CA2244922A1 - Dew-resistant retroreflective traffic sign having a textured glass surface - Google Patents

Abstract

A dew-resistant retroreflective traffic sign having an attached, front-facing glass plate. The glass plate has a textured surface exposed to the air. The textured surface imparts anti-dew properties to the sign. A method of making the dew-resistant traffic sign is also described.

Description

W O 97/31357 PCT~US97J00628 D E W ~tESISTA N T R ET R OEUEF~E CI~E
T~L~FFIC SIG N 13L~IN G A T ~X 1U~D G L ASS S UEUFA CE

~NTRODUC~ON
In order to be effective, roadway traffic signs must be visible to motor vehicle drivers at night as well as during the day. Because it is impractical toe all traffic signs with external li~hti~ a co~ Qn approach to F .~h~ the visibility of roadway trafflc signs is to use ~ u-t;flective graphics on the traffic sign.
Retroreflective signs have the unique ability to return a ~ Js~ ;Al portion of incident light back toward the light source. At ni~httime, light frommotor vehicle hf~ htc strikes the l ~;ll orenective graphics and is r~llulenected to the motor vehicle driver. The bright image displayed by the rc~.ult;flective sign makes the sign easier to read and gives motorists more time to react.
A cignific~nt problem that has been encountered with rc~L~u-~ective trafflc signs is the ac~m~ fior- of water droplets on the surface of the signs.
Dew is an e~l~e~ y commûn source of water d,uple~s and can be particularly problematic because it occurs predf~ ly at ni~httime when the ~l,u.~nective signs are ope,aLive. When present on a traffic sign in the form of small beaded water ~l.o~lel~, dew can seriously disrupt the path of in~.identand reL.o,t;nected light. This can make h~ru",.aLion on traffic signs much more difficult for passing motorists to read. This problem is very well known, and there have been a variety of attçmrtc aimed at removing dew or p~ ling dew from rO-,. h-~, on the surface of trafflc signs.
One method of e~ ;ng dew from reL,orc;nective signs is to heat the u~enective elempnt Gerrnan Patent NO. 4226266 to Gubela discloses an electrical heating element placed behind the retroreflector.
European Patent Application NO. 0155572, ~cci~ned to Biersdorf AhiPn~PsPllc~h~ ~iccloses a ~Llu~nective traffic sign having a heat radiator of Anotli7ed All~minllm that is mounted directly on the upper edge of the traffic sign. The advantages attributed to this device include the lack of a need for a hydrophilic surface coating, a heat retaining layer on the back of the sign, or an artificial energy supply.
J~p~nese Ko~ai Patent Pl-hlicAtion No. 7-3731 ~.ei~n~d to Ter.hnonijuichi K.K. di~rloses an anti-dew reflective road traffic sign that has a heat storage co,llaine. tightly bonded to its back side. The heat storage cf~ CQ..~ a heat storage agent, typically a gel co~ ;..;..g a liquid glycol that collects heat and radiates that heat towards the sign's front surface.
U.S. Patent No. 5,087,508 to Beck di~ClQsçs a reLlulGnective trafflc sign having a thermal reservoir layer located behind the ~el-ulenective surface.The thermal reservoir cc,nLains a material that undergoes a phase change b~ ,n - 20~C and 40~C. The energy barrier of the phase trAn~ision pie~ s the sign from cooling rapidly.
JApa~u~e Patent ApplicAtir~n 0614961A ae~igne(l to Kawai Musical In~ll Ulll~ll ~g. Co., dierlQsee an anti-fog mirror in which a vibration ~ellel~Lor is ~ hed to the rear surface of a mirror. J~p~n~se Patent Appli~tion 07259024A ~ ned to ~tellchh~ Denki Sangyo KK, ~ie~loses an anti-fog mirror for road safety in which solar energy is stored in a heater unitlocated behind the mirror's surface.
Another method that has been used to impart anti-dew characLe,isLics to ,ellulGnective traffic signs is to apply a water-spreading, Lydlùphilic coating to the surface of the sign. The hydrophilic coating spreads the moisture over the sign's surface and thus makes the sign easier to read because the resulting thinwater layer does not alter the path of incidçnt and lel~ulelle~ e light to as great an extent. U.S. Patent Nos. 5,073,404, 4,844,976 and 4,7S5,425 to T.
Huang disclose a lellulenective ~ e~ g that has a ll~"~are.,l coating CClll~ illg colloidAl silica and a pûlymer selected from ~liph~ttc pol~/ult;lt~Alles7 polyvinylchloride co-polymers and acrylic polymers. The colloid~l si~ica is disposed in the polymer at about 10-80 weight % (10-70 weight % in the case of polyacrylates). The l,~nsl ale"~ coatings provide superior dew repellency, allowing the r~forcnective ~h~eting to retain a higher percentage of its original bri,~htne,cs aflLer being exposed to moisture.
There are numerous ~ pl~ of anti-dew, water-spreading layers that are made with inorganic colloidal particles disposed in a polymeric binder.
S U.S. Patent No. 4,576,864 to Krautter et al. rliC~loses a water-spreading layer that is composed of colloidal particles of a metal or silicon oxide in which thewater-spreading layer is adhered to a plastic substrate by an adhesive c~u~ g a non-water-soluble, organic-solvent-soluble, and e~eentiAlly non-~ ,llable, polar-group-c~ .g-polymer. U.S. Patent No. 4,478,909 to TAni~-r.hi et al. discloses an anti-fogging coating having finely divided silicaparticles disposed in a matrix of polyvinyl alcohol and an or~Anf silicon alkoxycompound or hydrolysates thereo~ A similar coating is also described in U.S.
Patent No. 5,134,021 to Hosono et al.
In the area of pavement marker technology, it has been known to bond a flat glass plate to a .Gllorènective chçeting's surface to improve abrasion reCict~n~e U.S. Patent Nos. 4,232,979, 4,340,319 and 4,596,622 to Heenan et al., ~ close road pavement IlI&~kG.:i that have a glass sheet fixed to a l~,t~u,~nective cheeting~s front face. The glass is plefelled to be an l~nt~mp~red and ~ FAled sheet about 2-15 mils thick. There is a particular need for improving the abrasion . ~ ce of pavement markers since they must be able to resist tire impacts in the p- e~ence of abrasive materials such as grit and sand as well as roadway çh~.mi~Alc and telllpèl~ re and weather eAllènles.
Unlike pavement markers, however, trafflc signs are not disposed on road sllrfAcFc and, as a conse.luence, do not require the extreme abrasion reci~Ance that is required of pavement III~.Iktl~. For trafflc signs, loss of intensity caused by P.t;~ ;Qn, ~spe~i~lly dew, is of great concern.

