RETROREFLECTIVE ARTICLE THAT HAS PHENOLIC RESIN IN CHEMICAL ASSOCIATION WITH REFLECTIVE ALUMINUM LAYER

Abstract: A retroreflective sheeting (10, 30) comprising a retroreflective constituent (12, 32) that includes optical elements (14, 34) and a reflective layer (16, 36). The reflective layer (16, 36) contains elemental aluminum and is disposed behind the optical elements (14, 34). A phenolic resin is placed in chemical association with the elemental aluminum. The phenolic resin has a total hydroxyl number of at least 90. The placement of the phenolic resin in chemical association with the reflective layer provides the retroreflective sheeting with improved durability, demonstrated by prolonged retroreflective performance.

Description: RETROREFLECTIVE ARTICLE THAT HAS PHENOLIC RESIN IN CHEMICAL ASSOCIATION WITH REFLECTIVE ALUMINUM LAYER TFΓMΠCAJ. FTFTΠ The present invention pertains to a retroreflective article that employs a layer of aluminum as a retroreftector and that has a phenolic resin in chemical association with the aluminum layer. BACKGROUND OF THEINVENΗON Retroreflective articles have the ability to return a substantial portion of incident light in the direction from which the light originated. This unique ability has promoted widespread use of retroreflective articles on signs and markers along roadways. The articles display information to motor vehicle drivers at nighttime by retroreflecting light from the headlamps of motor vehicles. Retroreflective articles typically ar^ ' "■ the form of a sheeting, and there are two major categories of retroreflective sh- j. The first category often is referred to as "beaded lens sheeting". In beaded leru> sheeting, a monolayer of beads (also referred to as lenses, lens elements, and microspheres) is employed as optical elements that collimate light so that the reflected light is returned in a direction substantially parallel to the direction from which the light entered the sheeting. One configuration of beaded lens sheeting is "exposed lens" sheeting in which the lens elements are exposed to the ambient environment. Another configuration is an "enclosed lens" sheeting such as disclosed in U.S. Patent 2,407,680 to Palmquist, where the lens elements are covered by a flat transparent cover film. In another configuration, sometimes referred to as "encapsulated lens" sheeting, the lens elements are encapsulated in the form of a multitude of hermetically-sealed air chambers; see for example, U.S. Patents 3,190,178, 4,025,159, 4,025,159, 4,896,943, 5,064,272, and 5,066,098. In all of the embodiments of beaded lens sheeting, the monolayer of lens elements is embedded in a supporting surface which is commonly referred to as a binder layer. The second major category of retroreflective sheeting is referred to as cube- comer sheeting. Examples of cube-comer sheeting are shown in U.S. Patents 3,684,348, 4,801,193, 4,895,428, and 4,938,563. In cube-comer sheeting, a multitude of cube-comer elements, each having a shape resembling the comer of a room, project from a body portion (also known as a land). The multitude of cube- comer elements act as the optical elements which collimate light so that the reflected light is directed out of the sheeting in a direction substantially parallel to the direction from which the light entered the sheeting. Light which enters the front of the cube-comer sheeting passes through the body portion, strikes the backside of the cubes, and is reflected to pass through the body portion of the sheeting in the direction of the light source. In both of the major categories of retroreflective sheetings, a metal layer often is disposed behind the optical elements as a reflector. Aluminum frequently is the metal of choice for use in the metal layer. The aluminum layer together with the optical elements comprise the retroreflective constituent of the sheeting. The sheetings that use lenses as the optical elements typically have the aluminum layer located on the embedded surfaces of the lenses. In the cube-comer sheetings, the aluminum layer typically is disposed on the back side of the cube-comer elements. In known retroreflective sheetings, the aluminum in the metal layer often becomes oxidized. Oxidized aluminum transmits light, whereas elemental aluminum reflects light. Thus, extensive oxidation of the aluminum in the reflective layer can lead to loss of in retroreflectivity. In U. S. Patent 4,248,748 to McGrath and Johnson, a heat-activated adhesive is disclosed that contains an acrylate polymer and a tackifying resin. In addition to these ingredients, it is disclosed that the adhesive may contain other materials such as phenolic resins to control pre-adhesion and to strengthen the heat- activated bond. No disclosure is provided in regard to the phenolic content of the phenolic resin. In examples 8-15 of the patent, a "a low-reactivity phenolic resin" Super Beckacite™, Reichhold Chemicals Inc., was added to the heat-activated adhesive. It is believed that this phenolic resin has a total hydroxyl number of about 470. The patent discloses that the heat-activated adhesive can be used on a retroreflective sheeting. Phenolic resin adhesives have been used in the aircraft industry