ANTI-DEW ADHESIVE FILM

Abstract: An anti-dew article with an adhesive layer than results in improved durability to outdoor conditions is described. The anti-dew layer has a polymeric base layer and inorganic oxide particles surface layer. The adhesive that bonds the anti-dew film to a substrate is a cross-linked acrylate pressure sensitive adhesive with a low glass transition temperature and essentially no tackifiers. The adhesive is especially valuable for adhering an anti-dew film on a retroreflective traffic sign. The resulting sign exhibits excellent durability in a variety of weather conditions including cold weather.

Description: ANTI-DEW ADHESIVE FILM FIELD OF THE INVENTION This invention pertains to articles that have an anti-dew film bonded to a substrate using an adhesive layer and to methods of making these articles. The anti- dew articles are particularly useful on retroreflective articles. INTRODUCTION Retroreflective articles have the unique property of being able to return a substantial portion of incident light back toward the light source over a wide range of incoming light angles. A primary use of retroreflective articles is on traffic signs. At night, light from motor vehicle headlights strikes the traffic sign and is retroreflectβd back 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. Retroreflective signs are commonly displayed outdoors and thus are frequently exposed to moisture droplets that form on its surface and to weathering conditions such as heat, cold, hail, etc. These environmental conditions can sometimes make it difficult for motor vehicle drivers to read the information on the signs. Investigators in the retroreflective art have recognized the existence of these problems for many years. For example, in 1946 Palmquist et al. in U.S. Patent No. 2,407,680 taught the need for weatherproof retroreflective signs and retroreflective articles with exterior water-spreading surfaces to minimize the effects of water droplets. More recently, traffic signs have been used that have an exterior anti-dew film (of the type described by Krautter et al. comprised of inorganic oxides bonded to a clear polymeric film by a soluble organic functional tie layer) bonded to the surface of a retroreflective traffic sign in which the pressure sensitive adhesive is made of a cross-linked ester acrylate/acrylic acid with tackifier. We have found that in outdoor applications, the anti-dew film peeled off the sign, described above, but left the psa on the sign surface. This caused accumulation of dirt and reduced light transmission from the retroreflective sheeting. In addition, the psa appeared yellow from UV degradation, and fuzzy, spotty ghost images appeared caused by phase separation of the tackifier from the acrylic adhesive. Thus, despite long and intensive efforts in this area, there remains a need to make retroreflective articles with high weather-durability and an anti-fogging or anti-dew exterior surface. SUMMARY OF THE INVENTION The present invention provides a durability improved article with an anti- dew film on an exterior surface. The inventive article comprises, at least, an anti- dew film, and an adhesive layer disposed on the anti-dew film. The adhesive comprises a cross-linked pressure sensitive adhesive (psa). The psa is essentially without a tackifying resin because it has been found that the presence of a tackifier in the adhesive layer causes undesirable properties. It is preferred that the adhesive layer has a glass transition temperature, T g , below about -15 C. The anti-dew film has an anisotropic structure with a polymeric base layer and a water-spreading surface film that contains inorganic oxide particles. The inorganic particles in the water-spreading layer have a lower thermal expansion coefficient than the polymeric base layer; the anti-dew film is anisotropic in that the water-spreading layer has a different coefficient of thermal expansion than the surface layer. As a result, changes in temperature cause the anti-dew film to curl. Thus the anti-dew film tends to peel off substrates such as retroreflective traffic signs. The adhesive of the present invention has sufficient strength to counteract this curling tendency, resulting in more durable dew-resistant articles. The invention further provides a method of making a durability improved article with an anti-dew film in which an ester acrylate, a copolymerizable monoethylenic monomer and a cross-linking agent are combined to form a composition that is essentially without tackifiers, and the composition is cured to form a pressure sensitive adhesive. The pressure sensitive adhesive is disposed between a substrate and an anti-dew film, wherein the anti-dew film comprises a polymeric base layer and a surface layer wherein inorganic oxide particles occupy at least a portion of the exterior surface. The present invention provides an excellent balance of properties and numerous advantages, even after outdoor exposure for several years, including: peel strength, shear strength, clarity/non-phase separation, nonyellowing, nonpeeling, impact resistance, economy and simplicity of construction and preparation, durability at extremes of heat and cold, and lasting dew resistance. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a cross-sectional view of a retroreflective sign according to a preferred embodiment of the invention. Fig. 2 is a graph of data showing the increase of shear strength with increasing concentration of cross-linker. DETAILED DESCRIPTION OF THE INVENTION The construction of a preferred embodiment of the invention is shown in cross-sectional view in Fig. 1. The illustration in Fig. 1 shows the order of layers in a preferred embodiment of the invention but is not drawn to scale and does not accurately depict the relative thicknesses of each layer. In this embodiment, retroreflective sheeting 4 is mounted on backing board 2. Typically the retroreflective sheeting is mounted on the backing board by an intervening pressure sensitive adhesive layer (not shown). Colored overlay 6 with cutout regions 7 is bonded on the retroreflective layer by another pressure sensitive adhesive (not shown). Adhesive layer 8 bonds anti-dew-film 10 to overlay 6. In an alternative arrangement (not shown), where an overlay is not present, the adhesive layer 8 bonds the anti-dew-film to the retroreflective layer. The surface of the anti-dew- film 10 consists of water-spreading layer 12 that contains hydrophilic inorganic oxide particles 14. In a preferred embodiment, the article of the present invention is a retroreflective traffic sign that contains a layer of 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 VIP 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. The surface of the retroreflective sheeting facing the anti-dew film is a polymeric material, preferably an acrylate, more preferably polymethymethacrylate. The surface may be impregnated with color pigments. A series of colored High Intensity Grade, Diamond Grade VIP, and Diamond Grade LDP Scotchlite™ retroreflective sheetings in colors such as yellow, orange, brown, blue and green are available from 3M, St. Paul, MN. The surface facing away from the anti-dew film is typically coated with an adhesive layer, preferably a pressure sensitive adhesive. The back board 2 is typically a metallic, wooden or polymeric material. Preferably, the back board is a rigid material, with aluminum being the most common. The inventive article may also have a flexible polymeric material backing material or a combination in which a flexible polymeric material is mounted onto a rigid material such as aluminum or plywood. The back board is usually preferred to be opaque. Typical examples of commercially available back boards 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 back boards are commonly used in traffic sign industries and are available from the Lyle Sign Company, Eden Prairie, MN. The colored overlay 6 is preferably a plastic sheet about 25 μm to 125 μ , more preferably about 75 μm in thickness. Cut-outs are made in the overlay layer. Typically the cut-outs are in the shape of letters, numbers or other graphic information. The colored overlay blocks out light of selected wavelengths while the cut-outs allow unimpeded transmission of light to and from the retroreflective sheeting. Colorless regions may substitute for cut-outs in the colored overlay. The overlay is preferably an acrylate, more preferably polymethylmethacrylate. The overlay is usually adhered to the retroreflective sheeting by an adhesive, preferably a pressure sensitive adhesive. The adhesives used to bond the retroreflective sheeting to the substrate or bond the colored overlay to the retroreflective sheeting may include the adhesives described herein or those otherwise known in the art. Indeed, colored overlays with adhesives of the type described herein, with release liners, are commercially available for application to retroreflective sheeting in traffic signs. The adhesive layer disposed between the retroreflective sheeting and the colored overlay is preferred to have high clarity and at