SU~LURY OFTHE~NVE~ON
The present invention provides an anti-dew reL.uleflective sign comprising a glass plate having a textured outer surface disposed over a r~,L.ulenective graphic. The textured glass surface is exposed to air, and, in W O 97/31357 PCT~US97/00628 dew con~lition~ spreads ple~ lion in a thin film over the glass plate's surface.
The present invention also provides a method for making an anti-dew u~cnective sign in which a glass plate having a textured outer surface is placed over the le~lolt;nective graphic.
The present invention provides an economical and elegantly simple method of making an anti-dew .~ ilective traffic sign. The anti-dew characteristics of the inventive ~ ,nec~ e traffic sign are achieved without the need for cle~l.ical inputs, heating flF~ , heat storage layers, infrared bearn radiators, phase transition materials or vibration gc~ . Other advantages provided by the textured glass surface include~ e to organic solvents, thus f~r.ilit~ting removal of graffiti from the sign; ~ allltl;llg rPcict~nce and protection from ultraviolet (UV) light, thus ~yt~n~li~ the lifetime of underlying polymers and inks.
These and other features of the present invention are more fully shown and desclll,ed in the dli~wh~gs and detailed description of this invention, where like It;rtl~,nce numerals are used to repleselll similar parts. It is to be understood, however, that the description and d~wi~ s are for the purposes of illu~ ion and should not be read in a manner that would unduly limit the scope of this invention.

BRUEF DESCR~r~ON OFTHE DRA~NGS
FIG. 1 is a front view of a It;llolenective sign of the present invention in which the r~,l-urenective graphic is "St. Paul 10 km".
FIG. 2 is a cross-sectiorl~l view taken along line 2-2 of FIG. 1.
FIG. 3 is a plot of light inlells;ly versus time of day from rellu~ ective ~heeti~ having a flat glass surface.
FIG. 4 is a plot of light ;llh.l~.ly measured versus time of day from u- ellective ~h.oeting having a textured glass surface.
FIG. S is an SEM photomicrograph of the surface of a flat glass plate.

