CA2252854A1 - Formed ultra-flexible retroreflective cube-corner composite sheeting with target optical properties and method for making same - Google Patents

Abstract

A retroreflective sheeting having a multiplicity of discrete, cube-corner elements cured in situ on a transparent, polymeric overlay film deformed into a three-dimensional structure so that base edges of a plurality of cube-corner elements are non-planar with respect to one another. The retroreflective article preferably has at least one target optical property. The present invention is also directed to a method of deforming the retroreflective sheeting to form a retroreflective article in which the base edges of a plurality of cube-corner elements are non-planar with respect to one another.

Description

CA 02252854 1998- 1o- 19 FORMED ULTRA_FLEXIBLE RETRORE~ECTIVE
CUBE-CORNER COMPOSITE SH~ G WITH TARGET
OPTICAL PRO~ ~11~;S AND METHOD FOR MAKING SAME

S FIELD OF~ 70W
The present invention relates to a flexible, rel,orenective ~heetin~
deru""ed to produce target optical p~upellies and to a process of defo"nil,g a ull;nective sheeting into a three~ nr onal article with such optical p~olpe~lies.
BACKGROUND 0~ T~E INVENTION
Cube-corner r~llortllectors typically cG""~,ise a sheeting having a generally planar front surface and an array of cube-corner elem~Pnts protruding from the back surface. Cube-corner reflecting elements comprise generally 15 l,ihed,~l structures that have three appro~;A~ely mutually perppn~lic~ r lateral faces meeting in a single corner, i.e., cube-corner. Light inridPnt to the frontsurface enters the sheet, passes through the body of the sheet to be internally reflected by the faces of the elemPnts so as to exit the front surface in a direction substantially toward the light source. The light rays are typically 20 reflected at the cube faces due to either total internal reflection ("T.I.R."), or reflective coatings such as a vapor-deposited ~ min~m film. Use of met~lli7.~
al~mimlm coating on the cube-corner F~ ..e.-~s tends to produce a gray coloration to an obse.ver in a.~bicnt light or daylight conditions, and is thus considered aecthetic~lly undesirable for some applications.
25A very common r~trolenective sheeting uses an array of cube-corner elf ~.e.~1~ to l~llorenect light. Figures 1 and 2 illustrate an example of such a ult;nective sheeling~ noted generally by numeral 10. The array of cube-corner Plem~ont~ 12 project from a first or rear side of a bûdy portion 14 that includes a body layer 18 (also rer~,led to in the art as an overlay) and may also 30include a land layer 16. Light illustrated as arrows 23 enters the cube-cornershoeting 10 through the front surface 21; it then passes through the body CA 022.728.74 1998 - 10 - 19 WO 97/41463 PCT/USg6/14034 portion 14 and strikes the planar faces 22 of the cube-corner ~le~ s 12 to return in the direction from which it came.
Figure 2 shows the back side of the cube-corner ele~..f~-lc 12, where each cube-corner element 12 is in the shape of a trihedral prism that has three 5 exposed planar faces 22. The cube-corner el~,. c.~ls 12 in known arrays are typically defined by three sets of parallel v-shaped grooves 25, 26, and 27.
~djncent planar faces 22 on n~ cent cube-corner Pl~ments 12 in each groove form an external dihedral angle (a dihedral angle is the angle formed by two intc. ~eclh~g planes). This eAternal dihedral angle is con~lanl along each groove 10 in the array. This has been the case for the variety of previously produced cube-comer arrays.
The planar faces 22 that define each individual cube-corner el~ ~.c .l 12 generally are subsla"l;ally perp~n~iic~ r to one another, as in the corner of a room. The internal dihedral angle -- that is, the angle between the faces 22 on 15 each individual cube-corner ~lem~nt in the array -- typically is 90~. This internal angle, however, can deviate slightly from 90~ as is well known in the art; see for eY~mple, U.S. Patent No. 4,775,219 to Appeldom et al. Although the apex 24 of each cube-corner eletnent 12 may be vertically aligned with the center of its base (see, for eA~"ple, U.S. Patent No. 3,684,348) the apex also 20 may be offset or canted from the base center as disclosed in U.S. Patent No.
4,588,258 to Hoopman. Other cube-corner configurations are disclosed in U.S. Patents 5,138,488. 4,066,331, 3,923,378, 3,541,606, and Re 29, 396, 3,712,706 (Stamm), 4,025,159 (McGrath), 4,202,600 (13urke et al.), 4,243,618 (Van Arnam), 4,349,598 (White), 4,576,850 (Martens), 4,588,258 (Hoopman), 25 4,775,219 (Appeldorn et al.), and 4,895,428 (Nelson et al.).
Where the cube-corner ~ olellective cheeting is likely to be used in an emrilon"~enl where it could be eA~Josed to moisture or other ele~.f~lc e.g., outdoors or in high humidity, it may be pr~,fe"ed that cube-corner e~ are en~srs~ ted with a CG~ IC sealing film. The a~ol~ ;Qned U.S. Patent 30 No. 4,025,159 ~ loses encapsulation of cube-corner ele~..e~ using a sealing film.

Basic cube-corner el~ ..e~ have a low angularity such that the element will only brightly ~,t~urcnc~l light that i".pinges on it within a narrow angular range centering approYin~ely on its optical axis. The optical axis is the trisector of the internal space defined by the faces of the el~Pmçnt T~.~p:.~g;.~g S light that is inc.lined substantially away from the optical axis of the element strikes a face at an angle less than its critical angles, thereby passing through the face rather than being rçflected Figure 3 is a graph in polar coordinates of the optical profile of a basic cube-corner r~tloicIlective sheet, having six m~imA and six minima at 30~
A7i-nl-thAI intervals. The h~lensily of the ret,ùfcnective beam from a cube-corner rclro~ cne~ e sheeting is greatest when the in~i~ent beam has an angle of inridçnce of 0~ (normal to the plane of the shçeti~). At higher angles of incidence (appro~ çly greater than 30~) the brightness of the retroreflected beam is a fi~nction of the angle about an axis normal to the sheet called the 1 S ~7im~thAI angle. When the angle of incidçnce of a light beam is held constant at a value of, for e,~n,vle 60~ from normal, and the ~7inn~thAI angle of the inci~ent beam is varied from 0~ to 360~, the intensity of the r~trorcnected beamvaries as illustrated in Figure 3.
There are a number of applications for cube-corner tcl,orcnective ~heetine with non-standard or cu~tomi7ed optical profiles. For example, more un,ro,ll, rcl,~renectivity or wider retroreflective angularity than shown in Figure 3 is often required. For some applications it may be desirable to limit eL,or~,nectivity to a narrow band of angularity and/or along a specific sepn~çntof the ~imllthAI angle.
One method of ~ the optical profile of cube-corner PlPmPntC is to cut the master or mold formed thereon into pieces and reA~.sc...l)ling the pieces in a pattern that produces differing zones of orientation on the el,orelle~,re ~heeti~ For example, an optical profile with wide I ~tl orenective angularity in multiple viewing planes can be achieved by rotating 30 a~c~Pnt pieces of the mold or master 30~ or 90~ about an axis normal to the plane of the r.le.... ~-l S (rotalillg the pieces 60~ or any mllltiple thereof effects no net change in o~je.~ m of the cube-corner e~ Csv-~h!;~g the pieces of the mold or master with the necess~ry precision, however, is time con.~ , and cA~,cnsi~e. A method of l~ ~es~ g a master mold is disdosed in U.S. Patent Application Serial No. 08/587,719 filed January 19, S 1996.
Another method of çh~l*;~ the optical profile of cube-corner elP.~.e~l~ is to tilt or cant the optical axes of cube-corner elemrnts with respect to one another. FIG. 4 illustrates a cube-corner ~1e..~ 30 with three mutually pe.~ ;c~ r faces 31a, 31b, and 31c that meet at the cube's apex 34. The 10 cube's base edges 35 are generally linear and generally lie in a single plane that defines the base plane 36 of the e4ment 30. Cube-corner elem~nt 30 also has a central or optical axis 37, which is the tri-sector of the internal angles defined by lateral faces 3 la, 3 lb, and 31c. The optical axis may be disposed perpçnAic.~lqr to base plane 36, or it may be canted as described in U.S. Patent15 No. 4,588,258 to Hoopman and U.S. Patent No. 5,138,488 to ~7~7ech The cost of creating tooling ~ece~s~ ~r to practice the invention of Hoopman is relatively high. Moreover, this technique does not lend itself to rapid prototyping of customi7ed optical profiles or angularity.
Therefore, what is needed is a method of creating lt;llolenective 20 articles with prototype or target optical properties without the need for c,.~,s;.~e tooling.

5~4RY OFI~VENTIO~
The present invention relates to a flexible, lellu~;nec~ e sheeting 25 d~ro.."ed tû produce target optical p~ûpe,lies. The present invention is alsodirected to a process of defo,l,...-g a r~ nective ~heeting into a three-~;"~ L on~l article having such optical p, ~,p~- lies.
The relrore;llective sheeting inc.1~ldes a multiplicity of discrete, cube-comer r1c..~ cured in situ on a l~ale~lt, polymeric overlay filrn. The 30 l~l,orenective sl~ g is d~ro.",ed into a three-.i;.~-el-~;or~ structure so that base edges of a plurality of cube-corner e~ c are non-planar with respect to one another to produce at least one target optical propc. Iy. The target opticalprope.lies may be a desired optical profile, angularity, three-d;,l,ensional appP~ce, whitçness, glitter-effect, or conlb:~J~;Qnc thereof. The retroreflective eh~eting is pl~fc~ably a single, unitary sheet.
S The base edges of a plurality of ~jacçnt cube-corner Ple~.. P,l-~ c may be non-planar or tilted with respect to one another. The base edges of one or more cube-corner f~ 1S are preferably not parallel to a front surface of the overlay film. The cube-corner elements may have a variable density across a portion of the relror~nective article. ~djae~nt cube-corner P4~n~ e across a 10 portion of the lcl~orcllective article may have a variable sF~cing The overlay film may have a thickness that varies across a portion of the rclrorellective article.
The present retroreflective article may be used as a master to produce tooling for forming a~ tion~l rctro,cllective articles.
The three-d;-~ -~r;cn~l structure may have one or more embossed symbols. The ~ l urelle~ re chçetin~e may optionally include a specular reflector coated on the cube-corner P~c ~ c. The .ellorenective sheeting may optionally include a sealing film ~Ytçndin~ snb~ lly across the cube-corner ~le..~ s opposite the overlay film. Met~11i7ed cube-corner elemenLs may 20 optionally be b~~~filled with a co~tin~ such as a polymeric material, resin or adhesive. In one embodinnçnt, the coating may be applied uniformly or in a pattern, such as p,h~ling symbols in one or more colors.
The polymeric overlay film plef~,.~ly has a first elastic mo~ull~e and the cube-corner c~ p~fc~bly have a second elastic modulus greater than 2S the first elastic mod~ s. The cube-comer ~lo....~ s plefe,ably are constructed from a thermoset polymer. The polymeric overlay film is preferably constructed from a thc,l,,ofol -'le polymer. The overlay film may be s~lected from the group cQneicting of the following: ionom.,.ic ethylene copolymers, pl~tiri7çd vinyl halide polymers, acid-functional ethylene copolymers, aliphstic30 polyurethanes, aromatic polywcll-a~lP~ other light l,~ s:.~e ~IsctQmers~ and cor,lbh~dlions thereof. The cube-comer ~ P~S may be selected from the .. . .

