DEW-RESISTANT RETROREFLECTIVE TRAFFIC SIGN HAVING A TEXTURED GLASS SURFACE

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: DEW-RESISTANT RETROREFLECTIVE TRAFFIC SIGN HAVING A TEXTURED GLASS SURFACE INTRODUCTION 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 to illuminate all traffic signs with external lighting, a common approach to enhancing the visibility of roadway traffic signs is to use retroreflective graphics on the traffic sign. Retroreflective signs have the unique ability to return a substantial portion of incident light back toward the light source. At nighttime, light from motor vehicle headlights strikes the retroreflective graphics and is retroreflected to the motor vehicle driver. The bright image displayed by the retroreflective sign makes the sign easier to read and gives motorists more time to react. A significant problem that has been encountered with retroreflective traffic signs is the accumulation of water droplets on the surface of the signs. Dew is an especially common source of water droplets and can be particularly problematic because it occurs predominantly at nighttime when the retroreflective signs are operative. When present on a traffic sign in the form of small beaded water droplets, dew can seriously disrupt the path of incident and retroreflected light. This can make information on traffic signs much more difficult for passing motorists to read. This problem is very well known, and there have been a variety of attempts aimed at removing dew or preventing dew from forming on the surface of traffic signs. One method of eliminating dew from retroreflective signs is to heat the retroreflective element. German Patent No. 4226266 to Gubela discloses an electrical heating element placed behind the retroreflector. European Patent Application No. 0155572, assigned to Biersdorf Aktiengesellschaft discloses a retroreflective traffic sign having a heat radiator of anodized aluminum 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. Japanese Kokai Patent Publication No. 7-3731 assigned to Technonijuichi K K. discloses an anti-dew reflective road traffic sign that has a heat storage container tightly bonded to its back side. The heat storage container contains a heat storage agent, typically a gel containing 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 discloses a retroreflective traffic sign having a thermal reservoir layer located behind the retroreflective surface. The thermal reservoir contains a material that undergoes a phase change between - 20°C and 40°C. The energy barrier of the phase transition prevents the sign from cooling rapidly. Japanese Patent Application 0614961 A assigned to Kawai Musical Instrument Mfg. Co., discloses an anti-fog mirror in which a vibration generator is attached to the rear surface of a mirror. Japanese Patent Application 07259024A assigned to Matsushita Denki Sangyo KK, discloses an anti-fog mirror for road safety in which solar energy is stored in a heater unit located behind the mirror's surface. Another method that has been used to impart anti-dew characteristics to retroreflective traffic signs is to apply a water-spreading, hydrophilic 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 thin water layer does not alter the path of incident and retroreflective light to as great an extent. U.S. Patent Nos. 5,073,404, 4,844,976 and 4,755,425 to T. Huang disclose a retroreflective sheeting that has a transparent coating comprising colloidal silica and a polymer selected from aliphatic polyurethanes, polyvinylchloride co-polymers and acrylic polymers. The colloidal silica is disposed in the polymer at about 10-80 weight % (10-70 weight % in the case of polyacrylates). The transparent coatings provide superior dew repellency, allowing the retroreflective sheeting to retain a higher percentage of its original brightness after being exposed to moisture. There are numerous examples of anti-dew, water-spreading layers that are made with inorganic colloidal particles disposed in a polymeric binder U.S. Patent No. 4,576,864 to Krautter et al. discloses a water-spreading layer that is composed of colloidal particles of a metal or silicon oxide in which the water-spreading layer is adhered to a plastic substrate by an adhesive comprising a non-water-soluble, organic-solvent-soluble, and essentially non- swellable, polar-group-containing-polymer. U.S. Patent No. 4,478,909 to Taniguchi et al. discloses an anti-fogging coating having finely divided silica particles disposed in a matrix of polyvinyl alcohol and an organosilicon alkoxy compound or hydrolysates thereof. 