W 0971313~7 PCT~US97/0~628 FIG. 6 is an SEM photo,l"crograph of the surface of a textured glass plate.
FIG. 7 is an SEM photomicrograph of the surface of a highly textured glass plate.

DETAILED DESCRnY~ON OFTHE ~NVE~ON
In FIG. 1, a I t;ll ùrt;nective sign 2 of the present invention is shown that co~ .C il~ulll.alion in the form of a lc;~lolcnective graphic 3. In this case, the l~t.o.~le~ilive graphic is in the form of lettering that spells "St. Paul 10 krn".
In FIG. 2, the ~ ùlenective graphic 3 is disposed on substrate 4. An h,l~laye~ 5 overlies the ~~ I.or~ nective graphic 3. At the sign's top, outer layer lies a glass sheet 6 having one major glass surface 7 that faces the substrate and a second major textured glass surface 8 that is exposed to the ~tmosphto~re.
Retroreflective graphics of the present invention are defined as ~ c;llective ~hFel;~ g or .~tlu-enective ei~ a-,~lu~ed in the form of Ch&~nl;~el'~ .llllbtl'~ or symbols. Retroreflective graphics do not include a ullirOllll lt;~lurenective cheeting or layer over the entire surface. Thus, gl ~-are not merely plain reflectors such as a pavement marker; however, the lell Ul ~aective graphics can be disposed over a uniform l el- urenective bacL~uulld. The ~ ù~lective graphics can also be an inverse design such as a ~~l~ult;flective bacL~,.ound for non-leL-ù~flective characters, numbers or syrnbols such as a non-reflective profile of a cow or deer. In the latter case, the l~,l-u.t:necli~e graphic would include both the I~I-u-enective bacl~uund and the non-reflective profile.
The l~lr~ Ile~;Li~e graphic 3 is typically bonded to the substrate 4 or to a background materiai by an adhesive or by mechqnic,q~l means such as qno-li7ed "h~ rivets. Adhesives are p-~;r~ id, and p-cis~.l'e sensitive adhesives are esrer~iqlly prefellt;d. The lelru-tinective characters, numbers or symbols can be bonded to a re~lu~nective bac~roul,d. For example, the ~el-u,ellective A 30 characters, numbers or symbols can be cut from white l~L,ur~,nective sheetin~
and bonded to a bacl~lound of l~;Llu~ellective ~heeting that has been overlaid with a clear, colored polyrner film such as an acrylic film. Common background colors are green, brown or blue. Another way of making the r~h orGllective graphic is to cut out letters, numerals or symbols from a L,~l..s~,alenl colored polymer film, and l~min~te the colored letters, numerals or symbols onto white lGI.orGnective ~hf~etin~ A suitable, commercially available clear colored acrylic film is ScotchliteTM Elc_l.u.~c Cuttable Film Series 1170,available from 3M, St. Paul, MN. In still another alternative, the rGllulGnective graphic can be produced by printing over portions of a lGLlurGllec~ e sheet.
For ~ .~ n ~'e, a stop sign graphic can be made by screen printing a red clear ink with a negative legend on a white lGl~ulGflective .~;h.~?eting The reL-ol~nective graphics typically contain ,el.~-Gnective .~l-c~