WO 97/41463 PCT/USg6/14034 group conq ~ g of monofilnction~ ifimctionql, and polyfunctional acrylates or combinations thereof.
The present invention is also directed to a method of forming a re~orellective article having at least one target optical pl~p~,. ly. A cube-corner 5 retlol~e~ re shee~ g is pl~pared having a ml~ltip!i~ity of discrete, cube-comer el---..P,-~ls cured in situ on a l~a,.i~,ar~.~t, polymeric overlay film. The flexible r~l~orenective sl~ 8 is deformed into a three-dimensional confi~ration so that the base edges of a plurality of cube-corner chF-.~enls arenon-planar with respect to one another.
The step of derol l~ g may include tilting the base edges of the plurality of a:ljacent cube-corner el~-..ç~ls with respect to one another. The step of dero."ling is pr~felably selected from the group consisting of thermo-fo,~lling,vacuum-forming, embossing, and combin~tionc thereof. The step of dero.lllll-g may include folmil.g a three--l;...e~ on~l symbol in the rellolenective ~eetinf~15 altering the density and/or spacing of at least a portion of the cube-corner r4-..~nlS, or stretching the l~l,ol~Ilective sheetin~ in at least one direction. The step of stretching may include uniformly (or non-uniformly) stretching or bi-axially stretching the r~orenective cheet;~ The step of dero,l.ln~g may include altering the base edges of one or more cube-corner el~ ..e--ls so that 20 they are not parallel to a front surface of the overlay film.
The cube-corner el~ ..e~-ts may optionally be coated with a spectral reflector. A sealing film may optionally be bonded subst~T-ti~lly across an exposed surface of the cube-corner el~...F.~ls either before or after the step of deforming the n,l,u~t;nective sheetine In an alternate embo-limPnt, a mold is formed from the cube-corner el~mP~nts of the derûllllcd l~l,olenective article. A polymeric material is applied to the mold and the polymeric material is at least partially cured. The polymeric material is then removed from the mold so that a second f~,olenective article is produced.
As used herein:

D~ro~ g refers to therrno-forming, vacuum-forming, embossing, molding, ~ .p ~, elastic or in.ols~tic stretchine uniformly or non-un,~o,lnly stretching, or co,..bi~ ion~ thereo~
Syrnbol refers to any qlphsnllm~ric character, logo, seal, geometric S pattern or co~ ionc thereof Target Optical Flopc,lies refers to a desired optical profile, angularity, three-.l;....l-v:onal appea.~ce, whiteness, glitter-effect, or co..lbi~ t;on~
thereo~

BRlEFDESCRrPTlON OFDRA U~NG
The invention is further e~cplvsined with reference to the drawing, wl~ereill:
Figure 1 is a sectionsl view of a prior art cube-corner rtllorenective sheetine, Figure 2 is a bottom view ofthe retroreflective $hçeting of Figure l;
Figure 3 is a graph in polar coordinates of the optical profile of a cube-corner clP-.If nl having six mql~ims and six minima at 30~ s7imllth?1 intervals;Figure 4 is an isometric view of a cube-corner ClC---f -~1 that may be used in a rellor~nective ~heetin~ ofthe invention;
Figure 5 is a bottom view of a retlorellective article acco~ding to the present invention;
Figure 6 is a sectional view of a rc;lrulenective article taken along lines 6-6 of Figure 5;
Figure 7 is a sectional view of the l~tlurc;nective article taken along lines 7-7 of Figure 6;
Figure 8 is a sectiollql view of a r~ll û- t;nective article having a seal film secured to the ba&~Q;de ofthe rel,orenective ~heetine Figure 9 is a srh~...AIic illustration of a method for p.epa.ii,g a r~tlor~nective vheetin~;
Figure 10 is a sche,.. ~ic illustration of an alternate method for ple~,~il.g a l~lrorenective sheetin~

Figure 11 is a sch -~ ';c illustration of a method of prep&;i.g a t~ortllect*e article;
Figure 12 is a s~ ic illustration of an alternate method of prepa.iilg a rellolellective article;
Figure 13 is a photograph of an ~ ~n-pl~-y rellort;llective article;
Figure 14 is a photomicrograph of a d~s~;on on the lelrorcnective article of Figure 13;
Figure 15 is a photomicrograph of a depression on the ~ -u~llective article of Figure 13;
Figure 16 is a photograph of an eAe",?la-y lel,olellective article;
Figure 17 is a photomicrograph of a protrusion on the l~t~ol~llective article of Figure 16;
Figure 18 is a photomicrograph of a protrusion on the lel,orellective article of Figure 16;
Figure 19 is a photograph of an çYçmrl~ry re~ror~nective article co~ ing a symbol;
Figure 20 is a photograph of a plurality of çYrmr~ el,or~Ilective article;
Figure 21 is a photomicrograph of a lelrurenective article co~ in~ a 20 ~9 symbol;
Figure 22A is a graph of e.ltlance angle versus brightness for various sperimen~;
Figure 22B is a graph of observation angle versus brightn~sc for the "~e~ of Figure 22A;
Figure 23A is a graph of e~.llance angle versus b.;~l.l .ess for various s~c~
Figure 23B is a graph of observation angle versus brightn~ for the specimen~ of Figure 23A;
Figure 23C is a bar graph of change in whitçnç~s of various ~eç;...~ .s 30 after defo.,.,at;on;

CA 02252854 l998- lO- l9 Figure 24A is a graph of entrance angle versus brightness for various s~ec.. ~, Figure 24B is a graph of observation angle versus brightne~s for the ~eC;~cnc of Figure 24A;
S Figure 25A is a graph of cnl-~ce angle versus brightnesc for various eç~
Figure 25B is a graph of observation angle versus brightness for the s}~P~ en4 of Figure 25A;
Figure 26A is a graph of e~ ce angle versus brightnFss for various 10 s~e~ c; and Figure 26B is a graph of observation angle versus brightness for the sper;~.~enC of Figure 26A.
Figure 27A is a graph of cnll~ce angle versus bnghtncss for various Sl~P~ P ~
Figure 27B is a graph of observation angle versus brightness for the syec;lllF-~s of Figure 27A;
Figure 27C is a graph of entrance angle versus brightnPcs for various co.................................................. P.cial reflectors; and Figure 27D is a graph of observation angle versus brightnesc for the 20 conli"crcial reflectors Figure 27C.

DETAI~ED DESCR1PTION O~PREFERRED EA~BODM~ENTS
The present invention relates to a rcllo,ellective article forrned from a flexible, l~t,ul~Dective ~hc~ing to produce target optical prop~.lies and to a 25 process of delolllling a ret~u,~neclh~e sl.e~ p. into a three-dim~nsiQnal article.
The l ~l urellective ~hçetin~ has a mUItirli~ity of discrete, cube-corner Ple ~
cured in situ on a l-~lspare.ll, polymeric overlay film. The let~ulenective ~h~ g is deformed into a three-~ .e~;rn~l structure so that the base edges of a plurality of cube-corner e~ F~ are non-planar ~,vith respect to one 30 another.

CA 02252854 1998- lo- 19 The r~Llolenective article of the present invention has the ability to reflect substqntiq-l quantities of in~ çnt light back towards the light source while cAl~;l;ng target optical prop_llies. The present rel~olenective article isslitn~le for being inco,~G,~ied into a variety of products, such as clothing, 5 shoes, license plates, signs, vehicle markings, cone sleeves and barrel wraps.Methods of making a glittering l~l,orellective articles are ~licrl~sed in the following related applications filed on the same day h_lcwilh: "Method of Mq~in~ Glittering Retroreflective .Sh~etin~" attorney docket No.
52374USAlA, Serial No. 08t641,129; "Mold for Producing Cl;ll~ling Cube-10 Corner Retroreflective Sheeting" ~Uoll,~ docket No. 52471USA5A, SerialNo. 08/640,383; and "Glittering Cube~Comer Rel~orelle~;li.~e Sheeting"
attorney docket No. 52373USA3A, Serial No. 08/640,326.
Figure 5 shows the bn~Qide of a unitary cube-corner sheeting 60 that has been deformed to produce at least one target optical propelLy. Cube-15 corner cle.-~nls 30 are similar to those depicted in Figure 4. Each cube-corner elem~nt 30 meets, but is not necessqrily connPcted to, an ndj~qcent cube-corner clF ~ t at a base edge 35. The array inchldes three sets of parallel grooves 45,46, and 47. The external dihedral angles (decignq-ted as a in Figure 6) between faces 31 of a~ijncent cube-corner F~I-FIments 30 vary along the grooves 45-47 in20 the array. The base edges 35 of the cube-corner cle~ l s in the array are non-planar. Consequently, the apex 34 of one cube, such as cube 30a may be relatively close to another apex such as cube 30b, but the apex of cube 30b may then be further away from another n~ljncent apex such as the apex of cube 30c.
Figure 6 is an ~Y~mplqry illustration of ~ictq~nces the base edges 35 are 25 offset or tilted with respect to one another, or with respect to the front surface 51. For cube-corner el~-..e .ts that are about 50 to 200 micrometers high, the variation in height b.,l~een ndj-~-nt base edges typically is about 0 to 50 micrulllc,tels. It will be understood that the present rellorenective article may be derolll,cd on a micro or macro level. As will be ~iiccucsed in the Fy~nnplcs~30 the lelrolenective sheeting may be deformed over coated abrasive paper co~ nil~g abrasive grains with d;~ ..cle~ of about 100 to 550 microllletel~.

CA 022~28~4 l998- lo- ls Abrasive grains of this size have radii of curvatures of about 50 to 225 micrometers. The lc;l~or~llecli~e shF~ g may be deformed over smaller structures, in the range of about 10 to 50 micro~ e,~ thollgh the change in the optical prope.lies may be ".~ l It is believed that the change in the 5 optical prope~L;es of the lel~u~ ective cheeting when d_fol",ed over micro structures in the range ûf about 250 to 10 microns is a function of the size of the cube-corner c~ e-.l~ and the thickness of the overlay film. For example, smaller cube-corner elr-.~P~-Is and/or a thinner overlay film may be more s~ceptil~le to defu~mdlion over micro structures within this range.
Figure 6 is a sectionq-l view ofthe cube-corner .sl-e~l;ng 60 of Figure S
showing the position of one cube apex relative to another. Additionally, Figure 5 shows tilting or canting of the base edges 35 relative to one another and relative to front surface 51. The base edge 35 of one cube may be disposed closer to or further away from the front surface 51 of overlay film 58 than the 15 base edges of other ~ q. ent cube-corner elFmçntc due to dt;f~"",alion of theoverlay film 58. If the unitary cube-corner sheeting 60 possesses a land layer 56, it is also not un,fo-lnly spaced from the front surface 51. The cube-corner ~heeting 60 preferably does not have a land area 56, such that each cube-corner Ple...~nl 30 is a discrete entity. When the cube-corner Plemçntc are tilted, the20 base edges 35 of many of the cube-corner clen~e..lc 30 do not reside in the same plane as the front surface 51. Additionally, the edges 35 of one or more cube-corner ~FkF!~,.e~.lc 30 are not parallel to the front surface 51. Either surface of the overlay film 58 may optionally contain symbols printed on or formed therein.
Figure 6 also shows the ~t-F~ ql dihedral angle, a, that defines the angle bcl~cel- faces 31 of a1jarçnt cube-corner elF~ s 30. Angle a may vary along all grooves in a single parallel groove set, it may vary along all grooves in two parallel groove sets, or it may vary along grooves in all three groove sets in the array. In an array of randomly tilted cube-comer Pl~.."~
30 angle a varies randomly a~.or~g~l a~jac-Fnt faces of a~ljacP-nt cube-comer Ple ..~ .ls througho~lt e~s-Fntiqlly the whole array.