least moderately good adhesion and shear strength. Types of ester acrylates used to make the adhesive layer 8 include the C - Cio alkyl esters of acrylic and methacrylic acid. The ester acrylate preferably comprises acrylate or methacrylate ester monomers including ethyl acrylate, n-butyl acrylate (BA), isobutyl acrylate, 2-methyl butyl acrylate, 2-ethylhexyl acrylate, 2- ethylhexyl methacrylate, n-octyl acrylate, n-octyl methacrylate, isooctyl acrylate (IOA), isodecyl acrylate (IDA), isooctyl methacrylate, isononyl acrylate, isodecyl methacrylate, and mixtures thereof. The ester acrylate may also include hydroxyethyl methacrylate, hexyl acrylate, hexyl methacrylate and may also include, vinyl acetate and combinations thereof with acrylates. The ester acrylate is most preferably isooctylacrylate. Preferably about 85 to about 99, more preferably about 87 to about 98, and most preferably about 90 to about 97 weight percent ester acrylate, based on the total weight of resin precursors (i.e. the weight of starting materials not including solvents) is used in forming the adhesive layer . Preferred copolymerizable monoethylenic monomers include acrylic acid (AA), methacrylic acid, itaconic acid, crotonic acid, acrylamide, acrylonitrile, methacrylonitrile, N-substituted acrylamides, hydroxy acrylates, N-vinyl pyrrolidinone, maleic anhydride fumaric acid and the like. The copolymerizable monoethylenic monomers include N,N-dimethyl acrylamide, N,N-dimethyl methacrylamide, N,N-diethyl acrylamide, N,N-diethyl methacrylamide, N-vinyl caprolactam, and the like. The most preferred monoethylenic monomers are acrylic acid and methacrylic acid. Preferably about 1 to about 15, more preferably about 2 to about 12, and most preferably about 3 to about 10 weight percent copolymerizable monoethylenic monomers based on the total weight of resin precursors are used in forming the adhesive layer. Examples of cross-linking agents that can be used in the present invention include those described in U.S. Patent Nos. 3,440,242, 4,418,120 and 3,301 ,835. Preferred cross-linking agents are di and tri functional aziridines such as trimethylolpropane-tris-(B-N-aziridinyl)propoionate. Most preferred are bis amides such as N,N-bis-propyleneisophthalimide. As shown in Fig. 2, too little cross- linking agent results in unacceptably low shear strength; on the other hand, too much cross-linking agent causes a loss of adhesion due to interfacial failure. Preferably the cross-linking agent or agents are used in about 0.01 to about 0.1, more preferably about 0.02 to 0.08, and most preferably 0.03 to 0.06 weight percent of solids. The cross-linked acrylic pressure sensitive adhesive may be formulated to cross-link with UV or electron beam radiation. Typical radiation cross-linked psas are described by Martens in U.S. Patent No. 4, 181,752. Small amounts of UV absorbers and hindered amine light stabilizers can be incorporated in the pressure sensitive adhesive to prevent UV degradation in outdoor settings. Suitable UV absorbers are benzotriazole compounds including Tinuvin™-328, Tinuvin™-P, and hindered amines include Tinuvin™-292, Tinuvin™-144, and Tinuvin™-622LD, available from Ciba-Geigy Co., Hawthorne NY. Other suitable UV absorbers include benzophenone compounds such as Uvinul™-400, Uvinul™-490 and Uvinul™-N539, available from BASF Co., Clifton NJ. Preferably, less than two weight percent of each the UV absorbers and hindered amine light stablizers are incorporated in the pressure sensitive adhesive. Kealy and Zenk in U.S. Patent No. 4,418,120 teach that crosslinked IOA/AA psas should contain an added tackifying rosin ester to achieve an excellent balance of tack, peel adhesion and shear properties in a psa that adheres well to low energy surfaces. Surprisingly, and contrary to the teaching of Kealy and Zenk, it is a discovery of the present invention that the presence of tackifiers in the adhesive layer bonding the anti-dew layer results in undesirable properties such as yellowing, peeling, phase separation between adhesive and tackifier, loss of transparency (i.e. cloudiness) with aging, and overall poor durability of the dew resistant retroreflective article. Failure of the tackified adhesive layer has been found to be most severe in regions where seasonal temperature changes vary over a broad range such as from -40°C to 35°C. This may be due to the psa changing from a rubbery phase to a glassy phase at low temperatures, causing a reduction in adhesion strength. Thus, the adhesive layer should be substantially free of tackifiers; that