FY~mrles of col"--le~.iially available l~l- ur~lective ~heetin~ that may be used to make the graphic include ScotchliteTM Reflective Sh~etin~ High Intensity Grade Series 3870, ScotchliteTM Reflective Sheeting Diarnond Grade VIP Series 3990, and ScotrhliteTM Reflective SheetinE~ Diamond Grade LDP Series 3970, available from 3M, St. Paul, ~i~-l-f.~ Retroreflective ~l~e~ typically Cûl~ )li3eS a reflective surface and optical fJe~ ; The reflective surface serves to reflect inr;~nt light, and the optical f~lPm~nt~ serve to redirect theinr,idçnt light toward the light source. The reflective m~t.~?ri~l may _om~lise a ~ e~;~.lAr metal reflector such as ~hlmim~m or silver (see, for e.Y~mrle, U.S.
Patent No. 5,283,101) or a diffilse .Gne~ lor such as a heavy metal pigment or apolymeric material wherein reflect~n~e is caused by a di~G-t;--ce in refractive indices at an interfiace (frequently a plastic-air interface~. Optical ,el~ s typically come in one of twû forms: beaded lens ~.?lçmlo.nt~ and cube corner f~ E~ -ples of ~I.urGnective ~h~etin~ that employ beaded lens ~ " ,l~ have been ~ ;losed in U.S. Patent Nos. 2,407,680, 3,190,178, 4,025,159,4,265,938,4,664,966,4,682,852,4,767,659,4,895,428,4,896,943, 4,897,136, 4,983,436, 5,064,272 and 5,066,099. Fy~ es of rGtlu.~nective ~llee~ g that employ cube corner el~...e .I.s have been ~licc~1osed in U.S. Patent - 30 Nos. 3,684,348, 4,618,518, 4,801,193, 4,895,428, 4,938,563, 5,264,063 and WO 97/31357 ' PCT~US97/00628 S,272,562. Disclosures of the patents cited in this paragraph are incorporated in their entirety here by ~GrelG.,ce.
The substrate 4 is typically a mP.t~llic, wooden or polymeric material.
~ Preferably, the sulJ~ Le is a rigid material, with ~Illmimlm being the most co,.. noll The substrate may also be a flexible polymeric material or a co~ on in which a flexible polymeric material is mounted onto a rigid material such as ~I.,.,.;.,~.i~, or plywood. The substrate is usually prGrGllGd to be opaque. Typical ~ -ples of com~nercially available subsL.,lles include: a 2 mm (~ çrs) thick acid etched and deglcased ~ panel, a high density 2 cm (cf~ntim~otf~rs) thick plywood, or a 4 mm thick rll)e~gl~s.,-l~,hlru~ied plastic panel; all these substrates are cûll~nonly used in trafflc sign industries and are available from the Lyle Sign Co~ , Eden Prairie, MN.
In some embodim~nt~ ,I,ol ~nective signs of the present invention can be produced without a substrate. In this case, the . G~S Ol Gnective sign would be com~,ised of a textured glass sheet ~tt~h~d to the rGllu-Gnective graphic. In one embodiment, the glass sheet is att~h~d to the .GLrolenective graphic by a clear adhesive. In another embodiment, a clear pressure sensitive adhesive with releasable liner is att?~(~hed to the back of the ~ellult;nective graphic. Release liners are typically sheets of a non-stick polymer such as a fluoropolymer or a silicone-treated polyethylene, polypropylene, poly(ethylene tele~ te), etc.
For ~ ition~ lel~h and rigidity, the leLlorenective sign can subsequ~ntly be mt l-nted onto a rigid substrate. Alternatively, the .GIlorGnective graphic and LulGd glass plate (with or without an adhesive layer) can be molmted in a frame.
The lcLlùlcnec~ e sign of the present invention further c~llLa;ns an interlayer ~ over the leL~ulcnective graphics. In general, the interlayer may beany light-Lla..c...i~-:ble layer. In one prerG--cd embodiment described above, the interlayer comprises an adhesive that bonds the lcllûrcnective graphic and the glass plate. In another prc~c-.ed embodiment, the interlayer comprises an 30 air gap.