CA 02252854 1998- lo- 19 The overlay film 58 in body portion 54 typically has an average thickness of app~o,;...-lçly 20 to 1200 micrometers, and preferably is about 50 to 400 micrometers. The cube-corner e~ CI~ typically have an average height of about 20 to 500 mi~ t~, more typically of about 25 to 200 5 micrometers. The optional land layer 56 preferably is kept to a minimsl thickness of 0 to l 50 ll..cro,l.elets, and is preferably as close to zero as possible so that the strain gen~"aled during dGru~ alion does not propagate laterally through the land area. A coating may optionally be applied to the exposed metslli7ed cube-corner fl ~e~le 30 to provide the dcr~ alions of the 10 reL.o.enective article 60 with additional structural support. For some applications, it may be desirable for the tel~orellective article to be a free-stsn~ self-sul,po,ling structure. In one embodiment, the coating is a polymeric material, resin or an adhesive. The coating may optionally contain a pi~y...-..-l or dye of one or more colors. Additionally, the coating may be 15 applied uniformly or in a pattern contP~ g symbols using a variety of printing techni~lues. l~etslli7ed fel~olGflective eheeting generally ..~A;..I~ .e higher brightness after dGro.".aLion because T.I.R. tends to break down in the ~Inee-sled cheel;.~g Figure 7 shows cube-corner el~rn~nts intersected by a plane that is 20 parallel to the front surface 5 l . As illustrated, the plane does not intersect each cube to produce a triangle 62 of thè same cross-sectional area. One cube may be tilted or offset from the front surface 51 to such an extent that the inte.~e~,l.ng plane only passes through a tip of the cube, resl-ltin~ in a smalltrisng~lsr cross se.,lion -- whereas, a cube that stands upright may be 25 intersected such that the triangle reC~Iltin~ from the cross-section is relatively large. Thus, even though the cube-corner elP~.e-.ls in the array may be of similar size, they can produce triangles of random sizes when intersected as des~;.il,ed because of the manner in which the cubes are tilted or offset with respect to a r~,fe, ence plane. It will be understood that the spacing ~t~. cen the 30 cube-comer ~ s 30 can vary, as will be ~iccussed below, slthollgh ;l-olenectivity tends to decrease as spacing inc1~,ases.

CA 02252854 1998- lO- l9 Figure 8 shows a rello~eIlective article 61 that has a seal film 63 disposed over the backside of cube-corner rlc...e~-~s 30, such as is disclosed in U.S. Patent No. 4,025,159. The seal film 63 is bonded to the body portion of the cheeting through the cube-corner elo~ 30 by a plurality of seal lines 64.
5 The bond;,.g pattern produces a plurality of herrneti~qlly sealed chambels 65 that prevent moisture and dirt from contectin~ the bac~r;de of the cube-corner Pl~n.~ c (~hc~ e~s 65 enable the cube-air interface to be ~n~ cd to prevent loss of relro,ellectivity. The cube-corner ~l~ment~ 30 may optionally be coated with a reflective material on the surface 67, such as vapor depositing10 or rh~m;rs11y depositing a metal such as alllm;rlllm, silver, nickel, tin, copper, or dielectric materials as are known in the art of cube-corner ~el.oreflective artides. It will be understood that the ~el~or~;nective cheeting 61 will typically have a metal layer on the surface 67 or a seal film 63, but not both.
Preferably, the sealing layer co",~.ises a thermop!-3~ctic material with a 15 similar low elastic modlllns as the overlay film 68. Illustrative examples include ionol"e.ic ethylene copolyrners, pl~cti~i7~d vinyl halide polymers, acid functional polyethylene copolymers, ~3liph3tic polyul elhol~e, aromatic polyur~ 3~e~ snd col~ n~l;onc thereo~ In certain applications, the optional sealing layer 63 can provide Sigl ific3-nt protection for the cube-corner cle.,.~,nls 20 of the coll")osite material from envi~ on~.F~ 3l effects, as well as ...3~ g a sealed air layer around the cube-corner el~ ,nls which is es~enti~l for creatingthe refractive index di~relenlial needed for total internal reflection. As a result of the decoupl;ng of cube-corner e~ .f~ 30, the sealing layer 63 may optionally be adhered, at least in part, directly to the overlay film 68 bet-.~,en 25 ;~depFndent cube-corner e1~F~enI~
The seal film may be bonded to the cube-corner ek-..el-ls in the body portion of the ~heeting using known techniqlles; see for example, U.S. Patent 4,025,159. Sealing technique examples include radio frequency welding, thermal fusion, con<luctive heat sealing, ultrasonic welding, and reactive 30 welding. When applying a seal film to the ~ c;de of a r~l~o.enective sheeting cons;derable attention must be paid to the composition and physical CA 022~28~4 1998- lo- 19 propc~lies of the seal film. The seal film must be able to securely bond to the b..rl~Q;de of the cube-corner sheel;ng . nd should not contain components that could adversely affect r~ olcnectivity or the appea,dl-ce of the rcliorenective product. For ~YDmrle, the seal film should not contain components that could S leach out (e.g., dyes) and contact the ~ac~Q;de of the cube-corner el~ nts The sealing film typically comprises a th~lll,oplasLic material because such materials lend ll~c"~selves well to fusing through relatively simple and colllll,only available thermo-bonding te~hniquçs Figure 9 is a sCl~r~ c illustration of an appa.~lus 120 for casting and 10 curing rctl ol cflective sl ~e~ e s lit~ble for use in the present invention. Overlay film 121 is drawn along guiding roller 122 or from a stock roll of material to nip roller 123, e.g., a rubber coated roller, where overlay film 121 contacts suitable resin formulations 124 previously applied to pattemed tool roll 125 through coating die 126. The excess resin eYtçn~ine above the cube-corner 15 el~ment forming cavities 127 of tool 125 is ~ ;7ed by setting nip roller 123 to a gap setting that is effectively less than the height of the cube-corner fo-ll"l.g el~m~nts oftool 125. It will be understood that the gap setting may beachieved by applying pressure to the nip roller 123. In this fashion, me~-.hDn~
forces at the interface between nip roller 123 and tool 125 insure that a ... ~ .... amount of resin 124 extends above cavities 127 of tool 125.
Depen~l;uE on the flexibility of overlay film 121, film 121 may be optionally supported with suitable carrier film 128 that provides structural and ~..e~l~A~-icD-l durability to overlay film 121 during casting and curing. The carrier film 128 may be stripped from overlay film 121 after the ~hee~ g is removed from tool 25 125 or left intact for further proce~ing of the rellolellective sl,eeling. Use of such a carrier film is particularly pre~" .,d for low modulus overlay films.
The resin composition that forms the l~l,olenective array of cube-corner ele..~ s can be cured in one or more steps. Radiation sources 129 expose the resin to actinic radiation, e.g., ultraviolet light, visible light, etc.
30 depel-~;ug upon the nature of the resin in a primary curing step through the overlay film. As can be appreciated by one of skill in the art, the s~lected WO 97/41463 PCTrUS96/14034 overlay film need not be completely or 100 percent l~1sps,enl to all possible wave~ hc of actinic radiation that may be used in curing the resin.
Alte.,.aliv~ly, curing can be p~,Ço"..cd by irradiation through a lI~nS~Ja1e~It tool 125, such as ~icrlosf~d in U.S. Patent No. 5,435,816.
The tool 12S has a mol~ling surface having a plurality of cavities opening thereon which have the shape and size suitable for forrning desired cube-corner ele-.f ~ls. The cavities, and thus resultant cube-corner cl~mf~
may be three sided pyra nids having one cube-corner each, e.g., such as are disclosed in the U.S. Patent No. 4,588,258, may have a rect~n~ r base with two rect~n~ r sides and two tri~ne~ sides such that each ~ e-~l has two cube-corners each, e.g., such as are disclosed in U.S. Patent No. 4,938,563 (Nelson et al.), or may be of other desired shape, having at least one cube-corner each, e.g., such as are disclosed in U.S. Patent No. 4,895,428 (Nelson etal.). It will be understood by those skilled in the art that any cube-corner _If ~ may be used in accordance with the present invention.
The tool 125 should be such that the cavities will not deform undesirably during fabrication of the composite article, and such that the arrayof cube-corner _IF.~f.~lS can be sepa~ated the.er~oll, after curing. Materials useful in forming tooling 125 Pre~IabIY ~C~;nÇ cleanly without burr formation, exhibit low ductility and low gl, ;n;l~ess7 and m~int~in dimensional accuracy after groove formation. The tool can be made from polymeric, met~ili57 composite, or ceramic materials. In some embodiments, curing of the resin will be pelroln~cd by applying radiation through the tool. In such Çf.'$, the tool should be sufficiently ~ ~~,~ enl to permit irradiation of the 25 resin ll~e.~llough. Illustrative eY~mrles of materials from which tools for such embodimentc can be made to include polyolefins and polycarbonates. Metal tools are typically preferred, however, as they can be formed in desired shapes and provide eyccllent optical surfaces to ~.s~ 7e l~lorenective pclrullllance of a given cube-corner cle.l,e,ll confi~ration.
The p~ .a,y curing can co".pl~tely or partially cure the cube-corner e4.~e-~1s A second radiation source 130 can be provided to cure the resin after cl.P.~ g 131 has been removed from tool 125. The extent of the second curing step is depenAent on a number of variables, among them the rate of feed-through of the materials, composition of the resin, nature of the crosslinking hlilidtGls used in the resin formulation, and the geG",cl,y of the 5 tool. Illustrative examples include eleel~on beam exposure and actinic r~iqtion e.g., ultraviolet rndi~tion visible light radiation, and infrared radiation.
Removal of the lellolénective s~eetin~ 131 from the tooling 125 typically ~enc.~les s~lffici~nt me~h~rlical stresses to f ~ lure the minirn~l land 10 area bët~.~en the cube-corner ~ if any, that exists bêl~.~en the individual cube-corner el~....c-lts of the she~t; ~p The decouple~ inAep~n~Açnt nature of the discrete cube-corner ~lemPntc and strong bond of each indepen~nt clc.ll~,ll to the overlay film gives the lelrorcnective shçeting sub~t~nti~l flc,.il,ilily, while ret~ high levels of le~lorellective pc.~""ance 15 afterundergoing l--eç~ l defolmalionstresses.
Heat ~ u..~ l ofthe .~l.ee~ g 131 may optionally be pe,~""ed after it is removed from the tool. Heating serves to relax stresses that might have developed in the overlay film or cube-corner rle~ ~le, and to drive off unreacted moieties and reaction by-products. Typically, such tre~tmP.nt 20 involves heating the she~;n8 to an elevated tc~llpclal~re~ e.g., above the glass transition te."~ctalure of the subject resin. Typically a ,cheeting will exhibit an increase in, elroré{lective brightness after such tre~tment Figure 10 illustrates an alternate appa"~lus for casting and curing ~el,u~;nective Cl~e~ g suitable for making the present rel~o,eIlective article.
25 Resin composition 124 is cast directly onto overlay film 121. The resin-film co,.,~inalion is then cont~cted with paUelltcd tool roll 125 with ples~lle beingapplied through app,op,iale setting of nip roller 123. As in the configuration illustrated in Figure 9, nip roller 123 serves to minitni7e the amount of resin eYten~lin~ above the cube-corner fol",ing cavities 127 of tool 125. The resin 30 can be cured by exposure to actinic radiation from a first radiation source 129, and optional second radiation source 130. The actinic radiation from first rn1iation source 129 must first pass through overlay film of the .~hee~ g beforein.pi~ on the resin.
The individual or discrete cube-corner rle.~e~ are ç~sçn~ially totally decoupled from each other, providing the ultra-flexible character of the S cG.,.pos.le lelluienective ~l~eeti~g The de~o ~le~ cube-corner clPn~t;nls are no longer mechanically con.,~lai..ed by the effect of any land area"l~;n;...;~ g the mechanical stresses that might tend to deform them and lead to degradation of ~llor~Ilective p~,.ru...-ance. The discrete cube-corner ~l~.ne~ of ret~olt;llective sh~eting retain a high degree of r~lloreflective bri~htnesc after 10 being dero-..,ed.
Rellorelle~ ve sheeting prepared according to the above method exhibits a .~l.orellective b..~,hl..eS~, i.e., a co~ffirient of retroreflection, of greater than about 50, ,>rerelably greater than about 250, and more pr~labiy greater than about 500, cqndelq/lux/square meter, measured at an e~lt~ance 15 angle of-4~ and an observation angle of-0.2~, when the .$heetin~ is in a planar, non-defo.,.,cd configuration. By planar it is meant that the sheeting is permitted to lay flat and by non-deforrned it is meant that the s~lenin~e has not been mec'~ qily sllessed after decoupling ofthe cube-corner ele~..~...l~
The resin composition and overlay film are prefe,ably such that when 20 the resin composition contacts the overlay film it penetrates the overlay film so that after the primary curing l,~ln.~nl an inte~,enel,a~ g network between the material of the cube-corner ~lerne~lts and the material of the overlay film is formed. The array of cube-corner eletn~nts preferably co",l"ises a material that is thermoset or extensively crosslinl~eA~ and the overlay film pl~re-ably 25 co,nplises a ll~cl."opla~ic material. The superior ch~,."ic~l and mechanical prope.lies of lhellllO5Cl materials yield cube-corner ele~n~ optimally capable of~ edesired,~ o-.,nectivity.
A critical criterion in the selection of these components is the relative elastic mod~iu$ for each con,ponenl. The term "elastic modulus" as used 30 herein means the elastic modulus determined according to ASTM D882-75b using Static Weighing Method A with a 12.5 ce ~I;...eter (5 inch) initial grip separation, a 2.5 c~ ter (1 inch) sample width, and a 2.5 c~ er/minute ( 1 one inch/minute) rate of grip sepa, alion.
Alle",ali~ely, elastic moduh~s may be del~.,..ncd . ~ Jrdi~g to standard~ed test ASTM D882-75b using Static Weighing Method A with a 5 five inch initial grip separation, a one inch sample width, and an inch per minute rate of grip separation. Under some ~;ir~ cç.~ the polymer may be so hard and brittle that it is ~liffic~lt to use this test to ascertain the modulus value precisely (~lthol-gh it would be readily known that it is greater than a certainvalue). If the ASTM method is not very sl~ita~le, another test, known as the 10 'lN~nninrlçnt~tion Technique" may be employed. This test may be carried out using a microind~nt~tion device such as a UMIS 2000 available from CSIRO
Division of Applied Physics In~tit~te of Industrial Technologies of T.intlfiel~1New South Wales, Australia. Using this kind of device, p~lct~a~ion depth of a Berkovich pyramidal diamond indenter having a 65~ included cone angle is 15 measured as function of the applied force up to the m~yimum load. After the x;l~.. load has been applied, the material is allowed to relax in an elastic manner against the ind.ont~r. It is usually a~Y.med that the gradient of the upper portion of the unloading data is found to be linearly proportional to force. Sneddon's analysis provides a relationship between the indentin~ force 20 and plastic and elastic components of the penetration depth (Sneddon I.N. Int.
J. Eng. Sci. 3, pp. 47-57 (1965)). ~rom an 1.;~ iOIl of Sneddon's equation, the elastic modulus may be recovered in the form E/(1-v2). The c~lcul~tion uses the equ~til~n E/(1-v2)-(dF/dhe)F~ /(3.3hp t~n(~ )) 25 where:
v is Poisson's ratio of the sample being tested;
(dF/dhe) is the gradient of the upper part of the unloading curve;
F~ is the m~xim~lm applied force;
hp,~,aX is the maximum plastic pen~ Lion depth;