is, the tackifier should not be present in amounts sufficient to lower durability of the dew resistant article. Preferably, the tackifier should not be present in amounts sufficient to raise the T g of the psa above about -15 C. The adhesive layer preferably has less than 5 weight % of tackifiers. More preferably, the adhesive layer contains no tackifiers. Tackifiers are typically naturally occurring materials, which are typically complex mixtures of high-molecular-weight organic acids and related neutral materials. Tackifiers include wood resins, or modified forms of such naturally occurring resins, e.g., hydrogenated or esterified resins, polymers of terpene, pinene, etc. Tackifiers include dark amber natural gum resin (Nelio N available from Union Camp), pale thermoplastic resin derived from the polymerization of mixed olefins (Super Sta-Tac 80 available from Reichhold Chemicals, Inc.), synthetic polyterpene (Wingtack Plus available from Goodyear), medium soft thermoplastic coumarone-indeme resin R 17 supplied by Neville), te ene-urethane resin (Iso-Terp 95 available from Schenectady Chemicals Inc.), and pentaerythritol ester rosin (Foral 85 available from Hercules). The anti-dew layer comprises an organic polymeric base layer and a surface layer containing inorganic oxide particles. The inorganic oxide particles are hydrophilic and at least a portion of these particles are exposed to the atmosphere. The base layer can contain some inorganic oxide particles but it has a lower concentration than the surface layer. In one preferred embodiment, the base layer is a polymer, preferably a non-biaxially oriented flexible acrylate, that is essentially free of inorganic oxide particles, and the surface layer contains greater than 90% oxide particles, more preferably greater than 98% oxide particles. The inorganic oxide particles are preferably silica, alumia, silicoaluminate or mixtures thereof. Examples of anti-dew layers useable in the present invention include those described in U.S. Patent Nos. 4,755,425, 4,844,976, 5,073,404, 4,478,909 and 5,134,021; the water-spreading layers described in these patents may require a surface treatment to increase the concentration of oxide particles at the surface. U.S. Patent No. 4,576,864 to Krautter et al. describe a particularly preferred anti-dew layer. In a preferred embodiment, silica particles in the surface layer are treated with an aluminate solution to improve durability. The retroreflective articles are made using assembly techniques known in the art. Typically, retroreflective sheeting with pressure sensitive adhesive and release liner is laminated onto a backboard by removing the release liner and passing the laminate through a nip roller. A colored overlay having cut-out regions is then applied over the retroreflective sheeting. Typically, the colored overlay has an adhesive layer and a release liner over the adhesive layer. The colored overlay is kiss cut using electronic cutters and the undesired areas are weeded out. A removeable premask tape is often applied to the weeded colored overlay to facilitate transfer to the sign face. The release liner is stripped off and the overlay is pressed onto the retroreflective sheeting. After lamination, the premask (if present) is removed from the sign face. The reflective sign at this stage is ready for application of the dew resistant film with pressure sensitive adhesive. The adhesive used to apply the anti-dew film can be prepared by mixing appropriate amounts of an ester acrylate(s) and copolymerizable ethylenic monomer(s) in an organic solvent or mixture of organic solvents. A cross-linker is added with mixing. The resulting solution is then coated, preferably by roll coating onto a silicone-coated paper release liner. Solvent is removed by evaporation and the adhesive is cured. In a preferred embodiment, an IOA/AA adhesive with a bis amide cross-linker is cured by heating at about 150 C for about 10 min. An anti- dew film is applied onto the adhesive layer, for example by passing through a nip roller. The thus-produced anti-dew film with adhesive and release liner can be stored for future use. The release liner can be peeled off and the exposed adhesive pressed onto the traffic sign. The psa for the anti-dew film preferably has strong adhesion over temperatures ranging from -40 C to 35 C. The retroreflective sign preferably exhibits no visual yellowing after 10 years of outdoor exposure. The adhesive should retain good clarity. The acrylic copolymer in the psa is cross-linked to provide good peel strength and