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 retroreflective sheeting's surface to improve abrasion resistance. U.S. Patent Nos. 4,232,979, 4,340,319 and 4,596,622 to Heenan et al., disclose road pavement markers that have a glass sheet fixed to a retroreflective sheeting's front face. The glass is preferred to be an untempered and unannealed sheet about 2-15 mils thick. There is a particular need for improving the abrasion resistance of pavement markers since they must be able to resist tire impacts in the presence of abrasive materials such as grit and sand as well as roadway chemicals and temperature and weather extremes. Unlike pavement markers, however, traffic signs are not disposed on road surfaces, and, as a consequence, do not require the extreme abrasion resistance that is required of pavement markers. For traffic signs, loss of intensity caused by precipitation, especially dew, is of great concern. SUMMARY OF THE INVΈNΉON The present invention provides an anti-dew retroreflective sign comprising a glass plate having a textured outer surface disposed over a retroreflective graphic. The textured glass surface is exposed to air, and, in dew conditions, spreads precipitation in a thin film over the glass plate's surface. The present invention also provides a method for making an anti-dew retroreflective sign in which a glass plate having a textured outer surface is placed over the retroreflective graphic. The present invention provides an economical and elegantly simple method of making an anti-dew retroreflective traffic sign. The anti-dew characteristics of the inventive retroreflective traffic sign are achieved without the need for electrical inputs, heating elements, heat storage layers, infrared beam radiators, phase transition materials or vibration generators. Other advantages provided by the textured glass surface include: resistance to organic solvents, thus facilitating removal of graffiti from the sign; weathering resistance; and protection from ultraviolet (UV) light, thus extending the lifetime of underlying polymers and inks. These and other features of the present invention are more fully shown and described in the drawings and detailed description of this invention, where like reference numerals are used to represent similar parts. It is to be understood, however, that the description and drawings are for the purposes of illustration and should not be read in a manner that would unduly limit the scope of this invention. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a front view of a retroreflective sign of the present invention in which the retroreflective graphic is "St. Paul 10 km". FIG. 2 is a cross-sectional view taken along line 2-2 of FIG. 1. FIG. 3 is a plot of light intensity versus time of day from retroreflective sheeting having a flat glass surface. FIG. 4 is a plot of light intensity measured versus time of day from retroreflective sheeting having a textured glass surface. FIG. 5 is an SEM photomicrograph of the surface of a flat glass plate. FIG. 6 is an SEM photomicrograph of the surface of a textured glass plate. FIG. 7 is an SEM photomicrograph of the surface of a highly textured glass plate. DETAILED DESCRIPTION OF THE INVENΗON In FIG. 1, a retroreflective sign 2 of the present invention is shown that contains information in the form of a retroreflective graphic 3. In this case, the retroreflective graphic is in the form of lettering that spells "St. Paul 10 km". In FIG. 2, the retroreflective graphic 3 is disposed on substrate 4. An interlayer 5 overlies the retroreflective 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 atmosphere. Retroreflective graphics of the present invention are defined as retroreflective sheeting or retroreflective elements arranged in the form of characters, numbers or symbols. Retroreflective graphics do not include a uniform retroreflective sheeting or layer over the entire surface. Thus, graphics are not merely plain reflectors such as a pavement marker; however, the retroreflective graphics can be disposed over a uniform retroreflective background. The retroreflective graphics can also be an inverse design such as a retroreflective background for non-retroreflective characters, numbers or symbols such as a non-reflective profile of a cow or deer. In the latter case, the retroreflective graphic would include both the retroreflective background and the non-reflective profile. The retroreflective graphic 3 is typically bonded to the substrate 4 or to a background material by an adhesive or by mechanical means such as anodized aluminum rivets. Adhesives are preferred, and pressure sensitive adhesives are especially preferred. The retroreflective characters, numbers or symbols can be bonded to a retroreflective background. For example, the retroreflective characters, numbers or symbols can be cut from white retroreflective sheeting and bonded to a background of retroreflective sheeting that has been overlaid with a clear, colored polymer film such as an acrylic film. Common background colors are green, brown or blue. Another way of making the retroreflective graphic is to cut out letters, numerals or symbols from a transparent colored polymer film, and laminate the colored letters, numerals or symbols onto white retroreflective sheeting. A suitable, commercially available clear colored acrylic film is Scotchlite™ Electronic Cuttable Film Series 1170, available from 3M, St. Paul, MN. In still another alternative, the retroreflective graphic can be produced by printing over portions of a retroreflective sheet. For example, a stop sign graphic can be made by screen printing a red clear ink with a negative legend on a white retroreflective sheeting. The retroreflective graphics typically contain retroreflective sheeting. Examples of commercially available retroreflective sheeting that may be used to make the graphic include Scotchlite™ Reflective Sheeting High Intensity Grade Series 3870, Scotchlite™ Reflective Sheeting Diamond Grade VLP Series 3990, and Scotchlite™ Reflective Sheeting Diamond Grade LDP Series 3970, available from 3M, St. Paul, Minnesota. Retroreflective sheeting typically comprises a