W O 97/31357 PCTrUS97100628 The interlayer may also comprise a polymeric material. One preferred polymeric material is poly(methyl methacrylate). Other suitable polymers inc1ude: ~iiph~tic polyurethane, {meth)acrylic acid and ethylene copolymers, or a flexible poly(vinyl chloride). The polymeric material may also be a S copolymer, polymer blend, or a multilayer film. The polymeric material is preferably ~ J~Glll and will ~ 1slllil more than 80% of the inci~çnt visible light; more pleîGl~bly more than 90%. For ~dt1ition~l stability, the polymeric material may contain W absolbGl~ and free radical scavengers. Common of such addit;ves include hindered amines, b~,l~ophf..ol-Pc bel~o~ c,les, o~nilides and alyll)~n~o~tec mples of COIIIIIIGI C;a1IY available hindered amines include Chim~ccorb(TM) 944, Tinuvin(TM) 144, 622, and 770 available from Ciba-Geigy Corp., Hawlllollle, New York. Common e~lnples of W absc,-l,tl~ are b.,~ li~oles, such as Tinuvin(TM) 327, 328, 1130, or P, available from Ciba-Geigy Corp., Hawthorne, New York; n~nilicle~, such as Sanduvor(TM) EPU
or VSU, available from Sandoz Ch~m;c~l~ Corp., Charlotte, North Carolina;
and arylbe~l~oal~c such as W-Chek AM-340, available from Ferro Corp., Cleveland, Ohio. The polymeric layer may also contain coloring agents or fluorescent compoul-ds for m~mlf~ctllring various colored such as yellow, orange, brown, green, blue, fluorescent orange or yellow-green ~ ul~ective ~heetings The polymeric layer is preferably about 0.05 to 2.5 mm thick.
The glass plate 6 is a silica-based glass, preferably soda-lime glass. The glass plate can not be an organic polymeric material. It has been discovered that organic polymeric materials (with or without textured .s~ es), such as poly(methyl methacrylate), do not provide the full range of de~ }le characteristics inr,~ ing anti-dew p-u~G~lies~ durability, ~w~alll~ bility (e.g., re.~i~t~nee to mlcrobes), le~ ce to organic solvents, etc. that are provided by the textured glass plates of the present invention. The glass plate is light tr~ncmi~eible, and plG~el~bly capable of ~ .g at least 80%, more plt;r~,.ably 90%, of the intensity of visible light pel~G .~liy~l~rly incident to the W0 97/31357 PCT~IJS97~00628 glass plate. The thicL-nPc~ of the glass plate is preferably 0.1 to 10 mrn; morepreferably 0.5 to 6 mm; and still more pl~re,ably I to 4 mm.
The glass plate has two major surfaces. In the le~lo.enective sign of ~ the present invention, the outer major surface of the glass plate is exposed to the air. The outer surface is â textured glass surface having rnicroscopic surface v~ on~ of at least about 3 nm (n~nolnete~)~ The textured surfaces are pre~.~bly defined as co..~ g ~"~ upOl ~ having rli~m~ters in the size range of between about 0.003 to 10 ~m (ll,icl~ll.el~.~), more pltir~ bly b~,h.~n about 0.005 to 1 ~m, still more p ,~r~l~bly b~v~ n about 0.01 to 0.5 llm, and even more p-t r~,~ly between about 0.01 and 0.05 ,um.
The surface rnicropores can be better understood with l~r~l~,nce to Figs. 5-7 which show sc~nning el~ rui- mi~"oscopc (SEM) photomicrographs of three di~lellL glass s~ ces Fig. 5 shows an ~ ed, flat glass surface which appears re~ ss under SEM analysis. Fig. 6 shows a textured glass surface with ~ic(opG~es having ~1i~...... ;le. ~ in the size range of about 10 to 60 nm (the size scale is shown in the lower right hand corner of each photomicrograph~. Fig. 7 shows a more highly textured glass surface; the surface of this glass appears frosted when viewed by the unaided eye. The texturing can be either patterned or random, but is plerel~bly random (i.e., is without a regular pattem).
In a pl~rt;.led embodim~nt the textured glass has the characteristics of scallops, islands and micropores described in U.S. Patent No. 4,944,986, illccl~ol~led herein by reference. The scallops are generally in the range of 100 to 2,000 ~m. The islands are in the range of 10 to 120 ~m. These scallops and islands tend to diffluse the ;.. co,.. ;.-~ visible light. For higher clarity, the etched glass should have fewer scallops and islands, but more of the microporous surface texture. In a p,ert;"cd embodiment, the textured glass plate is AR glass purchased from Zuel Company, St. Paul, Minnesota.
The textured glass surface may alternatively be defined by its water spreading p,upe,lies. Thus, in a p,erellt;d embodiment, the static contact angleof deionized stationary water droplets on the textured glass surface at 25~C
g remains below 40~, more I~lG~lably less than 30~, and still more plcrGlably lessthan 20~. Static contact angles can be measured on a deionized water droplet of O.O1 ml with a contact angle goniometer.
At least the outer major surface 8 of the glass plate must be textured.
S The glass plate can be textured either before or after it is affixed to the sign.
The surface of the glass can be ~C~IUl ed by physical means such as grinding or sand blasting or by chemicai means. P~,rel~bly, the glass is etched with an acid, typically hydrofluoric acid. In a particularly pl~,f~ ed embodim~nt, the glass is etched with an aqueou~ solution of hydrofluoric acid, ~.. lo~.;.. l1~ bifluoride and a water-soluble organic compound such as sorbitol. The inner major surface of the glass 7 may be either smooth or ttAIu~cd. In a prere,~d embodiment, the inner surface is also textured in order to reduce cost or ,~"I~ r.e l~al~ JalGl~y.
The .t:L....e~lective signs of the invention may also include adhesive layers. The adhesive can serve to bond any of the layers in the sign. For GA rlc, an adhesive layer may be disposed on the second major glass surface;
thus, bonding the glass plate to the leL-ulGne~iLi~e graphic. The adhesive layeror layers can be corltin~o~l~ or I~ol~Gl~ Qus The ~-ol-co.~ oll~ layer or layers provide an air gap bt~ween layers. In some emborlim~nt~ the major glass surface 7 is coated with a silane prior to col t~cting the adhesive (see U.S.
Patent No. 4,596,622, i.,co.~or~Led herein by reference). An adhesive layer can also be disposed on the outer surface of the substrate (i.e., the major substrate surface facing away from the graphic). In in~t~nces where adhesive is disposed on the outer surface of the substrate, the substrate is p.Gre.~bly a flexible polymeric sheet. In some preferred embo~imtonts~ adhesive can be disposed on the rear of the retroreflective graphic or on the outer surface of the sulJ~LIale and covered with a release liner made of a polymeric material such assilicone treated polyethylene. Types of adhesives usable in the leL.u.enective signs include, but are not limited to, hot melt and pressure sensitive adhesives.
Foam adhesives are especially advantageous in those embodiments in which an adhesive is used to bond the ~G~-o,GnecLi~/e graphic to the substrate, since foam W O 97/313~7 PCTnUS97~0062B

