow temp. - aL~lre to produce the production tool. Examples of ptefe~ I cd thermoplastic production tool materials include polyester, polycarbonate, polyvinyl chloride, polypropylene, polyethylene and cG",bindlions thereof. If a thermoplastic production tool is utili~erl~ then care must be taken not to genc. a~e excessive heat that may distort the tool.
The production tool may also contain a release coating to permit easier release of the structured surface 18 from the production tool. Examples of such release coa1il,g~ for metal production tools include hard carbide, nitrides or borides coatings.
"ples of release coatings for thermoplastics include silicones a~oroc~ Pmicals, either grafted onto, or i",l.,eg"ated into the thermoplastic, or overla~ onto the therrnoplastic.
One prefel ,ed method of making a first layer 14 such as illustrated in FIGs. 1-3 is to first coat a coat~ble composition onto at least one side of a bacL ing applied using one of the previously mentioned suitable techniques. One pref~, I ed bar.~ing is.a polymeric film, such as polyester film that contains an ethylene acrylic acid copolymer primer. Second, the coated backing is contacted with the outer surface of a patterned production tool. The composition wets the pattemed surface to forrn anintermedi~te article. Third, the composition is exposed to radiation in at least some c~c~g S~

CA 0220364~ 1997-04-24 portion of the W and/or visible spectrum, and other optional energy sources, as previously de3c,il,ed, which at least partially cures or gels the resin in the composition before the illt~ e article is removed from the outer surface of the production tool Fourth, the illt~....e~ e article is removed from the production tool The four steps are 5 prcrt;lably carried out corltinl-ol-cly.
Alternatively, the CG~ os;l;Qr~ may be first applied to the productiQn tool in the m~tho~c i~ustrated in FIGs. 6 and 7. In FIG. 6, b?-~ing 41 leaves an unwind station 42 and at the same time the production tool 46 leaves an unwind station 45.
Production tool 46 iS coated with comrositiQ~ by means of coating station 44. It is 10 possible to heat the co-l-?os;lion and/or subject the composition to ultrasonics prior to coating to lower the viscosity. The coating station can be any conventional coating means such as knife coater, curtain coater, die coater, drop die coater, or vacuum die coater. During coating the formation of air bubbles should be l..i.~;~ . .;~ed The pi ~f~ d coating technique is a vacuum fluid bearing die, such as disclosed in U.S Pat Nos