reflective surface and optical elements. The reflective surface serves to reflect incident light, and the optical elements serve to redirect the incident light toward the light source. The reflective material may comprise a specular metal reflector such as aluminum or silver (see, for example, U.S. Patent No. 5,283,101) or a diffuse reflector such as a heavy metal pigment or a polymeric material wherein reflectance is caused by a difference in refractive indices at an interface (frequently a plastic-air interface). Optical elements typically come in one of two forms: beaded lens elements and cube corner elements. Examples of retroreflective sheeting that employ beaded lens elements have been disclosed 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. Examples of retroreflective sheeting that employ cube corner elements have been disclosed in U.S. Patent Nos. 3,684,348, 4,618,518, 4,801,193, 4,895,428, 4,938,563, 5,264,063 and 5,272,562. Disclosures of the patents cited in this paragraph are incoφorated in their entirety here by reference. The substrate 4 is typically a metallic, wooden or polymeric material. Preferably, the substrate is a rigid material, with aluminum being the most common. The substrate may also be a flexible polymeric material or a combination in which a flexible polymeric material is mounted onto a rigid material such as aluminum or plywood. The substrate is usually preferred to be opaque. Typical examples of commercially available substrates include: a 2 mm (millimeters) thick acid etched and degreased aluminum panel, a high density 2 cm (centimeters) thick plywood, or a 4 mm thick fiberglass- reinforced plastic panel; all these substrates are commonly used in traffic sign industries and are available from the Lyle Sign Company, Eden Prairie, MN. In some embodiments, retroreflective signs of the present invention can be produced without a substrate. In this case, the retroreflective sign would be comprised of a textured glass sheet attached to the retroreflective graphic. In one embodiment, the glass sheet is attached to the retroreflective graphic by a clear adhesive. In another embodiment, a clear pressure sensitive adhesive with releasable liner is attached to the back of the retroreflective graphic. Release liners are typically sheets of a non-stick polymer such as a fluoropolymer or a silicone-treated polyethylene, polypropylene, poly(ethylene terephthalate), etc. For additional strength and rigidity, the retroreflective sign can subsequently be mounted onto a rigid substrate. Alternatively, the retroreflective graphic and textured glass plate (with or without an adhesive layer) can be mounted in a frame. The retroreflective sign of the present invention further contains an interlayer 5 over the retroreflective graphics. In general, the interlayer may be any light-transmissible layer. In one preferred embodiment described above, the interlayer comprises an adhesive that bonds the retroreflective graphic and the glass plate. In another prefe ed embodiment, the interlayer comprises an air gap. The interlayer may also comprise a polymeric material. One preferred polymeric material is poly(methyl methacrylate). Other suitable polymers include: aliphatic polyurethane, (meth)acrylic acid and ethylene copolymers, or a flexible poly( vinyl chloride). The polymeric material may also be a copolymer, polymer blend, or a multilayer film. The polymeric material is preferably transparent and will transmit more than 80% of the incident visible light; more preferably more than 90%. For additional stability, the polymeric material may contain UV absorbers and free radical scavengers. Common examples of such additives include hindered amines, benzophenones, benzotriazoles, oxanilides and arylbenzoates. Examples of commercially available hindered amines include Chimassorb(TM) 944, Tinuvin(TM) 144, 622, and 770 available from Ciba- Geigy Corp., Hawthorne, New York. Common examples of UV absorbers are benzotriazoles, such as Tinuvin(TM) 327, 328, 1 130, or P, available from Ciba- Geigy Coφ., Hawthorne, New York; oxanilides, such as Sanduvor(TM) EPU or VSU, available from Sandoz Chemicals Coφ., Charlotte, North Carolina; and arylbenzoates, such as UV-Chek AM-340, available from Ferro Coφ., Cleveland, Ohio. The polymeric layer may also contain coloring agents or fluorescent compounds for manufacturing various colored such as yellow, orange, brown, green, blue, fluorescent orange or yellow-green retroreflective sheetings. 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 surfaces), such as poly(methyl methacrylate), do not provide the full range of desirable characteristics including anti-dew properties, durability, weatherability (e.g., resistance to microbes), resistance to organic solvents, etc. that are provided by the textured glass plates of the present invention. The glass plate is light transmissible, and preferably capable of transmitting at least 80%, more preferably 90%, of the intensity of visible light peφendicularly incident to the glass plate. The thickness of the glass plate is preferably 0.1 to 10 mm; more preferably 0.5 to 6 mm; and still more preferably 1 to 4 mm. The glass plate has two major surfaces. In the retroreflective sign of the present invention, the outer major surface of the glass plate is exposed to the air. The outer surface is a textured glass surface having microscopic