and are known to be stable in hot wet climates. See Poole, P. Adhesion Problems in the Aircraft Industry, ch. 9 of Industrial Adhesion Problems, edited by D. Brewis and D. Briggs, Wiley Interscience, New York (1985). Modified epoxy-phenolic primers containing corrosion inhibitors have been shown to improve durability of aluminum structural adhesive joints. Id U.K. patent application GB 2,165, 165-A describes a process for coating aluminum to provide corrosion resistance and an increase adhesion of paints. The coating process involves use of a polyphenolic compound or its acidic salt. In Matienzo, L.J., et al., J. of Materials Sci., v. 21, pp. 1601-1608 (1986), it has been reported that organophosphonates and organosilanes protect aluminum from environmental corrosion. SUMMARY OF THE INVENTION The present invention provides a new retroreflective article, which comprises: a retroreflective constituent that includes optical elements and a reflective layer that contains elemental aluminum, the reflective layer that contains elemental aluminum being disposed behind the optical elements; and a phenolic resin in chemical association with the elemental aluminum, the phenolic resin having a total hydroxyl number at least 90; with the proviso that if the retroreflective article possesses an adhesive layer for mounting the article to a substrate, the phenolic resin is incorporated in the adhesive layer at less than 25 percent by weight. The present invention differs from known retroreflective articles in that a phenolic resin having a total hydroxyl number of at least 90 is placed in chemical association with the elemental aluminum in the reflective layer. The term "total hydroxyl number" means the sum of the hydroxyl number and the acid number, which are deteπriined in accordance with two standardized tests. The first standardized test determines the hydroxyl number and is ASTM E 222-88, where Test Method A (Pressure Bottle Method) is employed. The second standardized test determines the acid number and is ASTM D 465-92, where the potentiometric method is used. The term "chemical association" means the phenolic resin interacts with the elemental aliiminum to hinder its oxidation. It has been discovered in this invention that when the phenolic resin is placed in chemical association with the reflective layer, the durability of the retroreflective article is improved. The article is able to retain much of its initial retroreflectance for an extended time period. When the phenolic resin is in direct contact with the elemental aluminum, the phenolic resin can prevent oxidation of the aluminum in the reflective layer. When other layers are disposed therebetween, it still can be possible in some embodiments for the phenolic resin to chemically associate with the aluminum in the reflective layer. In addition to placing a damper on the oxidation of the aluminum in the reflective layer, it also has been discovered in this invention that the phenolic resin makes an encapsulated retroreflective sheeting more resistant to delamination. This is very important because delamination disrupts sheeting optics and allows direct attack by the elements of weather — including sunlight, moisture, dirt, and mildew — on the internal components of the sheeting, particularly around the sheeting's edges. The durable retroreflective sheeting of this invention allows good retroreflective performance to be maintained for an extraordinary duration, reducing the need to monitor and replace the sheeting as often. BRIEF DESCRIPΉON OF THE DRA WINGS In the drawings: FIG. 1 is a cross-section of an example of a beaded lens retroreflective sheeting 10 in accordance with the present invention; FIG. 2 is a top view of the beaded lens retroreflective sheeting 10 of FIG. l; FIG. 3 illustrates an example of a cube-comer retroreflective sheeting 30 in accordance with the present invention; and FIG. 4 is a bottom view of the backside of cube-comer elements 34 in a retroreflective sheeting. These drawings are idealized and are not drawn to scale. ΠFTATTFT DFSCRIPΉON OF THE PREFERRED EMBODIMENTS In the practice of the present invention, a retroreflective sheeting is provided that has longer lasting retroreflective performance and improved resistance to delamination. FIG. 1 illustrates an example of a beaded lens retroreflective sheeting 10 in which the lens elements 14 are encapsulated. Retroreflective sheeting 10 comprises a retroreflective constituent 12 that includes a monolayer of lens elements 14 and a reflective layer 16 that contains elemental aluminum. Retroreflective constituent 12 is embedded in a binder layer 18. Located behind binder layer 18 is an adhesive layer 20. A release liner 22 is placed over the adhesive layer 20 to protect the same until sheeting 10 is adhered to a substrate (not shown). In chemical association with the reflective layer 16 is a phenolic resin that has a total hydroxyl number of at least about 90. The phenolic resin may be present, for example, in the binder layer 18, the adhesive layer 20, on the front or first major surface 23 of binder layer 18, or it may be present on the substrate to which the retroreflective sheeting becomes secured. On the