CA 022~28~4 1998-10-19 ~ is the half-inrluded cone angle of the Berkovich pyramidal in~Fnter;
and E is the elastic modulus.
Values o~lail.et under the nanoindçnt~tion technique may have to be correlated 5 back to ASTM D 882-75b.
As ~liscucced above in relation to the fi1nd~mF~nt~l principles behind the optical plope.lies of cube-corner ekn~ ls~ even slight distortion of the geo".t;l~y of cube-corner el~ c can result in substantial degradation.of optical p.upe.lies of the cube-corner Fle,me~ Thus, higher elastic modulus 10 materials are preferable for the cube-corner elemPnts due to their increased reCict~nce to distortion. The overlay film of the composite r~orenective cheetine is preferably a polymeric material of somewhat lower elastic modulus.
During curing of the cube-corner component, depending on the composition of the cube-corner material, individual cube-corner elements may 15 experience a certain degree of sl,. ~ ~e. If the elastic modulus of the overlay film is too high, torsional stresses can be applied to the cube-corner elF rnents if they shrink during curing. If the stresses are sufficiently high, then the cube-corner Plemçntc can become distorted with a resulting degradation in optical pelro~.nal~ce. When the elastic mo~l.4~c of the overlay film is sufficiently lower 20 than the modulllc of the cube-corner element material, the overlay film can deform along with the shrinkage of cube-corner elem~Fnts without exerting such deru-...alional stresses on the cube-corner el~n,~n~s that would lead to undesirable degradation ofthe optical characteristics. The modul~s di~rt,cl.lialb.,~ ,Fn the overlay film and the cube-corner ~l~mentC should be on the order 25 of 1.0 to 1.5 x 10' pascals or more.
As the height ofthe cube-corner ~IF-~UI~S diminjchF~s, it is possible for this modulus .~ enlial to reach the low end of the range given ;~ di~tely above. However, it should be kept in mind that there is a practical lower limit to the modllll~c of the cube-corner element material. Below a certain level, 30 generally on the order of about 2.0 to 2.5 x 108 pascals for cube-corner e~l. "~ s about 175 microns (0.007 inches) in height, less for smaller cube-CA 022s28s4 1998- lo- 19 corner P~ c.~ the cube-corner Pl~-..e~.1s beco...c too flexible and do not possess s~lffi~i~-nt ..~ecl-~nical rigidity to plopci.ly fracture upon application of stress. The cube-comer elc~..P~s preferably have an elastic modulus of greater than about 25 x 108 pascals.
After curing, the thir.l~ness of the land area, i.e., the thic~n~ss of the cube-comer array material opposite the plane defined by the bases of the cube-corner ~kmel~ls, is preferably less than 10 percent of the height of the cube-comer e~ s, and more preferably less than 1 percent thereof.
Preferably the resin will shrink at least 5 percent by volume when 10 cured, more p,t;rerably b~l~.eel- 5 and 20 percent by volume, when cured. It has been found that by using resin compositions of this type, cube-comer arrays with minimqt or no land area thicknçss can be more easily fomled, thereby achieving the high flexibility. For ;~ nce, resin compositions that shrink when cured will tend to retreat into the cube-corner-shaped cavity, tending to leave a 15 land area that only connects adj~c~nt cavities and ther~;role ~djac~-nt cube-corners with a narrow portion if applied to the tool in appropriate q~lqntities The narrow portion is readily broken res.lking in decoupling of individual cube-comer el~ t~ as ~iicc~lssed below. ~heeti~l~ can in theory be formed with ~ssentiqlly no land area con~ecl;~ q~jqcçnt cube-corner elem~nts7 however, in 20 typical high volume mqm)f~~tllring a,,~ulgelllents~ a minimql land area having a thic~n~ss of up to 10 percent of the height of the cubes, p, erel~ly on the order of 1 to 5 percent, will be fomned.
Resins selected for use in the array of cube-comer ~1- .. n~ include cross-linked acrylate such as mono- or multi-functional acrylates or acrylated 25 epoxies, acrylated polyesters, and acrylated urethanes blended with mono- andmulti-functional l"ono...c;,~ are typically prere,l~,d. These polymers are typically pler~.,ed for one or more of the following reasons: high thermal stability, env.rol~ P,~Iq-l stability, and clarity, eYcellent release from the tooling or mold, and high receptivity for receiving a reflective co~q~tin~
F --~rl~S of materials suitable for rc,r,---ng the array of cube-comer ele~..P~.Ic are reactive resin systems capable of being cross-linked by a free radical poly...~,.i~lion ~~eCl~u~ic~ by ~YpOs ~re to actinic radiation, for exarnple, ele~,l,on beam, ultraviolet light, or visible light. ~ tion-qlly~ these materials may be pol~n.cl,Led by thermal means -with the addition of a therrnal il"lialor such as benzoyl peroxide. Radiation-initi-qted cationically 5 polymerizable resins also may be used. Reactive resins suitable for forming the array of cube-corner rle " ~1 s may be blends of photoinitiator and at least onecon,poulld bearing an acrylate group. Preferably the resin blend conlai"s a monofi-n~tionql, a ~ nctional~ or a polyfilnrtionql compound to ensure formation of a cross-linked polymeric n.,lwo. 1. upon irradi-qtion Illustrative ~;A~Ilples of resins that are capable of being poly"~e"zed by a free radical l"erh~nic~n that can be used herein include acrylic-based resins derived from epoxies, polyesters, polyethers, and urethqn~c ethylenically unsaturated compounds, aminoplast derivatives having at least one pendant acrylate group, isocyanate derivatives having at least one pendant acrylate 15 group, epoxy resins other than acrylated epoxies, and mixtures and co,.,billdlions thereo~ The term acrylate is used here to encompass both &cl~ldles and me~ c.ylates. U.S. Patent 4,576,850 (Martens) discloses PYq.npl~ ~ of crosQ~ ed resins that may be used in cube-corner el~Pment arrays.
Ethylenically unsaturated resins include both monomeric and polymeric 20 compounds that contain atoms of carbon, hydrogen and oxygen, and optionally nitrogen, sulfur, and the halogens may be used herein. Oxygen or nitrogen atoms, or both, are generally present in ether, ester, urethane, amide, and ureagroups. Ethylenically unsaturated compounds pr~;relably have a molecular weight of less than about 4,000 and pr~re~bly are esters made from the 25 ,.z~ion of compoun~ containing qliphqtic monohydroxy groups, !qlirhqtic polyl-~dro~ groups, and uns~lulaled carboxylic acids, such as acrylic acid, rylic acid, itaconic acid, crotonic acid, iso-crotonic acid, maleic acid, and the like. Such materials are typically readily available co.. e -,ially andcan be readily cross linked.
Some illustrative examples of compounds having an acrylic or ~ --rylic group that are suitable for use in the invention are listed below:

~ ................................. . .. . .

