CA2258288A1 - Anti-dew adhesive film - Google Patents

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

CA 022~8288 1998-12- l~

ANTI-DEW ADHESIVE FLM

FIELD OF THE l~vENr/o~r This invention p~.lail~s to articles that have an anti-dew film bonded to a 5 substrate using an adhesive layer and to methods of making these articles. The anti-dew articles are particularly useful on retl urellective articles.

INT~ODUCTIO~
Retroreflective articles have the unique property of being able to return a 10 substantial portion of incident light back toward the light source over a wide range of h~co~ing 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 relrorenect~d back to the motor vehicle driver. The bright image displayed by the r~llolenective sign makes the sign easier to read and gives motorists more time to 1 5 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 20 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 waterdroplets. More recently, traffic signs have been used that have an exterior anti-dew 25 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 iayer) 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 30 left the psa on the sign surface. This caused accumulation of dirt and reduced light tr~n.~miSsion from the retroreflective sheeting. In addition, the psa appeared yellow CA 022~8288 1998-12-1~

W0 98/5S~5 PCT/US97/17096 from W 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 ret,ul~nective articles with high weather-durability and an anti-fogging or5 anti-dew exterior surface.

S~RY OF THE I~NTION
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-10 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 esserltially without a tackifying resin because it has been found that the presence of a tackifier in the adhesive layer causes undesirable properties. lt is pl efe- ~ ed that the adhesive layer has a glass transition temperature, Tg, below about -15 C. The anti-dew film 15 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 PYpqn~;on 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 20 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 of~ 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 25 article with an anti-dew film in which an ester acrylate, a copolymerizable monoethylenic monomer and a cross-linking agcnt are combined to form a composition that is esscnt~ y without tackifiers, and the composition is cured to form a pressure sensitive adhe~i~e. The pressure sensitive adhesive is disposed between a substrate and an an~i-dew film, wherein the anti-dew film comprises a 30 polymeric base layer and a surface layer wherein inorganic oxide particles occupy at least a portion of the exterior surface.

., . ~

CA 022~8288 1998-12-1~

The present invention provides an eYce~e~t balance of prope,lies and "~e-ous adv~nt~-o~, even after outdoor exposure for several years, inclutling peel sl~engl}l, shear strength, clarity/non-phase separation, nonyellowing, nonpeeling, impact reCiq~nc~ economy and simplicity of construction and prepai alion~
5 durability at extremes of heat and cold, and lasting dew resict~nce.

BRI~FD~SC~UPTMN OF TI~EDRA~U~GS
Fig. I is a cross-sectional view of a r~l-orellective sign according to a l~le~"ed emho~ nçrlt 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 /~VENrlON
The construction of a preferred embodiment of the invention is shown in 15 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, retl orenective sheeting 4 is mounted on backing board 2. Typically the rellorellective sheeting is mounted on the backing board by an intervening pressure 20 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-25 film 10 consists of water-spreading layer 12 that contains hydrophilic inorganic oxide particles 14.
In a plert;..ed embodiment, the article of the present invention is a ellulenective traffic sign that contains a layer of retroreflective sheeting. Examples of commercially available retroreflective sheeting that may be used to make the 30 graphic include ScotchliteTM Reflective Sheeting High Intensity Grade Series 3870, ScotchliteTM Reflective Sheeting Diamond Grade VIP Series 3990, and ScotchliteTM

, CA 022~8288 1998-12-1~

Reflective .~hee ing Diamond Grade LDP Series 3970, available from 3M, St. Paul,e~ot?~ Retroreflective ~heeting typically colllpl;ses 8 reflective surface and optical ~ The reflective surface serves to reflect incident light, and theoptical el~ .C serve to redirect the incident light toward the light source. The5 reflective material may comprise a specular metal reflector such as ~hlminllm or silver (see, for ~".an,ple, U.S. Patent No. 5,283,101) or a diffuse reflector such as a heavy metal pigmçnt or a polymeric material wherein reflect~nce is caused by a dillèle.~ce in refractive indices at an interface (frequently a plastic-air interface).
Optical el~m~nt.c typically come in one of two forms: beaded lens elements and 10 cube corner elçm~-nt~ Examples of l~Liorenective sheeting that employ beaded lens elements have been disclosed in U.S. Patent Nos. 2,407,680, 3,190,17~, 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 comer ~lem~nt~ have been disclosed in U.S. Patent Nos. 3,684,348, 4,61~,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 iml)legnated with color pigments. A series of colored High Intensity Grade, Diamond Grade VIP, and Diamond Grade LDP ScotchlitelM
20 rt;l~oreIlective ,cheeting~ 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 2~ col~...-on. 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 ~lurnim-nl or plywood. The back board is usually preferred to be opaque. Typical examples of commercially available back boards include: a 2 n, ~ (miliimeters) thick acid etched and degreased aluminum panel, a high density 2 30 cm (centimeters) thick plywood, or a 4 mm thick fiberglass-reinforced plastic panel;

., , . . ~ .