shear strength over service temperatures of -40 C to 35 C. The psa should have high transparency, preferably the psa layer is greater than 85% transparent, more preferably greater than 90% transparent to visible light. The psa preferably has a T g , as measured by differential scanning calorimetry - ASTM E1356, of less than -15 C, more preferably less than -20 C. Preferably, the psa has a peel strength such that an article with a 3 mil thick adhesive layer has a 90 peel strength of greater than 3, and more preferably greater than 4 lbs/in at 0 C. Preferably the dew resistant retroreflective signs have an impact resistance such that a 1.39 kg m impact to the back of the sign at -10 C causes no delamination of the dew resistant film, more preferably a 1.39 kg m impact to the back of the sign at -22 C causes no delamination of the dew resistant film. Preferably, the retroreflective signs have good weatherability such that the sign retains more than 90% retroreflectivity and has a yellowness index of less than 5 after exposure to 3000 hours of repeated cycles of 4 hours UV light at 60°C and 4 hours condensing humidity at 50°C according to ASTM G53. EXAMPLES A film curl test was conducted to determine the curling tendency of the anti- dew film caused by variations in temperature. An anti-dew film (Folie farblos 99840, from Rohm GmbH, Chemische Fabrik, Germany - the film is an acrylic film about 50 μm thick with an inorganic oxide coating on the top surface that is about 0.15 μm thick) was cut in circles with a diameter of 7.62 cm and placed on a flat plastic surface and held at a specified temperature for 30 minutes. As a result of the thermal treatment, the sample curled with the inorganic oxide surface on the inner diameter, and the chord or, if rolled into a tube, diameter of the sample was measured. Surprisingly, the thermally-aged specimens remained curled after returning to room temperature. The results are shown in Table 1. In comparison, the acrylic base film without silica coating (Rohm No. 99845) remained flat at all temperatures. The colored overlay film without psa (Scotchlite™ Electronic Cuttable Overlay film 1170 Series, available from 3M, St. Paul MN) also remained flat at all temperatures. The curling behavior shown in Table 1 is caused by the difference in thermal expansion between the acrylic base layer and the silica surface layer, and by internal stresses created during drying of the silica coating. The degree of curl can be used to calculate the curling force; see S. Wu, Polymer Interface and Adhesion, Marcel Dekker, Inc., NY, pp. 465-473 (1982). As the coating shrinks, the substrate is bent with the coating on the concave side. In preferred embodiments, the adhesive is designed with sufficient strength to counteract the curling force that may be caused by heating and cooling the retroreflective article. To prepare a cross-linked, pressure-sensitive adhesive, a mixture containing 384.6 parts of adhesive solution (copolymer of 93:7 IOA/AA, 26% solids in a solvent mixture of about 46% ethyl acetate, 26% heptane and 2% cyclohexane) and 0.043 parts. N, N-bispropylene isophthalimide was knife coated onto a silicone- coated paper liner and heated in a forced-air oven at 170 F for 1.7 min, 270 F for 1.7 min, and 350 F for 1.7 min, to remove the solvents and produce a 2 mil thick (0.5 mm), dry, cross-linked adhesive layer. The anti-dew film (Rohm GmbH No. 99841) was then laminated coating side out to the cross-linked adhesive at 60-70 psig nip pressure using a set of 36" wide nip rollers with soft rubber covering. This produced a pressure-sensitive clear overlay film with dew resistant coating on the exposed outer surface. Samples prepared according to this procedure are referred to as Example 1. A comparative example was prepared using the psa taught by Kealy et al., U.S. Pat. No. 4,418,120. To a solution of 100 parts by weight IOA:AA (94:6) was added 30 parts rosin ester tackifying resin (Foral 85 - a glycerol ester of stabilized rosin having a softening point of 82°C, available from Hercules, Inc.) to form about a 45% solids mixture in a blend of acetone, toluene and heptane. Before coating, 0.075 parts N, N-bispropylene isophthalimide cross-linker per hundred parts of dry weight of the adhesive was added to the mixture and mixed well for 15-30 min with a propeller mixer. The resulting mixture was bar coated onto a release liner and dried and cured as in Example 1. The coated acrylic pressure-sensitive adhesives containing varying levels of bisamide cross linker from 0.025 to 0.060 parts per