(1) Monofi~nctional compounds:
ethylacrylate, n-butylacrylate, isobutylacrylate, 2-ethylhexylacrylate, n-hexylacrylate, n-octylacrylate, isooctyl acrylate, isobG,l.yl acrylate, tetrahydrofurfuryl acrylate, 2-phenoxyethyl acrylate, and N,N-dimethylacrylamide;

(2) Difilnctionq-l compounds:
1,4-b"l qn~iQl diacrylate, 1,6-h- - ~ne~liQl diacrylate, n eope.l~ylglycol diacrylate, ethylene glycol diacrylate, triethyleneglycol diacrylate, tetraethylene glycol diacrylate, and diethylene glycol diacrylate; and (3) Polyfi-nctionvql compounds:
~ "Glhylolp~opane triacrylate, glyceroltriacrylate, pentaerythritol triacrylate, pentaerythritol tetraacrylate, and tris(2-acryloyloxyethyl)isocyanurate.
Monofunctional compounds typically tend to provide faster penGIIalion of the material of the overlay film and difunctional and polyfunctional co"".ounds typically tend to provide more cros~lin'-e(l, sl-ollger bonds within and betweenthe cube-corner elementc and overlay film. Some leprese.ltali~re ~,~amples of other ethylenically unsaturated compounds and resins include styrene, divinylbGll7.~ne, vinyl toll~nP, N-vinyl fo,.. ~ de, N-vinyl pyrrolidone, N-vinyl 20 caprolactam, mono. llyl, polyallyl, and polymethallyl esters such as diallyl phthqiq~te and diallyl adipvqte~ and amides of carboxylic acids such as N,N-diallylq~ipYn~ide.
Illustrative eYqmrles of photopoly,l,c;,i~alion initiators that can beblended with acrylic compounds in cube-corner arrays include the following:
25 benzil, methyl o-bçn7Oqte, benzoin, benzoin ethyl ether, benzoin isoplopyl ether, bel~oill isobutyl ether, etc., be.~ophenone/tertiary amine, acetophenonessuch as 2,2-diethu~ ce~ophenone, benzyl methyl ketal, 1-hydro~y~,ycloheYylphenyl ketone, 2-hydroxy-2-methyl- 1 -phc.l~lpropan- 1 -one, 1 -(4-isopropylphenyl)-2-hydl or~y-2-methyl~ ~")~l- 1 -one, 2-benzyl-2-N,N-30 d;~ hylamino-1-(4-morpho!inoph .~rl)-1-butqnon~, 2,4,6-l~h~thylb~,nzoyl-diphe.lylphosph;rle oxide, 2-methyl-1~(."tll"~1Lo), phenyl-2-l,lo",holino-1-CA 02252854 1998- lo- 19 WO 97141463 PCI~/US96/14034 propanone, bis(2~6-Aimethoxybenzoyl)(2~4~4-tl imelh.~lpe.l~yl)phQsrhine oxide, etc. The compounds may be used individually or in co"lbil-alion.
Cationically polyn.c.i~able materials including but are not limited to materials co.~ g epoxy and vinyl ether functional groups may be used S herein. These systems are phol~i.;l;ated by onium salt in,lialols, such as triarylsulfonium, and diarylic~lonium salts.
Preferably, the overlay film used is a polymeric material selected from the group cQn~ e of io~...e ic ethylene copolymers, plsctiçi7~d vinyl halide polyJners, acid fi~n~ I;o~ polyethylene copolymers, ~ h~tic polylne~ n~s, lO aromatic polyur~h~es, other light tr~n~miccive elaslo,l,el, and combilldlionsthereof. Such materials typically provide overlay films that are imparted with the desired durability and flC-ib:' ~y to the resl~lt~nt rc;lrorellective sheeting while permitting desired prefe~ed pel-e~ ion by the cube-corner elenlçnt resin composition.
The overlay film pr~felably comprises a low elastic modulus polymer, e.g., less than about 13 x 108 pascals, to impart easy bending, curling, flexing, c(j~lllling, or stretching to the resultant r~l~orenective composite. Generally,the overlay film co,np,ises a polymer having a glass transition telllp~lalllre less than about 50~C. The polymer preferably is such that the overlay film retains 20 its physical integrity under the conditions it is exposed to as the resultantcomposite l-,llo~enective shçeting Is formed. The polymer desirably has a Vicat softening tc."pe,~ re that is greater than 50 ~C. The linear mold shrinkage of the polymer desirably is less than 1 percent, ~ltho~lgh certain co.~.b-~c~;onc of polymeric materials for the cube-corner elçmçntc and the overlay ~,vill tolerate a greater extent of shrinkage of the overlay material.
d polymeric materials used in the overlay are resistant to degradation by W light radiation so that the retroreflective sheetin~ can be used for long-term outdoor appl;~.Ptionc The overlay film should be light tr~ncmicsive and preferably is sub~lo~ y l,~r.sparenl.
The overlay film may be either a single layer or multi-layer cGlly)onenl as desired. Either surface of the overlay film may contain printed or formed (such as st~mred or embossed) symbols If multilayer, the layer to which the array of cube-corner eh .l~nl~ is bonded should have the p~ope.lies described herein as useful in that regard with other layers not in contact with the array-of cube-corner elc~rnl~ having ~Plected characteristics as l-ecess~y to impart S desired characteristics to the r~S~lt~nt co,.,pcs;le ~t;l.ortnective chç~ An alternate overlay is disclGsed in U S. Patent Application Serial No 08/516,165 filed August 17, 1995.
The overlay film should be s~lffic;çntly e~enQ;ble to achieve decoupling of the cube-corner el~ .~e ~lc as ~1;ccucsed herein It may be elastomeric, i e, 10 tend to recover to at least some degree after being elon~te~l, or may have s lbs1z~ y no tendency to recover after being elongated, as desired Illustrative examples of polymers that may be e..lployed in overlay films hereininclude:
(1) Fluorinated polymers such as poly(ch orotrifluoroethylene), for example KEL-F800 Brand available from Mim1esola Mining and ~nnf~ct~lring, St Paul, Mh~llc30la; poly(tetrafluoroethylene-co-heY~fllloropropylene), for .~Y~mple EXAC FEP Brand available from Norton Pe.ro-.l.dllce, Brampton, ~CQ ch~setts; poly(tetrafluoroethylene-co-perfluoro(alkyl)vinylether), for eAa...ple, EXAC PEA Brand also 20 available from Norton P~.~ance; and poly(vinylidene fluoride-co-heY~fllloropropylene), for e,~-lple, KYNAR FLEX-2800 Brand available from Pennwalt Coll~o-alion, Philadelphia, Pennsylvania;
(2) Tonomçric ethylene copolyrners such as: poly(ethylene-co-mçth~crylic acid) with sodium or _inc ions such as SURLYN-8920 Brand and STlRLYN-9910 Brand available from E.I. duPont Nemours, Wilmington, Del~.~e, (3) Low density polyethylenes such as low density polyethylene; linear low density poly~lhylcne; and very low density polyethylene;

(4) Pls~ici~ed vinyl halide polymers such as plscti~i7~d poly(~h~ loride);