CA 022~8288 l998- l2- l~

W0-98t55555 PCT/US97/17096 all these back boards are comn~Qnly 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 ~lm to 125 ~Im, more preferably about 75 llm in thic~ness Cut-outs are made in the overlay layer.
5 Typically the cut-outs are in the shape of letters, numbers or other graphic i.~fo,...ation. The colored overlay blocks out light of selected wa~len~lh.c while the cut-outs allow ~ )eded llanC~ics;on of light to and from the rel.olellective ~l~eet;~ Colorless regions may substitute for cut-outs in the colored overlay. The overlay is plerelably 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 co.lll,.ercially available for application to retroreflective sheeting in trafflc signs The adhesive layer disposed between the retlorenective sheeting and the colored overlay is prerelled to have high clarity and at least modcrately good adhesion and shear strength.
Types of ester acrylates used to make the adhesive layer 8 include the C4 -Cl0 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 aclylate, isodecyl meth~crylate, 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 CA 022~8288 1998-12-1~

acrylate, based on the total weight of resin precursors (i.e. the weight of starting materials not inclu(ling solvents) is used in forming the adhesive layer .
Pl~r~ d copolymerizable monoethylenic monomers include acrylic acid (AA), ...ell.P-.,.ylic acid, it~conic acid, crotonic acid, acrylamide, acrylonitrile, 5 ."cl'-,rylonitnle, N-substituted acryl~mide~, hydroxy acrylates, N-vinyl pyrrolidinon~, maleic anhydride fumaric acid and the like. The copoly",~ al)le monoethylenic ",onor .e. s include N,N-dimethyl acrylamide, N,N-dimethyl m~th~-~.rylarnide, N,N-diethyl acrylamide, N,N-diethyl meth~crylamide, N-vinyl caprolactam, and the like. The most p~t;fc:~ed monoethylenic monomers are acrylic 10 acid and methacrylic acid. Preferably about I 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.
F.Y---rl~ 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.
Prere. I ~d cross-linking agents are di and tri functional aziridines such as trimethylol~lopane-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, toomuch 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 plefe.ably 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 form~ ted to cross-link with W or electron beam radiation. Typical radiation cross-linked psas are described by Martens in U.S. Patent No. 4,181,752.
Small amounts ~f UV absorbers an- ' .ndered amine light stabilizers can be inco"lo,~led in the pressure sensitive ac .ive to prevent UV degradation in outdoor settings. Suitable UV absorbers are benzotriazole compounds including TinuvinTM-328, TinuvinTM-P, and hindered amines include TinuvinTM-292, TinuvinTM-144, and TinuvinTM-622LD, available from Ciba-Geigy Co., Hawthorne . _ CA 022~8288 1998-12-1~

NY. Other suitable UV absorbers include benzophenone compounds such as Uvinul~'~00, Uvinu1TM-490 and Uvinul~-N539, available from BASF Co., Clifton NJ. Pl.,fel~bly, less than two weight percent of each the UV absorbers and hindered amine 1ight stablizers are incorlJol~1ed 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 oftack, peel adhesion and shear properties in a psa that adheres well to low energy surfaces. Surprisingly, and contrary to the te~ch~ 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 rel.olellective 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 ~0~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 sl~el~glh. 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 s~ cient to raise the Tg of the psa above about -15 C. The adhesive layer preferably has less than 5 weight % of tackifiers. More preferably, the adhesivelayer 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 ofterpene, pinene, etc. Tackifiers include dark amber natural gum resin (Nelio N
available from Union Camp), pale thermoplastic resin derived from the poly"~eli~lion of mixed olefins (Super Sta-Tac 80 available from Reichhold Chemicals, Inc.), synthetic polyterpene (Wingtack Plus available from Goodyear),me-iium soft thermoplastic coumarone-indeme resin R 17 supplied by Neville), .. . . ~., .. .. . . , .. ~ ..