hundred parts acrylic polymer were tested for shear adhesion (holding power) by the following modified version of PSTC-7 Shear Test (Pressure-Sensitive Tape Council Method): The adhesive was laminated to acrylic overlay film to make a film tape. The tape was cut into 1.27-cm wide x 8 cm long strips and applied to 3M Scotchlite™ Reflective Sheeting (Diamond Grade™ VIP 3990) which was mounted on a 70 mm x 70 mm x 1.0 mm thick aluminum panel. The length of adhesive overlap on the reflective sheeting was 2.54 cm giving an overlap area of 3.22 sq. cm. The unapplied tail of the film tape was reinforced with Scotch™ filament tape to prevent tearing under load. After conditioning the applied samples for 24 hr dwell time at constant temperature and humidity, a 1 kg load was suspended from the free end of tape, with the test panel in a vertical orientation. The time for the weight to fall (due to adhesive creep at room temperature) was recorded as the shear time to failure. As Figure 2 indicates, the shear time to failure increases as the cross linker loading is increased. Shear times (at room temperatures) of approximately 5,000 - 15,000 min are desired for good creep and shrinkage resistance of the applied pressure-sensitive overlay film. This range of shear adhesion is obtained with preferred cross-linker levels of about 0.03 to 0.06 parts bisamide per 100 parts acrylic copolymer. Test specimens for measuring reverse impact at low temperature were prepared by laminating an anti-dew film with psa on a retroreflective VIP (3990) sheeting (available from 3M, St. Paul, MN) on an aluminum panel of 6in x 6in (15cm x 15cm), having thickness of 0.080in (0.203 cm). All specimens were conditioned at room temperature for at least 24 hours to allow adhesion to fully develop. Then, the specimens were cooled in a cooler to -8° F (-22°C), 20 F (-7 C) and 40 F (4 C) for overnight prior to the test. In the test, specimens were placed on a steel metal ring having diameter of 3 " with the anti-dew film and retroreflective sheeting face down. A rounded steel bar weighing 1.98 kilograms was raised to 46.5 or 70 cm and let fall freely to punch the backside of the specimen to create an impact force of 0.92 or 1.39 kilograms-meters, respectively. Results are shown in Table 3. This test method is similar to ASTM 0-2794 with only a slight modification of the indenter diameter. The inventive films were subjected to a 90° peel test. In this test, samples were prepared according to Example 1 but with varying film thicknesses. The anti- dew films, 2.54 cm wide by 20 cm long, were laminated onto HIS 3870 sheeting (available from 3M Corp., St. Paul, MN) by passing through a nip roller then, the HIS 3870 sheeting was laminated onto a degreased aluminum plate. To ensure that failure occurred at the psa rather than the anti-dew film, the anti-dew film was reinforced with Scotch™ filament packaging tape prior to the peel test. The prepared specimens were let sit for at least one day, then cooled to the test temperature for at least two hours prior to testing. 90° peel strength was measured according to ASTM D3330-78. The cool chamber was attached to the Instron peel tester and Instron cross-head speed was controlled at 10 cm/min. The results are shown in Table 2. Pressure sensitive adhesive clarity and outdoor weathering resistance were measured from the retroreflection coefficient and yellowness index before and after an accelerated UV light exposure test in a weatherometer. Scotchlite™ High Intensity Grade No. 3870 retroreflective sheeting was laminated on an aluminum test panel, 7 cm wide, 28 cm long and 1 mm thick. The coefficient of retroreflection of the sheeting is 336 cdl/lux/m 2 , measured at a light entrance angle of 4° and an observation angle of 0.2°, in accordance with ASTM E810. Acrylic film without the inorganic oxide layer (Rohm No. 99845) was laminated with the above-described cross-linked psa at 60 psi nip pressure. The reflective panel yellowness index was measured with a Hunter LabScan Spectrocolorimeter, available from Hunterlab, Inc., Reston VA, in accordance with ASTM D1925. Weathering durability of the specimens was tested in a weatherometer in accordance with ASTM G53. The light source in the weatherometer was a fluorescent UV lamp with an intensity maximum at 313 cm '1 . Test specimens were remeasured after 3000 hours of repeated cycles of 4 hours UV light at 60°C and 4 hours condensing humidity at 50°C. The results are shown in Table 4. The film specimens using the inventive psa (Ex. 1) and the comparative psa both