(5) Polyethylene COpO~ inf.llldjn~ acid filnet ~ polymers such as poly(ethylene-co-acrylic acid) and poly(ethylene-co-meth~rylic acid) WO 97/41463 PCTtUS96/14034 poly(ethylene-co-maleic acid), and poly(ethylene-co-fumaric acid); acrylic functional polymers such as poly(ethylene-co-alkylacrylates) where the alkyl group is methyl, ethyl, prowl, butyl, et cetera, or CH3(CH2)n- where n is 0 to 12, and poly(ethylene-co-vinylacetate); and (6) Aliphatic and aromatic polyurell~&nes derived from the following mOnOm~S (1)-(3): (1) diisocyanates such as dicyclohc.~yllne~ e 1,4~-diisocyanate, isopho-une diisocyanate, 1,6-he-A~nell.ylene diisocyanate, cyclohexyl diisocyanate, d;phc.~ ne diisocyanate, and co,l.bil-Al;on.c of these diisocyanates, (2) polydiols such as polypentylene~lipqte glycol, polytel~a,~elhylene ether gylcol, polycaprolaclonediol~ poly-1,2-butylene oxide glycol, and coml)inalions of these polydiols, and (3) chain ~Ytenders such as butqneAiol and k~-q,l-erliol. Commercially available urethane polytners include: PN-04, or 3429 from Morton International Inc., Seabrook New ~ s~ e, or X-4107 from B. F. Goodrich Company, Cleveland, Ohio.
Coll,b; ~A~;onc ofthe above polymers also may be employed in the overlay film.
Pr~,felled polymers for the overlay film include: the ethylene copolymers that contain units that contain carboxyl groups or esters of carboxylic acids such aspoly(ethylene-co-acrylic acid), poly(ethylene-co-met~--.rylic acid), poly(ethylene-co-viny!~~etqte); the ionomeric ethylene copolymers; plqcti~i7ed poly(vinylchloride); and the qliphqtiC urethqr~es These polymers are p-efel.ed for one or more of the following reasons: suitable mecllqnical properties, good adhesions to the cube-comer layer, clarity, and en~h~n.-~F~-~vl stability.
Colorants, ultraviolet ("W") -bSolbt;lS, light st~bili7ers, free radical scavengers or sntioYiAsnt~ procec~ing aids such as s~ntibloc~i~ agents, releac;~e agents, lubricants, and other additives may be added to one or both ofthe r~t~oleflective layer and overlay film if desired, either uniformly in the configuration of a symbol. The particular colorant selectFd depFnds on the desired color; colorants typically are added at about 0.01 to 1.5 weight percent30 for a given layer. W absorbers typically are added at about 0.5 to 2.0 weightpercent. Illustrative examples of suitable W absoll,el~ include derivatives of bel~ol,iazole such as TINUVIN Brand 327, 328, 900, 1130, TINUV~-P
Brand, available from Ciba-Geigy COl~Jolalion, Ardsley, New York; çhemir.~l derivatives of bellzopheu~ne such as UVINIJL Brand M40, 408, D-50, available from BASF Col~Glalion, Clifton, New Jersey; SYNTASE Brand 230, 5 800, 1200 available from Neville-Synthese Organics, Inc., Pittsburgh, Pen,lsyl~rania; or ch~m;csl derivatives of diphenylacrylate such as UVINUL
Brand N35, 539, also available from BASF COl~lOlaliOn of Clifton, New Jersey. Light stabilizers that may be used include L..ldeled amines, which are typically used at about 0.5 to 2.0 weight percent. Examples of hindered amine 10 light ~l~h ' ,~.~ include TINUVlN Brand 144, 292, 622, 770, and CHIMASSORB Brand 944 all available from the Ciba-Geigy Corp., Ardsley, New York. Alternate hindered amines are disclosed in U.S. Patent No.
5,387,458. Free radical scavengers or antioxid~ntc may be used, typically, at about 0.01 to 0.5 weight percent. Suitable antioxidants include hindered 15 phenolic resins such as IRGANOX Brand 1010, 1076, 1035, or MD-1024, or IRGAFOS Brand 168, available from the Ciba-Geigy Corp., Ardsley, New York. Small ~ "O..~ of other proceseine aids, typically no more than one weight percent of the polymer resins, may be added to improve the resin's processability. Useful proceseing aids include fatty acid esters, or fatty acid 20 amides available from Glyco Inc., Norwalk, Connecticut, metallic stearates available firom Henkel Corp., Hoboken, New Jersey, or WAX E Brand available from ~oech.ct Celanese Corporation, Somerville, New Jersey.
The present ~ "~lenective article can be made in accol.lance ~,vith two di~,~;nt techniques. In the first technique, a ret,oreIlective article is made by 25 providing a first cube-corner cheetine that has the cubes a"anged in a conventional co.-fi~ alion, namely, a non-random orientation, and der~""..hlg this ch~eting under heat and/or pressure. In the second technique, the dero,.l.ed r~t.o~eile~ /e article can be used to create tooling. The tooling maybe used as a mold to cast or form additional l et,ul ellective articles In one e.~lbo~ .l the rl_l.orc:nective article of the present invention is made by thermo~-,--n~g the cube-corner letru'~nective ~hC~ P over a CA 02252854 1998- lo- 19 WO 97/41463 PCr/US96/14034 structured three-dimensional surface of a mold, such as illustrated in Figures 11 and 12. In Figure 11, the cube-comer elçm~nt~ 150 are placed over the structured surface of a mold 152. The overlay film 154 is located opposite an i~ol~tion web 156 to prevent the overlay film 154 from melting or adhering to 5 diaphragm 158. Altematively, the diaphragm 158 may have release plope-lies that pt;lro---. the fi~ncfion of the isolation web 156. Heat and/or pressure areapplied to the r~,tlor~llective sheefine 160 through the the-.noro...,lng diaphragm 158. The three-d;~.e~ onsl shape of the mold 152 may also include a variety of embossed symbols.
In an altemate embodiment illustrated in Figure 12, overlay film 170 is placed on the structured surface of a mold 172. The cube-comer elements 174 is located opposite an isolation web 176. Heat and/or pressure are applied to the rel,or~nective 5hee~ g 180 through the diaphragm 178. An appalalus suitable for themmoforming the l~t-o-enective sheetine to fomm the present 15 rellurenective article is available under the trade design~tion ScotchliteTM Héat Lamp Vacuum Applicator available from Dayco Industries, Inc. of Niles, MI or P.M Black Co. of Stillwater, MN.
Important thermofu.-"illg processi~e variables that may detemline the nature of the r.llorenecli~e article created include tempe,~lule, pressure, 20 duration of each, th~ ness and thermal characteristics of the thermofolll~,ngdiaphragm and the nature of the structured surface on the mold. The size, ulurolllllly and rigidity of the mold may also alter the processing specific~tions of the the..nofol.""~g process as well as whether the mold has an optical or a non-optical pattem. The construction of the rellorellective .l~ee~;np~ such as 25 the th:cl n~s~ son~ g t~"pe~ al~re and eYt~n~ ty of the overlay film, size ofthe cube-comer ~ n~..tc, the presence or ?.bsence of a vapor coat, ~L~lher a sealing film is present and the optical design of the ~ ol ellective ~heetine may also detemnine themmofomling proce~sine ~,al ~les.
Vacuum forming yields a rellorenective article in which the overlay 30 film becomes thinner in proportion to the ~ nce the sheet travels to contact the mold surface. Consequently, the spacing gradient between adjncent cube-CA 022~28~4 1998- lo- 19 corner ~IP ..c~ls incleases from the top of a protrusion on the mold toward the bottom of the deples~;on. The incleascd spacing generally produces lower retlorenectivity. Additionally, if the relrortnective .~hee1;~ incllldes a sealing film, the film is visible through the gap bel~e~ the cube-corner elem~nts The S sealing film may be applied either before or after dcfu~ alioll of the cube-corner shr~ g The sealing film may include one or more colors that would be visible during d~li",e viewing.
In an embodiment in which the cube-corner ele n-nle of the ~,orenective chee~ p are coated with a speclllqr reflector, a colored back 10 coating may be visible through separations between the cube-corner ~l~m~nts A colored back coating or adhesive serves to soften or alter the color and reduce the "grayness" of the specular reflector layer. Alternatively, the speculqr reflector may be a "non-silver" color, such as copper.
In an alternate embodiment, the l~lloltnective .sheeting may be 15 deformed by drape forming. The t~ ~neSs distribution of the overlay film using drape forming is opposite that of vacuum forming, so that the spacing gradient bel~.cen the cube-corner Plernents increases along the top of a protrusions during formation, while the spacing bt;l~ecn cube-corner .ol~m~nts along the bottom of a depless;on remains generally the same. The 20 r~,trolt;nective cheetin~ may also be stretched in one or more directions prior to or during d~""alion. Stretching increases the gap between adJ-qcent cube-corner elern~ontc and thereby reduces rel~olenectivity. Reduced r~lrolt;nectivity may be desirable for some applications.
In an qhP~r~qte embodiment of the present invention, the ~el-ul~nective 25 article of the present invention may be used to prepare a master tooling which can in turn be used to prepare a~iitiol~q-l re~rorene~ re articles. Retroreflective cheeting may be p.~,p~ed directly from the tooling. Use of such masters produces sheeting that is capable of ~ el~ ecting light and displays the target optical prop~;,lies of the original l~llol~nective article from which the tooling 30 was prepared. Images printed, deposited, or formed directly on the exposed CA 022S28S4 1998- lo- 19 back side of the cube-corner el~,...~...l~ by various techniques may also be replir~s-ted in the mold making process.

Angulan~
Angularity refers to the conc~l of how r~l~o.enectivity varies as the e~t~ance angle varies. Retroreflectivity varies nc~ d;ng to the entrance angle and the observation angle. The e~ ce angle is the angle bt;l~e~,n an on axis from a light source and a rel.ul~nector axis normal to the surface of the lel.orenective article. Entrance angle is usually no larger than 90~. Angularity is typically desc,;l,ed in terms of a plot of ret,u.ellectivity .on the vertical axis versus entrance angle on the ho-i~ol,ldl axis. When the m;nqtion axis, observation axis and rello~enector axis are in the same plane, the entrance angle can be considered negative when the leLrore;nector axis and observational axis are on opposite sides of the illl mins~or axis.
The observation angle is the angle bet~,el~ the illu.. ,;~;oll axis from the light source and the observation axis. The observation angle is always positive and is typically a small acute angle.

Optical Prof le Optical profile refers to the concept of rotational and o.i~nl~lional ~yn~etly of a lel.u~c;llective article. Rotational and orientational sylllnl~illy refers to how the l~lolenected light varies as the rt;~lolenective article is rotated about a normal pc;l~e~ ;c~ r to the lel~o~ellective surface. Plots of s-yn~ etly of rotation in~iic~te how the retroreflective pelrv~n~dnce of an article 2S will vary when oriented in varying directions about this axis. Figure 3 is an e of a plot of an optical profile.

E~4MP~S
Features and advantages of this invention are further explained in the following illustrative F~ s,. For purposes of these FY~mrle~ the r~lrolt;llective chpefing incl~ded cube-corner el~---Pi.ls with optical axes tilted CA 02252854 1998- lo- lg or canted with respect to one another, such as generally shown in U.S. Patent No. 4,588,258 to Hoopman.

Rtt~ lective Brightness Test The coerr.~ t of i.,l~o.~Ilection, RA, was measured in accordance with standa,di~ed test ASTME 810-93b. RA values are eAI.iessed in c~ndel~ per lux per square meter (cd lx l m 2).
For observation angle scans, the other test pa~"~lers were held consla,lt at:
entrance angle = -4.0 degrees ol;e.,l~lion angle = 0.0 degrees prese~ l;on angle = 0.0 For entrance angle scans, the other test pa, all~clers were held consla"l at:
orientation angle = 0.0 degrees observation angle = 0.2 degrees plesenL~Iion angle = 0.0 degrees.

Example 1 - Preparation of aflexible rch~ JIective sheet One percent by weight of Darocur Brand 4265 (50:50 blend of 2-hydroxy-2-methyl- 1 -phenylpl opan- 1 -one and 2,4,6-l,i,.lt;ll"~ll,enzoyldiphenylphosphine oxide, available from Ciba-Geigy Corp., Hawthorne, NY) was added to a resin blend of 40 percent by weight Photomer Brand 4035 (phe"o~yelhyl acrylate available from Henkel Corp. of Ambler, PA) and 60 percent by weight Photomer Brand 3016 (bis-phenol A
epoxy diacrylate av ~'~ble from Henkel Corp. of Ambler, PA), and 1 percent by weight Darocur 1173 (2-hydroxy-2-methyl-1-phe..ylplopan-1-one, available from Ciba-Geigy Corp., Hawthorne, NY). The resulting solution was used as a resin composition for ~",..ng cube-corner ~olelnçnt~
The resin composition was cast onto a 0.152 mm (0.006 inches) thick irh~iC polyurelllane overlay film (MORTHANE Brand 3429 urethane from Morton Intem~tior~ Inc., Seabrook, NH) on a polyethylene te~p~'~'ste (PET) carrier film. The coated film was passed b~ c~,n a polyule~ll&ne nip rol and the nickel ele~l,o~,l,lcd tool to create 62.5 r: 'OIIS (0.0025 inches) tall cube-corner elc~n~ ~s at 57~C (135~F). The gap bet~,veen the 90 durometer 5 polyur~l.ane rubber nip roll and the nickel tool was set to ...it.;...;~e the resin in the cavities. The resin was cured through both the overlay film and the carrier film with one AETEK nledillm pressure merculy lamp (available from AETEK
Intern~tionql of Pl~infiPl~ IL) set at 160 watts/cm (400 W/in). The feed rate ofmaterial through the cure station was 1.524 meters/min. (5 fpm). Upon 10 completion ofthe microreplication process and removal from the tool, the sideofthe composite with the cube-corner ~IF ~Pnl~ was post-cured by irr~ ting it with a medillm pressure mercury lamp (AETEK International) Opclaling at 80 watts/cm (200 wfin).