adhesives are likely to 'oe more durable. Adhesives ~ c1osed in U.S. Patent Nos. 4,906,S23 and 5,264,063 may be used, and are incorporated herein by reference.
lrlhion to, or in place of adhesives, the signs can use me~h~ni~
means to attach the glass plate to the signs. Examples of suitable ~l-e.h~
means include: clamps on the edges of the sign; a frame, preferably a heavy duty ~1.. ,;.. frarne; or screws through the glass plate. Additionally, gaskets or silicone sealer may be used around the edges of the sign to prevent l~ L~
or Cb~l;1l.l;l.~lll~ from e~ be~ween any ofthe sign's layers.
In its finished state, the ~cLlulGnective sign should ~ urenect light Pffici~ntly. Thus, using the ASTM E810-94 procedure described in the FY~ CS section, the ,~lrùrenective signs of the present invention ltL~ulenect p.e~e.al)ly at least 50%, more ~l~Çel~bly at least 70%, and most plt~,~ly at least 90% of in~ident light as col~l,ualed with leLlo~t;flective ~h~eti~ without a glass cover plate.

EXA~LES
The following non-limiting eY~mple~ have been selected to illustrate the invention.
In a colllp~a~ e test of dew l~ nl properties, various glass or plastic cover plates were mounted over identic~l sheets of l~;;llol~nective .l.ee~ , (ScotchlitelM Reflective Sheeting Diamond Grade - Visual Impact Pelrull~lallce, Yellow 3991, available from 3M, St. Paul, Mimleso~a) that was l~---;--~led onto an ~ .. panel via a plc~ule sensitive adhesive. The glass plates were held in place by upper and lower plywood mounts (61(L) x 3.8(H3 x 1.6(W) cm) having two parallel grooves (61 cm length x 1.3 cm depth x 0.4 cm width) sepa,~led by 0.62 cm. The surrounding edges were sealed with col~lll.able plastic tape to prevent moisture from c~mrlPn~ing on the interior surfaces. The glass-covered sheeting~ were tested for dew ,~c;~ ce loy pl~ ent side by side on an open deck on an autumn night in St. Paul, Minnesota. IllLe~ y of ,eLIc,rt;nected light having an el~Ll~u~ce angle of about CA 02244922 l998-07-30 W O97/31357 PCT~US97/00628 5~ from the main axis perpçn~icul~r to the test panel was measured using a retro-lu."il~o.lleLer (model 1980A, Spectra Pritchard) at an angle about 0.2~ off from the light source (i.e., a 0.2~ observation angle). The light source was a 500 watt floodlamp. Each test sign rc:~lulenection was measured in a 10 minute interval from 6 p.m. to 6 a.m. the following day. The ~ ul~nective data were then rec~lc~ ted to the standard unit of c~n-lPI~/lux/m2 by a calibration factor, which was obt~ine~i according to the method leco.. -~nrled in ASTM E810-94, from the fraction of ~~l~u~c:nected light measured by the photolnminomet~r without any con~lPnC~tion on test panel.
Fig. 3 shows the measurement of the intensity of light ~~l-u-~nected from ~heeting with a flat glass cover plate. The flat glass plate was 2.4 mm (95mil) thiclc, obtained from AFG Industries, Kingsport, Tennlsssee (average visible light ~.i.n~ ce of 91%). The initial intensity of letlulènected light was about 430 c~ndel~ per lux square meter (cd/lux/m2). At about 8:30 p.m., the formation of dew began to ~l;" ,;l~ cl. the intensity of light . èllvl ~nected from the ~h~etin~ and by 10:00 p.m., il.l~.nsily of r~l~o,enected light d;...;~ l.ed to about 50 cd/lux/m2. As shown in Fig. 3, intensity of retlorenected light gradually inc~.ased until it reached an i~lc;nsily of about 350 cd/lux/m2 at 3:00 a.m.
Fig. 4 shows the measurement of intensity of light lt;L-olcnected from ~h~etin~ with a textured glass cover plate. The textured glass was 2.4 mm (95 mil) thick, textured glass obt~ined from Zuel Company, St Paul, Minnesota (id~ ;r.ed as AR glass). At about 8:30 PM, the formation of dew began to ,I;..,:.~i~l, the intensity of light lel-u,t;nected from the ~heetin~ and by lû:00 p.m. the hllell"ly of rel,urenected light rliminiched to about 200 cdllux/m2. Asshown in Fig. 4, intensity of Icllu~;nected light then h~cl~,ascd, and by about 12:30 a.m."el~olt;nected light had recovered to its initial intensity.
At the same time, tests were co~ cted with cover plates of: coarsely textured (i.e., frosted) glass (obtained from Zuel Company, St. Paul, Minn~sota; identified as RR glass); textured poly(methyl methacrylate); and no cover plate. The frosted glass cover plate showed a general decrease in CA 0224492i 1998-07-30 WO 97/31357 PCT~US97/00628 ily (to about 330 cd/lux/m2) due to the ~osted character, but showed cellPnt anti-dew prope-lies that were very similar to the textured glass described above and in Fig. 4. Retroreflective sheeting without a cover plate orwith a textured poly(methyl IllGlha~ late) cover plate both showed a loss of inlen~ily to about 50 cd/lux/m2 and r~ h-ed at about 50 cdllux/m2 through 6:00 a.m.
Tests were also conrlucted of the r~llort;nectivity of ~l.Ç~ 8 under dry conflition~ with various cover plates measured at various observation angles from light having an ~..L,~ce angle of -4.0~. The measu.el..ellLs were p~"r~lll-ed accol.lin~, to ASTM E810-94. The ,el~u,-,nective ~heetin~ was Scotr~-liteTM Reflective Sheeting Diamond Grade LDP No. 3970. Glass cover plates were laid over the sheeting~ and the ~hee(;.~ with plate was held by a frarne. No adhesive was used. The results of these Illea:~UI t~ i are shown in Table 1.