3,594,865,4,959,265, and 5,077,870, all hlco-yo.aled herein by .~Ç~rence After the production tool is coated, the b~cl~ing and the composition are brought into contact by any means such that the composition wets the front surface of ~he bac~in~ In FIG 6, the composition is brought into contact with a "front" surface ofthe b7^~in~ by means of a contact nip roll 47. Contact nip roll 47 forces the resllhing construction against 20 support drum 43. A source of energy 48 providing radiation in at least some portion of the W and/or visible s~,~t,u"., and other optional energy sources, Il~~ ils a sufficient arnount of energy into the co-nposition to at least partially cure the ~d~litiQn-polyll.e.~Jcle resin in the composition. The term "partial cure" means that the resin precu, so, i are pol~...e. ized to such a state that the cured composition . ~leases from the 25 pro~uction tool. The partially cured resin can be more completely cured, once it is ~e...o~ed from the prod~lction tool, by an app~op.iale energy source Following this, the production tool is ~ewo~lnd on ,..andrel 49 SO that the production tool can be reused ~dition~lly, article 120 iS wound on ...andrel 121.
Alternatively the co~t-ble compositions can be coated onto the b;~c~in~
30 and not into the concavities of the prod~ction tool. The composition coated barl~in~ is then brought into contact with the production tool such that the composition flows into CA 0220364~ 1997-04-24 wo 96/14215 ~ 9S/12005 the concanlies ofthe production tool. The l~ A;~ g steps to make the article are the same as detAiled above.
Another method is illustrated in FIG. 7. Backing 51 leaves an unwind station 52 and the coAt^ble col-lpos;~ion 54is coated into the concavities of the productiQn tool 55 by means ofthe coating station 53. The coatable co...po~;~;on can be coated onto the tool by any one of many te~hn:quçs previously ...~ ;on ed Again, it is possible to heat the coal~Sle composition and/or subject the coatable composition to ulLl~sonics prior to coating to lower the viscosity. During coating the formation of air bubbles should be .-~ çd Then, the ba^~in.~ and the production tool colllA~ p the 10 coatable composition are brought into contact by a nip roll 56 such that the composition wets the front surface ofthe ba~ L-ing Next, the addition-polylllcli~dble resin in the - coatAble composition is at least partially cured by exposure to an energy source 57 providing radiation in at least some portion of the W and/or visible spe-i~l u,,l, and other optional energy sources. A~er this at least partial cure, the co?t~ble composition is converted to a composite 59 that is bonded or adhered to the baç~ing The res ~lting article is I clllo\/ed from the production tool by means of nip rolls 58 and wound onto a rewind station 60. In this method the p-efe.lcd bac~ing is a l-arls~ ent polyester film.
Regarding this latter method, the coatAble composition may alternatively be coated directly onto the front surface of the bacL-ing The coatable composition 20 coated bAçL ;.~p. is then brought into contact with the production tool such that the co2t~Lle composition wets into the concavities of the productiQn tool. The r~ ~ a;~ g steps to make the abrasive article are the same as detAiled above.
The radiation energy can be ~ ed through the production tool so long as the pr~,d~lcl;on tool does not appl~;~;ably absorb the radiation energy.~d~l;l;ol-qlly, the radiation energy source should not al)pre.;;ably degrade the productiol-tool.
Once layer 14 is produced, the method of adhering the structured layer 14 to the sep&alion layers 34 must be selected If a hot melt thermoplastic adhesive is to be employed, the s~,t,&~lion layer material may be printed onto a the.llloplaslic film 30 and the lhe",lol)laslic film heat-lA...;..~Ied in known fashion to !ayer 14, thus folllllng an article of the invention such as illustrated in FIGs. 2 and 3 . Of course, one skilled in the - art chooses the cGlllpos;lion of layers 14 and 24 kçepi~ in mind that the optical WO 96/14215 PcTruS9S/12oo5 p.up~,. lies of layer 14 must be intact in reflective areas 32 after the heat l~min~tiQn If an EVA hot melt adhesive is s~lecte~l the separation areas may be applied via p.i..li..g or other means to a plastic film to form an inl~....ç~ e~ and the illlell~ed;~e brought into contact with layer 14 in the presence of the molten EVA. The plastic film may then be 5 peeled away, revealing the EVA.

E~amples The invention will be further e-p!~ çd by the following illustrative examples which are int~nded to be nonlifniting Unless otherwise inflic~te-l all ~nounls 10 are ~,~y~essed in parts by weight.

-- E~ample 1 Four di~l e..l ll ~ s~rel-l first layers having a smooth surface and a structured surface, and optical se.;u.ily articles inco.~o.aling same, were made using the 15 following procedure. A first structured surface film (film 1 ) was prei)~ ed using a sl~dard p.is...~lic tooling having groove depth of 0.007 inch (0.178 mm) and pitch of 0.014 inch (0.356 mm), and three other structured surface films 2, 3, and 4 wereproduced using s~ ,hic p.i~...alic tools (4, 8, and 16 micrometer pitch). The structured surfaces were replicated from a nickel master onto a 0.005 inch (0.127 mm) 20 thick film known under the trade des;en~;on DSCT-PET from Douglas ~nCorl Co.
using the r(,llowing procedurê
A resin co...posi~ion was prepaled having a ViSCGSily of 1600 cps and concicted of 54.3 parts acrylate-capped polycaprolactone urethane oligomer; 16.3 parts N-vinyl pyrrolidonç; 11.3 parts [(ethoxy)-2-ethoxy]ethyl acrylate; 5.7 parts 1,6-h~.,.nç~iol diacrylate; 11.1 parts N-(isobutoxy methyl)acrylamide; 1.0 part tertiary amine mixture (known under the trade design~tion TINUVlN 292 from Ciba-Geigy, Ardsley, NY); and 0.25 part 1-hydroxy cyclohexyl acetophenone.
The nickel master was coated in each case with the liquid resin and overlaid with the PET film to f~ ilit~te curing. The excess resin was sq~leç~ed out by using a rubber roller. The tool and resin were i~ ed through the clear PET
backing using 2 passes of a metlil~m pre~ure Ill~,r~iuly lamp, providing an c~.~Josure of 110 mJ/cm2 of 350-380 nano."c~er light. The W cured replica was peeled from the CA 0220364~ 1997-04-24 master, typically ~l.owing ~Ycellent letloretlection when viewed pel~ c~ r to the smooth surface of the film A logo was reverse screen printed on a biaxially oriented pol~pfop~lene (130PP) filrn using a colorless acrylic ink known under the trade de~ tion RAGE-800 5 from Advance Process Supply of Ch:c?go The resl~h~nt logo was adhered to the p.ii....alic stNctured surface in each case using a desk-top l~min~tor heated to 149C, using an EVA hot melt adhesive. The BOPP film was removed and the adhesive layerplaced against and adhered to a photographic ;~stlale known under the trade designation DIJRA~LEX using the desk-top l~ or.
The first structured surface (film 1 )was too coarse, making it difficult for the adhesive to ndequ~tely fill the grooves, limiting the logo fidelity and resolution However, the image formed was very visible Films 2, 3, and 4 provided a better mix of logo b.;gl~1..ess and resQI ltion The sm~llest 4 mi~i-u--.eler pattern of film 2 was not reflective, likely a diffractive effect, and the logo image ~u~red The 8- and 16-15 micrc....ciler pitch films provided a good image Of the two, the 8 mic.ullletel film app~ed to be an opli,--u--- as the smaller pitch allowed better resolution E~ample 2 Based on the results of Example 1, two nickel production tools were 20 r~.icaltd having p.is...alic paUel..s oriented to shift the logo off-axis, as illustrated in FIG. 3. Groove pitches of 8 and 16 micror..ete,~ were chosen to be conc;cle~l with the opti.,.ulll id~ontified above. The groove depths were 5.6 and 11.2 ..,ic,c,...~ters, ~e~ cly. While the image of the logo was moved off-axis as desired in each case,the irnage visibility and its ability to complelçly cover the underlying doc~lm~nt was 25 reduced for both sa--.ple s As shown in Fig 3, this occurs as the tilted prisms do not allow 100% of surface 18 to be optically reflective Various modifications and alterations of this invention will beco...c appare..l to those skilled in the art without dep~ ling from the scope of the appended claims.