15 Example 2 - Vacuum-Formed Retroref lect ve Articles The l.,l~orellective sl~ec~ p of Example 1 was placed into a cl~mrine frame with the plano-side (overlay film) of the film facing upward on a vacuum-former Type Comet, Jr., Model lOX10 from Comet Industries, Inc..of Sanford, FL. After heating the film to approAi,.lalely 150 ~C using the 20 ~es ~ ce heater on the vacuum former, the film staned to sag (appi~,l,alely 20 seconds). The so~ened composite film was rapidly lowered onto a porous mold bearing a rect~n~ r array of 90 (g x 10) hemi-spherical ~1.59cm (0.625 inch) t~i~mPter depress;ons while a vacuum was being applied to the mold. The so~ened film formed a reflective sheet with .~trolenective hemi-spherical 25 cavities or depressions, shown in both a plan view and a perspective view in Figures 13. Figure 16 illustrates an alternate ~tlol~;nective article with hemi-spherical protrusions formed using the process of the present Example.
Figure 14 is a photomicrograph (50X) taken from the cube side of the derol,.led l~t~ ,nective ~heC~ g at the bottom of a vacuum-formed depless;on 30 of Figure 13. Figure 15 is a photomicrograph (50X) taken of a vacuum-formed depless;on from the overlay side. The cube-comer ~le ~ s are shown in dark CA 022s28s4 1998- lo- lg and the separations ~el~neen them is in white. The photomicrograph illustrates a ratio of the base edge of the cube-comer elem~nt~ to the sepalalions th~,-eb~l~.,ell is in the range of about 0.5:1 to 2:1. The cube-corner elc...~
are nomin~lly a1jllcent to one another prior to dero,lllalion. As is clear from S Figures 14 and 15, however, the vacuum fol....ng process s~ ,hes and thins the overlay film and increases the separation of the cube-corner clc ..~ at the bottom of a dcpress,on. The generally uniform separation bcl~.,el- the cube-corner ~ ls is ç~hAnced by heating the r~tlolenective ~heetir~ to soften the overlay film prior to vacuum for ning.
Example 3 The r~l,orencc~ e 5he~,l;i~g of Example 1 was placed into a clamping frame with the plano-side of the film facing downward. The film was heated using the method of FY~mple 2 until the film started to sag (appl o~ ely 10-15 15 seconds). The softened composite film was rapidly lowered onto a porousmold bearing a rect~n~ r array of go (g x 10) hemi-spherical depressions (~0.75 inch ~ ..e~er), such as illustrated in Figure 13, while a vacuum was being applied to the mold. The soflrçned film formed a reflective sheet with rellorenective hemi-spherical protrusions.
Figure 17 is a photomicrograph (50X) taken from the cube side of the de~olll,ed rel~ nective ~hçeting at the top of a vacuum-formed protrusion.
Figure 18 is a photomicrograph (50X) taken of a vacuum-formed protrusion from the overlay side. The cube-corner cl~m~ are shown in dark and the separations between them is in white. The cube-corner clemenls are nominally a~r~~nt to one another. As is clear from Figures 17 and 18, however, the vacuum fo.l.,.ng process stretches and thins the overlay film and inCl~iaSeS theseparation of the cube-corner element~ at the top of a ~ ;on. The separations ~et~,.cel- the cube-corner el-~n.~ are random due to non-uniform heating and draw, p.i...alil~ a filnction of the sho.l~.-ed heating cycle. Some 30 cube-corner ~l~.. ~,n~ are grouped together~ others are isolated. The randomsepalalion of the cube-corner ~Ir-~e"ls created a glittery visual appe&l~ ce. It .... _ CA 02252854 1998- lo- 19 will be understood that the separation between the cube-corner rlP....~nl s can be further altered by controlling the draw ratio of the overlay film over the mold.The present photc crographs of the r~l,orellec~ e cheeting with Pnhsnced glittering showed a sub~ Lially greater degree of cube-corner S P~ reGlif-~lA~;Qn and separation, than is present on undeformed rtl~o,~nective sheetin~ It is believed that the çnhanced glittering effect is related to the ndditiQn~l reflective paths available to light ineidçnt on the ~ljacent cube-corner rl~-.e4s. Acco~ .gly, there is a general range of glittering image forming abilities of the retroreflective article of the invention 10 which can be achieved by çh~ngin~ the proce~,cin~ ~a~ ~les .

Examyle 4 - l~ackf lled Formed R~ cJlective Article The lel-~,r~;nective sheeting of Example I was met~lli7P,d by vapor deposition of ~ minllm metal on the cube-corner PlemPnts. The met~lli7Pd 15 ~~lolellective cheetin~ was vacuum-formed with the plano-side of the film in contact with a mold to form a series of letters that spelled the word "VIPER"
as shown in Figure 19. While the forrned film was still in the mold, a two-part polyurelhane was poured into the cavity to backfill the cube-corner e1PmPnt$
and thermally cured. The individual letters were cut out and adhered to a steel 20 plate with a gloss black co~ting The lel~orenective ~heeting is generally planar, except along the transition edges of the letters. The ret,~renective article eYhibited s~dard let~or~nectivity along the planar surface. Some localized glitter-effect was noted along the transition edges of the letters.

25 Exany)le 5 - Preparation of a f lexible rcln~ ective s~e~ff~ g A mixture of 1 percent by weight of Darocur Brand 4265 (50:50 blend of 2-hydroxy-2-methyl-l-phenylprupan-1-one and 2,4,6-ell~ Jel~oyldiphe~lylphosphine oxide, available from Ciba-Geigy Corp., Hawthorne, NY) was added to a resin mixture of 19 percent by weight 30 PHOTOMER Brand 3016 (a bi~phP-nol A epoxy diacrylate, available from Henkel Corp., Ambler, PA), 49.5 percent by weight TMPTA

(lliln~lh~lol~Jropane triacrylate) and 30.5% Sartomer 285 (THFA is tetrahydrofurfuryl acrylate, available from Sartomer Corp.). This resin composition was cast at 57~C (135~F) b~ .een a tool with 85 .~ ons (0.0034 inches) tall cube-corner ~I..,,..,Ic and an qlirhqfic polyu-elhane overlay film 5 0.114 mm (0.0045 inches) thick (MORTHANE Brand 3429 ur~lhal1e from Morton I.lte...~;Qnql Inc., Seabrook, NH) on a polyethylene terephthql~te (PET) carrier film 0.51 mm (0.002 inches) thick. The rubber nip roll gap was set to ..-:n . ;~e the amount of resin composition over the cavities of the tool.
The resin was cured through both the overlay film and carrier film with one 10 AETEK medillm pres;,u~e mercury lamp (available from AETEK International of Plainfield, Illinois) set at 160 watts/cm (400 watts/in). The feed rate of material through the cure station was controlled to attain the desired degree of curing (exposure to 100 to 1000 millijoules/cm2). After the microreplication process was completed, the cube-corner side of the composite post-cured by 15 irrndiqting it with a mec~ium-pressure mercury lamp (AETEK International) operated at 80 watts/cm (200 Wrm).

Example 6 - Sealed Retroreflect~ve .~he~ g The l~tlor~nective sheetin~ of Example 5 was thermally sealed to a 20 white polyurethane sealing film as follows. A lqminqte sample of I t;L. Ol ~nective sheeting and sealing film was plepaled by first protecting it with a 0.025 mm (0.001 inch) polyester terepl~ qte film. This construction was then fed into a nip be~ el- a heated steel embossing roll and a 85 durometer rubber roll. The sealing film was a 0.05 mm (0.002 inches) thick white (TiO2) pig...el-led 25 aliphatic polyester ule~hd--e (MORTHANE Brand PNO3 supplied by Morton Internqtionql, Seabrook, New ~q....psl-;.e). The embossing pattern was of a chain link configuration and the embossing roll surface was 220~C (410~F).
The rubber roll surface te."pe.~l~re was 63~C (145~F). The rolls were turning at a surface speed of 6.09 meters/minute (20 feetlminute), and the force on the 30 nip was held at 114 Newtons/cr,~ r (65 pounds/inch). The polyester telepht~ te protective layers were removed from the ~mples prior to further use.

Example 7 - Pteparat on of ~icense plate A 152.4 x 304.8 mm (6" x 12") piece of the rel~orellective shee~ P
with a sealing film was prepdred as desc,;bed in Example 6. The sealed cube cheeti~ was then lPmit-sted to a pressure sensitive adhesive with a 1iner, product number 467 MP available from ~innesot~ Mining and M~rllfact-~ring Company of St. Paul, MN. The liner was removed and the sheeting was 10 l~min~ted to a flat, white license plate blank. The res~.lting article was embossed using conventional license plate embossing techniques. The sample ~mhossed very well and did not tent over the letters. In the view box, the sample was noticeably brighter and whiter than convention~l beaded license plate sheeti~ The can~ luYJsquare meter was 200 in the horizontal 15 direction and 300 in the vertical direction.

Example 8 - Flexibk r~ JIective s~i '',17~ g embossed over netting The l~tlorene~live ch-o~ting of F.Y~mple 6 using a pressure sensitive adhesive was embossed over five sdm~ s of small mesh industrial netting, as 20 shown in Figure 20. Heat l~rnin~tiQn of the retroreflective sheeting is preferable, because it helps the r~tlurellective cheeting co-~o..,. to the underlying netting. The industrial netting of Figure 20, viewed from le~ to right, is sold under the product dçcignq~ionc: NO 888 Regent - nylon 6.35 mm (0.25 inch) s~luare; NO 916 nylon delta 1.3 cm (0.5 inch) hex; 504-nylon 1.3 25 cm (0.5 inch) square; PE-101 polyester 1.59 cm (0.625 inch) hex; and the holizo..tally orient s~,cc;."en - NO 61339 polyester 3.175 mm (0.125 inch) hex, all available from Sterling Net Co. of Montclair, NJ.
The netting ch~nged both the angularity of the cube-corner Ple~..f.~lc and acted as a filler or cushion for the embossed lel~ore[lective cheeting The 30 portion of the ~t;l,o.~nective sh~e~ g defc""led by the netting is shown in white and the space between the netting is shown in black. A localized glitter-.. .. . .

effect was visible along the sharp transition regions in the rellorenective ~h~etine deru,l"ed over the netting. It will be understood that a met~ 7~d ~ t-o,enective sh~P~t;.~g with a suitable adhesive may alternately be embossed over the netting. One possible use could be in temporary pavement markings, S which need a Ji~ JII angularity from standard ~ orellective sheeting, as well as cushioning when run over by a car.