CA 022~8288 1998-12-1~

WO 98/5~i~55 PCT/US97117n96 t~.~,ene-urethane resin (Iso-Terp 95 available from SchPnect:~ ly 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 5 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 con~entration than the surface layer. In one preferred embotli~n~ont~ the base layer is a polymer, preferably a non-biaxially oriented flexible acrylate, that is essenti~lly free of inorganic oxide particles, and the surface layer contqin~ greater than 90%
10 oxide particles, more preferably greater than 98% oxide particles. The inorganic oxide particles are preferably silica, alumia, sil~co~lumin-q-te or mixtures thereof.
Examples of anti-dew layers useable in the present invention include those described inU.S. PatentNos. 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 l~eatll.enl to 15 increase the concertration 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 prerelled embodiment, silica particles in the surface layer are treated with an ~Illmin~te solution to improve durability.
The retlorellective articles are made using assembly techniques known in the 20 art. Typically, retroreflective sheeting with pressure sensitive adhesive and release liner is l~qm~ ted onto a backboard by removing thc release liner and passing the Iqn~ te through a nip roller. A colored overlay having cut-out regions is then applied over the rellol-eflective sheeting. Typically, the colored overlay has an adhesive layer and a release liner over the adhesive layer. The colored overlay is 25 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 Il ~nsrer to the sign face. The release liner is stripped off and the overlay is pressed onto the re.ro!!eflective s eeting. Afl:er lamination, the premas~; (if present):
removed from the sign face. The reflective sign at this stage is ready for application 30 ofthe dew resistant film with pressure sensitive adhesive.

.. ~ . . ... ...

CA 022~8288 1998-12-1~

The adhesive used to apply the anti-dew film can be prepared by mixing app~op,iale ~n~ountS of an ester acrylate(s) and copolymerizable ethylenic mon~ e.(s) in an organic solvent or mixture of organic solvents. A cross-linker is added with mixing. The res..l~ing solution is then coated, prefe-ably by roll coating 5 onto a silicone-coated paper release liner. Solvent is removed by evaporation and the adhesive is cured. In a plefe.-ed 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 10 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 te...p~.dlures 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 greaterthan 85% transparent, more preferably greater than 90% transparent to visible light.
The psa preferably has a T~, as measured by dif~erential scanning calorimetry -ASTM E1356, of less than -15 C, more preferably less than -20 C. Preferably, thepsa 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 Ibs/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 lesisla~.l film, more preferably a 1.39 kg m impact to the back ofthe 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%
~el~orellectivity 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.

.. ~. ~. i .. ... .. .. . . . .. .

CA 022~8288 1998-12-1~

EXAMPLES
A film curl test was cond~lcted to determine the curling tçndency of the anti-dew film caused by variations in temperature. An anti-dew film (Folie farblos 99840, from Rohm GmbH, Chemicche Fabrik, Germany - the film is an acrylic film about 50 ~m thick with an inorganic oxide coating on the top surface that is about 0.15 llm thick) was cut in circles with a diameter of 7.62 cm and placed on a flat plastic surface and held at a specified te"~pe,~ture for 30 nlin~ltes. As a result of the therrnal tre~tmPtlt, the sample curled with the inorganic oxide surface on the inner t~r~ and the chord or, if rolled into a tube, di~meter 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) rçm~ined flat at all ten.pe,alllres. The colored overlay film without psa (ScotchliteTM Electronic Cuttable Overlay film 1170 Series, available from 3M, St. Paul MN) also remainedflat at all temperatures.
The curling behavior shown in Table I 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 lnterface 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 wa~ knife coated onto a silicone-coated paper liner and heated in a forced-air oven ~ .70 F for 1.7 min, 27~ F for 1.7 min, and 350 F for 1.7 min, to remove the solvents and p~ duce a 2 mil thick(0.5 mm), dry, cross-linked adhesive layer.

CA 022~8288 1998-12-1~

The anti-dew film (Rohm GmbH No. 99841) was then l~nin~ted 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 co-,.pa-d~ e 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 ofthe adhesive was added to the mixture and mixed well for 15-30 min witha propeller mixer. The resulting mixture was bar coated onto a release liner anddried 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 ScotchliteTMReflective Sheeting (Diamond GradeTM VIP 3990) which was mounted on a 70 mm x 70 mrn x 1.0 mm thick aluminum panel. The length of adhesive overlap on the reflective ,cheetine was 2.54 cm giving an overlap area of 3.22 sq. cm. The unapplied tail of the film tape was reinforced with ScotchTM filament tape to prevent tearing under load. After conditioning the applied samples for 24 hr dwell time at constant temperature and humidity, a I 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 -., ~

CA 022~8288 1998-12- 1~
WQ 9815s555 PCT/US97/17096 15,000 min are desired for good creep and shrinkage rç~ict~nce of the applied pressure-sens;li~/e overlay film. This range of shear adhesion is obtained with prere,led cross-linker levels of about 0.03 to 0.06 parts bisamide per 100 partsacrylic copolymer.
5Test speçi-.... en~ for measuring reverse impact at low temperature were pr~paled by l~ g an anti-dew film with psa on a rellor~flective VIP (3990) she~ (available from 3M, St. Paul, MN) on an alllminu-n panel of 6in x 6in (lScm x 15cm), hav.ing thickness of 0.080in (0.203 cm). All specimens were con~ition~d at room temperature for at least 24 hours to allow adhesion to fullydevelop. 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 sheetin~ 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 l .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~e;nrolced with ScotchlM filament packaging tape prior to the peel test. The ple~ared spe~ en~ were let sit for at least one day, then cooled to the test te"~pe~tLIre 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 areshown in Table 2.
Pressure sensitive adhesive clarity and outdoor weathering resistance were measured from the retroreflection coefficient and yellowness index before and after CA 022~8288 1998-12-1~