_ W O 97/31357 PCT~US97/00628 Table 1: Intensity of Retrorçtlt~c~l Light (cd/luxlm2) vs. Observation Angle (degree) Observation Angle 0.20~ 0.30~ 0.50~ 1.00~ 1.50~ 2.00~
No cover 1402 967 442 29.7 7.6 3.3 Flat glass cover 1153 797 364 26.1 6.8 2.9 Textured AR glass cover 1274 886 403 28.3 7.3 3.3 Textured RR glass cover 3S3 312 228 76.8 24.4 8.5 As can be seen in Table 1, both flat glass and textured glass cover plates are acceptable in terms of the intensity of r~Llol~nected light. Flat glass cover plstes, however, do not perforrn as well as textured glass plates because l-,ho,~nectivity &~,I"G.,;ably deçlines under dew conditions (see, for ~ .ple, FIG. 3).
The frosted glass plate (i.e., textured RR glass) is acc~ al,ie for use on ,_L~olt;nective traffic signs because of its good anti-dew plopt;,Lies. It, however, is less desirable than a less coarsely textured glass because of its reduced r~;L,ulcnective intensity (see Table 1).
As ~iiccll~sed above, flat glass and textured glass surfaces were analyzed by SEM. S~mples of flat glass from AFC inthlctri~, textured AR glass from Zuel Co",pa,-y and tci~.Luled RR glass from Zuel Colll~Jally were vapor coated with a thin layer of pl~timlm (less than 3.5 nm) by conventional techn~ es The s~mr'e~ were then analyzed at 100,000X m~gnific~tion using a Hitachi Model S-4500 Field Emission Sç~nning Electron Microscope. The resulting SEM photomicrographs are shown in Figs. 5, 6 and 7 (flat glass, L~,ALu,ed AR
glass and textured RR glass, l e~,cc~ ely).
Various mo~iifiç~tiQn~ and alterations of this invention will become appa,~." to those skilled in the art without depalL;,lg from the scope and spirit 2S of this invention. For example, lt:L.or-;nective signs of the present invention can be made with or without a polymeric interlayer or can include additional WO 97/3~357 PCTllJS97J00628 layers such as adhesive layers between the graphic and substrate or graphic and glass. It should thel~îo,~ be understood that this invention is not unduly limited to the illustrative embo~iment~ set forth above, but is to be controlledby the limit~tions set forth in the claims and equivalents thereof.