Example 9 The ,.,l~orenective ~hP,~t;l~g of F ~~~lple 1 was vacuum formed on a 10 mold bearing a ~ symbol appro~il"alely 6.35 mm in di~nnet~r. Figure 21 is a photomicrograph (50X) taken from the overlay side of the rc;l.oJenective .~hs~ g The cube-corner elements are shown in black and the separations in white. The aa~ t~y of the ~ symbol prevented a uniform draw, resl-lting in subst~nti~l lal~do~ ;Qn ofthe cube-corner ~ m~ntc Exam~le 10 An llnce~led ,et-or~{lective cheeting according to Example 5 with cube-corner ek ~le"ts 0.086 mm (0.0034 inches) high was thermo-formed over 60, 100, 150 and 220 grit coated abrasive paper available from Minnesota 20 Mining and ~S~nuf~G~-ring Company of St. Paul, MN using the ScotchliteTM
Heat Lamp Vacuum Applicalor ~~icc~.~sed above. The cube-corner elP...~en~c were po.sitioned opposite the coated abrasive paper. The bake cycle in~ 4Ided ~allllil~g the applicalor to appro~ ely 118 ~C and baking for about 1.5-2.5 minlltes The lamp bank was raised at the end of the bake cycle to cool the 25 rel,oreilective articles.
Figure 22A is a plot of the relative brightnecc versus ~ tl ~lce angle for the res..lting l~t~olenective articles. Figure 22B is a plot of the relative brightness versus the observation angle. The control plot is the und~u-lllcd lell~"t;nective ~ g The rellorenective article had a glittery appe~nce 30 presumably due to the high level of randomization of the base edges of the cube-corner ~1~ .--enl~c CA 02252854 1998- lO- ls Exam~le 11 An !~nse~led ..,t,ol.,nective sheeting acco~ding to F.Y~mple 5 with cube-corner el~n~ç~c 0.086 mm (0.0034 inches) high was met~ 7ed by vapor deposition of ~ minllm metal on the cube-corner rle;..r~ $. The n~et~ ed 5 rel.urenective sl f~ g was thermo-formed over 60, 100, 150 and 220 grit coated abrasive paper a~zo .li.,g to the method of Example 10. The grit dçcigrqtions refer to abrasive particles with ~iDmetp~rs no larger than 551 microns, 336 microns, 169 microns and 100 microns, respectively. The cube-corner Pk nf--~C were positiQned opposite the coated abrasive paper. Figure 10 23A is a plot of the relative bl;~l~n~r~s versus entrance angle for the recl.lting r~l~ù~enective articles. Figure 23B is a plot ofthe relative brightnecs versus the observation angle. The control plot is the undero",.ed met~lli7çd -orenective che~
The let-o~enective article had a glittery appe&ance presumably due to 15 the high level of r~nclomi7~tion of the base edges of the cube-corner clem~n~-;
The rel~ol~;llective sheeting was also therrno-formed over a beaded pavement marker available under product dçcignDtion 5160 ScotchlaneTM foil backed tape from M;mlesota Mining and l~nllfr~cturing Company of St. Paul, MN
according to the method of Example 10. Figure 23C is a bar graph showing 20 the increase in whitçness of the let.oreflective cheeting after the thermo-fo"~"ng process for the four coated abrasive paper speçinlP-ns and the beaded pavement marker. Whitçne,cs is measured using a spectrophotometer with a bi-dileclional optical measuring system accordi..g to ASTM E 1349-90.
Whiten~sc is believed to be an a~pluAi".ale measure of the glittery appearance 25 of r~tlorenec~ e cheeting The level of wl.;lel-~sc for the rc;lrol~nective article thermo-formed over the 100 grit coated abrasive paper is believe to be a function of the size of the cube-corner elemçnts relative to the grit of the coated abrasive paper. That is, the 100 grit coated abrasive paper provided the greatest level of randomi7~tion of the base edges of cube-corner PlF~f."c 30 0.086 mm high.

EJcample 12 An l~nce~led rellorenective sl~tel;..g according to F.Y~mrle 5 with cube-corner elempnts 0.086 mm (0.0034 inches) high was thermo-formed over a series of ~cr~ n.~ using the method of Example 10. The spec;...~- in~ ded 5 a beaded pavement marker available under product de. g~ iQn 5160 Scotchlane~{ foil backed tape and a raised pa~e.,.cnl marker available under product design~tion A381 StamarkTM high p~.~""ance tape, both from Milulesota Mining and l~f~nl.f~ctl.ring Company of St. Paul, MN; a tool for msn-lf~~ring rèlroi~;nective sheel;-g with cube-corner P1e....~ e 0.178 mm 10 (0.007 inches) high; and a light diffuser available under the product cleci~n~tiQn Clear ~lis",alic from PlD~ 1;fe, Inc. of Coll.mhus OH. The cube-comer PlPm~nts were positioned opposite the sl)ec;~ lC.
Figure 24A is a plot of the relative brightn~Ps.s versus el.lrance angle for the resl~lting letl~rellective articles. Figure 24B is a plot of the relative 15 bri~htness versus the observation angle. The control plot is unde~ll~,ed retroreflective sheeting Variation in the glittely appeal ~1ce of the re~,orênective articles was pres~m-bly due to the various levels of r~n-lomi7~tion of the base edges of the cube-corner Plc ~ s.

20 Exany~le 13 An l~nce~led le~,o,ellective sheeting according to FY~mrle 5 with cube-corner e1empntc 0.086 mm (0.0034 inches) high was met~lli7ed by vapor deposition of al~ .. metal on the cube-corner r.l..."~ The met~lli7ed letlor~nective ~heetinp~ was therrno-formed over the beaded pavement marker, 25 raised pavement marker and light diffuser of Example 12 using the method of F ~~r~ le 10. The cube-corner elc~ were positioned opposite the spe ;...el-s.
Figure 25A is a plot of the relative bri~htnes~ versus entrance angle for the resulting l~llo~eilective articles. Figure 25B is a plot of the relative 30 b,;~ Pss versus the observation angle. The control plot is the undeformed rellorenective sheeting _.

CA 02252854 1998- lo- 19 Example 14 A relrolt;llective shee~ g according to Example 5 with cube-corner el~,....,.-~s 0.086 mm (0.0034 inches) high was meta!li7çd by vapor deposition of ~II.minl.m metal on the cube-corner el~ ..f nl~ The met~lli7ed r~t,olenective 5 sheeting was thermo-formed using the method of Example 10 over a poly~ro~yl~ne industrial mesh netting with a 1.27 cm (0.5 inch) hex pattern, sold under the product des~grstion NO916 by Sterling Net Co. of ~ontcl~ir~
NJ. The netting softened during the thermo-fo"....lg process and thus rem~in~d bonded to the ttl~r.nective c~ i \g The cube-corner e~ enls were 10 positioned opposite the ~specimpnc Figure 26A is a plot of the relative br;ghtness versus ~,.lr~nce angle for the resultine rtlror~nective articles. Figure 26B is a plot of the relative brightnçsc versus the observation angle. The control plot is the undeformed retlore~nective sheeting Example 1 f Three samples of the un~ealed r~tlultnective sheetine according to Example 5 with cube-corner ~lements of difI~.enl sizes were thermo-forrned over a beaded pavement marker available under product desien~tion 5160 20 ScotchlaneTM foil backed tape from Minnesota Mining and ~nl.factl.ring Company of St. Paul, MN. The cube-corner elemçnts were 0.0625 mm (0.002S inches); 0.086 mm (0.0034 inches) and 0.178 mm (0.007 inches) high, re.JI~e~ ely. The greatest glitter-effect was visible on the rc;Llorenec~ e .cheeting thermo-formed over the 0.178 mm cubes. The least amount of glitter-25 effect was visible on the .etrorenective ~hee.ti~ thermo-formed over the .0625 cubes.

F~onyle 16 An uncealed ~el~o~ellective sheeting acco-~l;ng to Example 5 with 30 cube-corner e~ ..v-.l~ 0.086 mm (0.0034 inches) high was me~t~lli7ed by vapor deposition of al~lminllm metal on the cube-corner elementC The m~t~ 7ed . .

CA 022s28s4 1998-lo-lg W O 97/41463 PCTrUS96/14034 rcllol~ilective sheeting was thermo-formed over the cube-corner side of three co.l""hcial reflectors. Reflector A was a 7.62 cm (3 inch) circular reflector divided into 6 pie-shaped wedges of cube corners, sold as Model V472R from Petc~son l~n~lf~ ring of Grandview, MO. Reflector B was a 7.62 cm (3 5 inch) circular reflector having about 20 ~iqmond shaped patterns 1.27 X 2.54 cm (0.5 X 1.0 inch) containing cube corner ~le...f.l-~.s~ sold as Model Sate-lite-30 from KyKu Products of Bedford ~eightc OH. The ll~ct~n~ r reflector 6.35 X 7.62 cm (2.5 X 3.0 inches) had vertical rows of cube comer ~Ir ~
offset from each other, sold as Model PEC 4200C from The Refractory of 10 Newburgh, NY.
Figure 27A is a plot of the relative brightn~c.c versus entrance angle for the res~lting ~lrolenective anides Figure 27B is a plot of the relative brightnçss versus the observation angle. The control plot is the undeformed met~lli7.od ,~llolenective sl~etin~ Figures 27C is a plot of the relative 15 bri~htnecc versus ~ llance angle for the commercial reflectors illustrated inFigures 27A and 27B. Figure 27D is a plot of the relative brightness versus the observation angle for the co"""t,~iial reflectors.
All patents and patent applications cited above are incorporated by l~Çerence in their entirety into this docllm~nt The present invention has now been described with I t;r~lence to several embodim~nts thereo~ It will be apparenl to those skilled in the art that many cl~ p~.s can be made in the embc~im~nts dcsclil,ed without departing from the scope of the invention. Thus, the scope of the present invention should not be limited to the structures desclibed herein, but rather by the structures desc,il)ed 25 by the lsn~ e of the claims, and the equivalents of those structures.

_

Claims (13)

What is claimed is:

1. A retroreflective article comprising a retroreflective sheeting including a multiplicity of discrete, cube-corner elements having base edges, the cube corner element being cured in situ on a transparent, polymeric overlay filmto form an interpenetrating network between a thermoset material of the cube corner elements and the polymeric overlay film, the retroreflective sheeting deformed into a three-dimensional structure so that the base edges of a plurality of cube-corner elements are non-planar with respect to one another.

2. The article of claim 1 wherein the retroreflective article produces a target optical property.

3. The article of claims 1-2, wherein the target optical property is glitter.

4. The article of claims 1-3 wherein the retroreflective article produces a target angularity.

5. The article of claims 1-4 wherein the base edges of a plurality of adjacent cube-corner elements are non-planar with respect to one another, wherein the base edges of a plurality of cube-corner elements are tilted with respect to one another, and wherein the base edges of one or more cube-corner elements are not parallel to a front surface of the overlay film.

6. The article of claims 1-5 wherein the cube-corner elements have a variable density across a portion of the retroreflective article, wherein adjacent cube-corner elements across a portion of the retroreflective article are variably spaced, and wherein the overlay film has a thickness that varies acrossa portion of the retroreflective article.

7. The article of claims 1-6 wherein the cube-corner elements of the retroreflective article are backfilled with a coating.

8. The article of claims 1-7 wherein the coating contains one or more colors.

9. The article of claims 1-8, wherein the retroreflective sheeting has its cube-corner elements arranged such that an angle .alpha. between faces of adjacent cube-corner elements varies therebetween throughout the sheeting to produce a target glitter property.

10. A method of forming a retroreflective article with at least one target optical property, comprising the steps of:
preparing a cube-corner retroreflective sheeting including a multiplicity of discrete, cube-corner elements having base edges, the cube corner elements being cured in situ on a transparent, polymeric overlay film to form an interpenetrating network between a thermoset material of the cube corner elements and the polymeric overlay film; and deforming the flexible retroreflective sheeting into a three-dimensional configuration so that the base edges of a plurality of cube-corner elements are non-planar with respect to one another.

11. The method of claim 10 wherein the step of deforming results in the base edges of the plurality of adjacent cube-corner elements to be tilted with respect to one another.

12. The method of claims 10-11 wherein the step of deforming is selected from the group consisting of thermo-forming, vacuum-forming, embossing, and combinations thereof.

13. The method of claims 10-12 wherein the step of deforming produces a three-dimensional symbol in the retroreflective sheeting.