-an accelerated W light exposure test in a weatherometer. ScotchliteTM High Intensity Grade No. 3870 l~lrorenective sheeting was l~min~ted on an ~ minum test panel, 7 cm wide, 28 cm long and I mm thick. The coefficient of l~t~oleflection of the sheetin~ is 336 cdUlux/m2, 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 lqmin~ted 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 spec.irnen~ was tested in a weathero,.,cler in accordallce with ASTM GS3. The light source in the weatherometer was a fluorescent W lamp with an intensity maximum at 3 l 3 cm '. Test specimens were reme~sured after 3000 hours of repeated cycles of 4 hours UV light at 60~C and 4hours condencin~ humidity at 50~C. The results are shown in Table 4.
The film s~ ecil.-el1s using the inventive psa (Ex. l) and the comparative psa both retained better than 90% of retroreflectivity. The retroreflective sheetingwithout a psa or film coating exhibited a yellowness index increase of 4.9 units, the ~heeting with the psa of Ex. 1 increased about 5.5 units and the specimen with the comparative psa increased about l 0 units. Visually, the specimen with the 20 col,.pa.~live psa showed yellowing while the specimens with the inventive psashowed 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 25 embo~iments, it is not limited to the specific embodiments and includes equivalents to what is described here.

, ., . ., . ~ ., . . , . ~ , CA 022S8288 1998-12- lS

FILM CURL TEST
Aging Specimen Curl Measurement Degree of 'unit in cm) Curvature Tempelalu,e Diqmeter Chord of Tube Curling (cm~l) (~C)(cm) Curl Diameter Radius -22 7.62 6.3Scm ----- 3.71 cm 0.27 21 7.62 4.45 cm ----- 2.24cm 0.45 54 7.62 ----- 2.22 cm 1.11 0.90 71 7.62 ----- 0.953 cm 0.476 2.10 PSA Test 90-Peel Adhesive Thickness Temp (Ib/in) (mils) (nc) Av.
Ex. 1 3 21 4.8 Ex. 1 2.6 21 3.9 Comp. Ex. 1.2 21 3.3 Ex. 1 3 1.7 5.3 Ex. 1 2.6 1.7 4.5 Comp.Ex. 1.2 1.7 1.8 Ex. 1 3 -7 4.1 Ex. 1 2.6 -7 3.4 Comp. Ex. 1.2 -7 1.5 COLD TEMPERATURE IMPACT TEST
PSA PSA Test Impact Observation - Coating Thickness Temperature Force of SampleII) Wt. (um) (~C) (kgm)Delamination (glm 2) of Anti-Dew Film Ex. 1 85 75 -22 1.39 No Ex. 1 60 50 -22 1.39 No Comp. Ex. 48 37 -22 1.39 Yes Ex. 1 85 75 -22 0.92 No Ex. 1 60 50 -22 0.92 No Comp. Ex. 48 37 -22 0.92 Yes Ex. 1 85 75 -7 1.39 No Ex. 1 60 50 -7 1.39 No Comp. Ex. . 48 37 -7 1.39 Partial Ex. 1 85 75 -7 0.92 No Ex. 1 60 50 -7 0 92 No g~m2) Comp. Ex. 48 37 -7 0.92 Partial (Standard, 96) Ex. 1 85 75 4 1.39 No Ex. 1 60 50 4 1.39 No Comp. Ex. 48 37 4 1 39 No , . . . .. . .. . ~

ACCELERATED WEATHERING
Sample PSA Cro~ PSA Weatber Retro- C~ ~-r ' Y~"
LD. Type linker Coating Te~t r :f1 e ' ~ P~ Inde~
LevelWeigbt (UV(cdl/lu~/m2) 1~
F'r~ ~) ment (pph)(~/m2)(hour) (~/O) 1. Ex. 1 0.026 50.3 0 305 91 -1.74 3,000 313 93 2.05 2. Ex. 1 0.026 56.5 0 306 91 -3.47 3,000 308 92 2.01 3. Ex. 1 0.030 50.3 0 306 91 -3.76 3,000 308 92 1.54 4. Ex. 1 0.043 50.3 0 309 92 -3.93 3,000 307 91 1.55 5. Comp 0.07548.1 0 315 94 -2.45 3,000 321 96 7.24

Claims (13)

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 8 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 animpact 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.