Claims (12)

CLAIMS:

1. An anti-dew retroreflective sign comprising: a retroreflective graphic and a glass plate having a major surface disposed over the retroreflective graphic; wherein the major surface is exposed to the atmosphere and is facing away from the retroreflective graphic; and further wherein the major glass surface exposed to the atmosphere is a textured glass surface.

2. The anti-dew retroreflective sign of claim 1 further comprising a substrate disposed under the retroreflective graphic.

3. The anti-dew retroreflective sign of claim 2 wherein the substrate is a glass plate is 0.5 to 6 mm thick, and wherein mechanical means are used to attach the glass plate to the retroreflective graphic.

4. The anti-dew retroreflective sign of claims 1-3 wherein a moisture resistant seal comprising an elastic gasket or silicone sealant is disposed around the edges of the sign to prevent moisture from condensing between the retroreflective graphic and the glass plate.

5. The anti-dew retroreflective sign of claims 1-4 wherein an interlayer is disposed between the retroreflective graphic and the glass plate;
and wherein the interlayer comprises as least one substance selected from the group consisting of air, adhesive, and organic polymer.

6. The anti-dew retroreflective sign of claims 1-5 wherein the retroreflective graphic comprises retroreflective sheeting in the shapes of letters, numbers or symbols bonded via an adhesive to a retroreflective background sheet having a clear, colored, polymeric overlay.

7. The, anti-dew retroreflective sign of claims 1-5 wherein the retroreflective graphic comprises a retroreflective sheet having printing thereover to produce contrasting regions such that the sign displays letters, numbers or symbols.

8. The anti-dew retroreflective sign of claims 1-7 wherein the textured glass surface comprises micropores that have diameters in the range of 0.005 to 1 µm.

9. The anti-dew retroreflective sign of claims 1-8 wherein the textured glass surface comprises micropores having diameters in the range of 0.01 to 0.5 µm.

10. The anti-dew retroreflective sign of claims 1-9 wherein the textured glass surface is water-spreading such that a 0.01 ml deionized water droplet on the textured glass surface has a contact angle less than 30° at room temperature.

11. The anti-dew retroreflective sign of claims 1-10 wherein the glass plate comprises soda lime glass.

12. A method of making an anti-dew retroreflective sign comprising: disposing a glass plate over a retroreflective graphic wherein the glass plate has a first major glass surface facing the retroreflective graphic and a second major glass surface facing away from the retroreflective graphic, wherein the second major glass surface is a textured glass surface.