retained better than 90% of retroreflectivity. The retroreflective sheeting without a psa or film coating exhibited a yellowness index increase of 4.9 units, the sheeting with the psa of Ex. 1 increased about 5.5 units and the specimen with the comparative psa increased about 10 units. Visually, the specimen with the comparative psa showed yellowing while the specimens with the inventive psa showed no observable yellowness. Thus the yellowness of the comparative example is primarily caused by the UV degradation of the tackifying resin. All patents mentioned herein are incorporated by reference as if reproduced in full below. While this invention has been described in connection with specific embodiments, it is not limited to the specific embodiments and includes equivalents to what is described here. TABLE 1 FILM CURL TEST TABLE 2 90 PEEL TEST TABLE 3 COLD TEMPERATURE IMPACT TEST TABLE 4 ACCELERATED WEATHERING

Claims: CLAIMS: 1. A durability improved article with anti-dew film on an exterior surface, comprising: an anisotropic anti-dew film comprising a polymeric base layer and a surface layer wherein inorganic oxide particles occupy at least a portion of the exterior surface; and an adhesive layer disposed on the surface of the polymeric base layer opposite to the inorganic-oxide-particle-containing exterior surface; wherein the adhesive layer comprises a cross-linked acrylate pressure sensitive adhesive, and wherein the adhesive layer is essentially without a tackifier. 2. The article of claim 1 wherein the adhesive layer has a T g between about -15°C and -50°C as measured by diffential scanning calorimetry according to ASTM E1356. 3. The article of claims 1-2 wherein the anti-dew film comprises a non- biaxially oriented acrylic film and a layer of inorganic oxide particles deposited on a surface of the film. 4. The article of claims 1-3 wherein the article is a retroreflective sign including a backing board, retroreflective sheeting and a colored overlay. 5. The article of claims 1-4 wherein the adhesive layer contains less than 5 weight percent tackifier. 6. The article of claims 1-5 wherein the anti-dew film, prior to joining with an adhesive layer, has latent curling tendency such that the film will exhibit a degree of curvature of at least 0.5 after heating to about 54°C for one hour. 7. The article of claims 1-6 wherein reactive materials used to make the adhesive layer consist essentially of about 90 to about 98 weight percent isooctylacrylate, about 2 to about 10 weight percent acrylic acid and about 0.01 to about 0.05 weight percent of a bis amide cross-linker. 8. The article of claims 1-7 wherein the surface layer of the anti-dew layer is more than 90 weight percent inorganic oxide particles, and wherein the base layer is essentially without oxide particles. 9. The article of claims 1-8 having a peel strength of at least 4 lb/in at 0°C and at least 2 lb/in at -10°C and having a shear strength of at least 5000 minutes as measured by the modified version of PSTC-7 Shear Test described herein. 10. The article of claims 1-9 having a yellowness index of less than 5 after exposure to 3000 hours of repeated cycles of 4 hours UV light at 60°C and 4 hours condensing humidity at 50°C according to ASTM G53 and having reverse impact resistance such that the anti-dew films exhibits no delamination after an impact force of 1.3 (kg)(m) at -8°F. 11. A method of making a dew resistant article comprising the steps of: combining an ester acrylate, a copolymerizable monoethylenic monomer and a cross-linking agent to form a composition; and wherein the composition is essentially without tackifiers; curing the composition to form a pressure sensitive adhesive; disposing the pressure sensitive adhesive on an anti-dew film, wherein the anti-dew film comprises a polymeric base layer and a surface layer wherein inorganic oxide particles occupy at least a portion of the exterior surface. 12. The method of claim 11 wherein the step of combining consists essentially of combining 92 to 98 weight percent of isooctylacrylate, 2 to 8 weight percent acrylic or methacrylic acid and a cross-linker selected from the group consisting of bis amides and aziridines. 13. The method of claims 11-12 wherein the anti-dew film is subsequently applied on a retroreflective traffic sign to form a dew resistant traffic sign.

Inventors: BERGESON DAVID, HUANG TZU-LI, BERGESON, DAVID, HUANG, TZU-LI
Applicants: MINNESOTA MINING & MFG, MINNESOTA MINING AND MANUFACTURING COMPANY
Application year: 1997
Main classification: C09J7
IPC codes: C09J7