surface variations of at least about 3 nm (nanometer). The textured surfaces are preferably defined as containing micropores having diameters in the size range of between about 0.003 to 10 μm (micrometers), more preferably between about 0.005 to 1 μm, still more preferably between about 0.01 to 0.5 μm, and even more preferably between about 0.01 and 0.05 μm. The surface micropores can be better understood with reference to Figs. 5-7 which show scanning electron microscope (SEM) photomicrographs of three different glass surfaces. Fig. 5 shows an untextured, flat glass surface which appears featureless under SEM analysis. Fig. 6 shows a textured glass surface with micropores having diameters 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 preferably random (i.e., is without a regular pattern). In a preferred embodiment, the textured glass has the characteristics of scallops, islands and micropores described in U.S. Patent No. 4,944,986, incoφorated herein by reference. The scallops are generally in the range of 100 to 2,000 μm. The islands are in the range of 10 to 120 μm. These scallops and islands tend to diffuse the incoming visible light. For higher clarity, the etched glass should have fewer scallops and islands, but more of the microporous surface texture. In a preferred 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 properties. Thus, in a preferred embodiment, the static contact angle of deionized stationary water droplets on the textured glass surface at 25°C remains below 40°, more preferably less than 30°, and still more preferably less than 20°. Static contact angles can be measured on a deionized water droplet of 0.01 ml with a contact angle goniometer. At least the outer major surface 8 of the glass plate must be textured. The glass plate can be textured either before or after it is affixed to the sign. The surface of the glass can be textured by physical means such as grinding or sand blasting or by chemical means. Preferably, the glass is etched with an acid, typically hydrofluoric acid. In a particularly preferred embodiment, the glass is etched with an aqueous solution of hydrofluoric acid, ammonium bifluoride and a water-soluble organic compound such as sorbitol. The inner major surface of the glass 7 may be either smooth or textured. In a preferred embodiment, the inner surface is also textured in order to reduce cost or enhance transparency. The retroreflective signs of the invention may also include adhesive layers. The adhesive can serve to bond any of the layers in the sign. For example, an adhesive layer may be disposed on the second major glass surface; thus, bonding the glass plate to the retroreflective graphic. The adhesive layer or layers can be continuous or noncontinuous. The noncontinuous layer or layers provide an air gap between layers. In some embodiments, the major glass surface 7 is coated with a silane prior to contacting the adhesive (see U.S. Patent No. 4,596,622, incoφorated 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 instances where adhesive is disposed on the outer surface of the substrate, the substrate is preferably a flexible polymeric sheet. In some preferred embodiments, adhesive can be disposed on the rear of the retroreflective graphic or on the outer surface of the substrate and covered with a release liner made of a polymeric material such as silicone treated polyethylene. Types of adhesives usable in the retroreflective 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 retroreflective graphic to the substrate, since foam adhesives are likely to be more durable. Adhesives disclosed in U.S. Patent Nos. 4,906,523 and 5,264,063 may be used, and are incoφorated herein by reference. In addition to, or in place of adhesives, the signs can use mechanical means to attach the glass plate to the signs. Examples of suitable mechanical means include: clamps on the edges of the sign; a frame, preferably a heavy duty aluminum frame; or screws through the glass plate. Additionally, gaskets or silicone sealer may be used around the edges of the sign to prevent moisture or contaminants from entering between any of the sign's layers. In its finished state, the retroreflective sign should retroreflect light efficiently. Thus, using the ASTM E810-94 procedure described in the Examples section, the retroreflective signs of the present invention retroreflect preferably at least 50%, more preferably at least 70%, and most preferably at least 90% of incident light as compared with retroreflective sheeting without a glass cover plate. EXAMPLES The following non-limiting examples have been selected to illustrate the invention. In a comparative test of dew resistant properties, various glass or plastic cover plates were mounted over identical sheets of retroreflective sheeting (Scotchlite™ Reflective Sheeting Diamond Grade - Visual Impact Performance, Yellow 3991, available from 3M, St. Paul, Minnesota) that was laminated onto an aluminum panel via a pressure sensitive adhesive. The glass plates were held in place by upper and lower plywood mounts (61(L) x 3.8(H) x 1.6(W) cm) having two parallel grooves (61 cm length x 1.3 cm depth x 0.4 cm width) separated by 0.62 cm. The surrounding edges were sealed with conformable plastic tape to prevent moisture from condensing on the interior surfaces. The glass-covered sheetings were tested for dew resistance by placement