48557.f~r

Claims

CLAIMS:

1. An optical security article comprising:
a) a first layer substantially transparent to visible light and having a first refractive index, the first layer having a structured first surface comprising a plurality of adjacent geometric concavities and peaks and a substantially planar second surface, b) an adhesive layer coterminating with said first layer, the adhesive layer substantially completely filling and adhesively contacting a first number of thegeometric concavities, the adhesive layer having a second refractive index which is substantially equal to the first refractive index, thus forming at least one transparent area, and c) [-]

2. An article in accordance with claim 1 wherein said separation layer is discontinuous, and the discontinuous portions of the separation layer have substantially the same refractive index.

3. An article in accordance with claim 1 wherein the separation layer comprises an imaging material.

4. An article in accordance with claim 1 wherein the refractive index of the first layer is RI1 and the refractive index of the adhesive layer is RI2, and wherein RI1 and RI2 are related by a ratio RI1:RI2 ranging from about 0.9:1.0 to about 1.1:1Ø

5. An article in accordance with claim 1 wherein said adhesive layer comprises a hot melt thermoplastic adhesive.

6. An article in accordance with claim 5 wherein a surface of said adhesive layer opposite the plurality of geometric concavities has a second layer removably adhered thereto.
[ a separation layer disposed between the adhesive layer and a second number of the geometric concavities to prevent contact of the adhesive with the concavities, and to form a plurality of pockets between the separation layer and

7. An article in accordance with claim 1 wherein said adhesive layer comprises a pressure-sensitive adhesive.

8. An article in accordance with claim 7 wherein a surface of said adhesive layer opposite the plurality of geometric concavities has a second layer removably adhered thereto.

9. An article in accordance with claim 1 wherein said plurality of geometric concavities is defined by a series of parallel linear prisms having planar facets.

10. An article in accordance with claim 9 wherein said linear prisms are asymmetrical.

11. An article in accordance with claim 1 wherein said first layer comprises a film which is substantially totally internal reflecting in the absence of contact between said first layer and said adhesive layer.

12. An article in accordance with claim 1 wherein said first layer comprises a diffraction grating.

13. An article in accordance with claim 1 wherein said pockets are air pockets.

14. An article in accordance with claim 1 wherein the separation layer contacts only the peaks of the structured first surface.

15. A tamper-proof article comprising the optical security article of claim 1 adhered by said adhesive layer to a substrate.

16. The optical security article of claim 1, wherein said structured first surface is a microstructured surface.