side by side on an open deck on an autumn night in St. Paul, Minnesota. Intensity of retroreflected light having an entrance angle of about 5° from the main axis peφendicular to the test panel was measured using a retro-luminometer (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 retroreflection was measured in a 10 minute interval from 6 p.m. to 6 a.m. the following day. The retroreflective data were then recalculated to the standard unit of candelas/lux/m 2 by a calibration factor, which was obtained, according to the method recommended in ASTM E810-94, from the fraction of retroreflected light measured by the photoluminometer without any condensation on test panel. Fig. 3 shows the measurement of the intensity of light retroreflected from sheeting with a flat glass cover plate. The flat glass plate was 2.4 mm (95 mil) thick, obtained from AFG Industries, Kingsport, Tennessee (average visible light transmittance of 91%). The initial intensity of retroreflected light was about 430 candelas per lux square meter (cd/lux m 2 ). At about 8:30 p.m., the formation of dew began to diminish the intensity of light retroreflected from the sheeting and by 10:00 p.m., intensity of retroreflected light diminished to about 50 cd/lux/m 2 . As shown in Fig. 3, intensity of retroreflected light gradually increased until it reached an intensity of about 350 cd/lux/m 2 at 3:00 a.m. Fig. 4 shows the measurement of intensity of light retroreflected from sheeting with a textured glass cover plate. The textured glass was 2.4 mm (95 mil) thick, textured glass obtained from Zuel Company, St. Paul, Minnesota (identified as AR glass). At about 8:30 PM, the formation of dew began to diminish the intensity of light retroreflected from the sheeting and by 10:00 p.m. the intensity of retroreflected light diminished to about 200 cd/lux/m 2 . As shown in Fig. 4, intensity of retroreflected light then increased, and by about 12:30 a.m., retroreflected light had recovered to its initial intensity. At the same time, tests were conducted with cover plates of: coarsely textured (i.e., frosted) glass (obtained from Zuel Company, St. Paul, Minnesota; identified as RR glass); textured poly(methyl methacrylate); and no cover plate. The frosted glass cover plate showed a general decrease in intensity (to about 330 cd/lux/m 2 ) due to the frosted character, but showed excellent anti-dew properties that were very similar to the textured glass described above and in Fig. 4. Retroreflective sheeting without a cover plate or with a textured poly(methyl methacrylate) cover plate both showed a loss of intensity to about 50 cd/lux/m 2 and remained at about 50 cd/lux/m 2 through 6:00 a m. Tests were also conducted of the retroreflectivity of sheeting under dry conditions with various cover plates measured at various observation angles from light having an entrance angle of -4.0°. The measurements were performed according to ASTM E810-94. The retroreflective sheeting was Scotchlite™ Reflective Sheeting Diamond Grade LDP No. 3970. Glass cover plates were laid over the sheeting, and the sheeting with plate was held by a frame. No adhesive was used. The results of these measurements are shown in Table 1. Table 1: Intensity of Retroreflected Light (cdΛux m 2 ) 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 353 312 228 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 retroreflected light. Flat glass cover plates, however, do not perform as well as textured glass plates because retroreflectivity appreciably declines under dew conditions (see, for example, FIG. 3). The frosted glass plate (i.e., textured RR glass) is acceptable for use on retroreflective traffic signs because of its good anti-dew properties. It, however, is less desirable than a less coarsely textured glass because of its reduced retroreflective intensity (see Table 1). As discussed above, flat glass and textured glass surfaces were analyzed by SEM. Samples of flat glass from AFC industries; textured AR glass from Zuel Company and textured RR glass from Zuel Company were vapor coated with a thin layer of platinum (less than 3.5 nm) by conventional techniques. The samples were then analyzed at 100,000X magnification using a Hitachi Model S-4500 Field Emission Scanning Electron Microscope. The resulting SEM photomicrographs are shown in Figs. 5, 6 and 7 (flat glass, textured AR glass and textured RR glass, respectively). Various modifications and alterations of this invention will become apparent to those skilled in the art without departing from the scope and spirit of this invention. For example, retroreflective signs of the present invention can be made with or without a polymeric interlayer or can include additional layers such as adhesive layers between the graphic and substrate or graphic and glass. It should therefore be understood that this invention is not unduly limited to the illustrative embodiments set forth above, but is to be controlled by the limitations set forth in the claims and equivalents thereof.

Claims: 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 at 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.

Inventors: BENSON GERALD M, HUANG TZU-LI J, BENSON, GERALD, M., HUANG, TZU-LI, J.
Applicants: MINNESOTA MINING & MFG, MINNESOTA MINING AND MANUFACTURING COMPANY
Application year: 1997
Main classification: G09F13
IPC codes: B32B3, B32B17, E01F9, G09F13