front or first side 25 of the retroreflective sheeting 10, a cover layer 24 can be secured to the retroreflective constituent 12 along a network of lines to form an encapsulated lens retroreflective sheeting; see, for example, U.S. Patent 3,190,178 to McKenzie. This forms a multitude of hermetically sealed cells 26 within which the lens elements 14 are encapsulated and have an air interface. As shown from a front view in FIG. 2, retroreflective sheeting 10 can have a multitude of hermetically sealed air chambers 26 bounded by a network of bond lines 28. Although FIGs. 1 and 2 illustrate an encapsulated retroreflective sheeting, the invention may be used in connection with the other beaded lens sheetings such as the enclosed lens and exposed lens constructions; see, for example, U.S. Patents 2,407,680; 4,968,063; 5,060,981; 5,080,463; 5,128,804; 5,200,262; and 5,283,101, the disclosures of which are incorporated here by reference. The lens elements 14 supported by the binder layer 18 are capable of altering the direction of light so that, in conjunction with alum um-containing reflective layer 16, incoming light can be retroreflected. As shown, the lens elements 14 can be microspheres which, preferably, are substantially spherical in shape in order to provide the most uniform and efficient retroreflection. The microspheres preferably also are substantially transparent so as to rninimize absorption of light so that a large percentage of the incident light is retroreflected. The microspheres often are substantially colorless but may be tinted or colored in some other fashion. The microspheres may be made from glass, a non-vitreous ceramic composition, or a synthetic resin. In general, glass microspheres are preferred because they tend to be less expensive, harder, and more durable than microspheres made from synthetic resin. Examples of microspheres that may be useful in this invention are disclosed in the following United States patents: 1,175,224, 2,461,011, 2,726,161, 2,842,446, 2,853,393, 2,870,030, 2,939,797, 2,965,921, 2,992,122, 3,149,016, 3,468,681, 3,946,130, 4,192,576, 4,367,919, 4,564,556, 4,758,469, 4,772,511, and 4,931,414. The disclosures of these patents are incorporated here by reference. Microspheres used in the present invention typically have an average diameter in the range of about 30 to 200 microns. Microspheres used in the present invention typically have a refractive index of about 1.7 to 2.3. The average diameter and refractive index of the microspheres may vary depending on whether the microspheres are encapsulated, enclosed, or exposed; see U.S. Patent 4,957,335 to Kuney, Jr., the disclosure of which is incorporated here by reference. The reflective layer 16 that contains elemental aluminum is disposed behind the layer of lens elements 14 such that the reflective layer 16 is capable of reflecting incident light that passes through the lens elements 14 from the front side 25 of the sheeting 10. In many embodiments, it is preferred that the reflective layer is disposed directly on the embedded portion of the lens elements. The reflective layer contains an effective amount of elemental aluminum. It is to be understood, however, that some of the aluminum may be in the form of metal oxide and/or hydroxide. The reflective layer that contains elemental aluminum can specularly reflect light and typically has a thickness of about 50 to 100 nanometers. The reflective layer may be produced by known techniques, including vacuum deposition, vapor coating, chemical deposition, or electroless plating processes. The binder layer, onto which the retroreflective constituent is supported, typically is a flexible polymeric material. A number of polymers may be used as a binder layer in a beaded lens sheeting. Examples of some of the polymers include: polyolefins, polyesters, polyurethanes, polyepoxides, natural and synthetic rubbers, acrylic resins, and combinations thereof The binder layer typically is a continuous fluid-impermeable, sheet-like layer but may be a fibrous structure, for example, as taught in U.S. Patent 5,128,804. In general, the binder layer has a thickness of about 1 to 250 micrometers. Typically, the thickness is about 25 to 75 micrometers. Thicknesses less than 25 micrometers may be too thin to adhere to both the substrate and the optical elements, and thicknesses greater than 75 micrometers may unnecessarily stiffen the sheeting and add to its cost. The binder layer also may contain other operational additives as are well-known in the art. Examples of operational additives include colorants (for example, pigments and dyes), stabilizers (for example, thermal and hydrolytic stabilizers and antioxidants, hindered amines, flame retardants, flow modifiers (for example, surfactants), viscosity adjusters (for example, organic solvents), rheology modifiers (for example, thickeners), coalescing agents, plasticizers, tackifiers, et cetera. Generally, the binder layer contains about 70 percent by weight up to about 100 percent by weight of polymeric material, with the remainder being operational additives in effective amounts. The adhesive layer preferably is a pressure-sensitive adhesive (PSA). As the term is used herein, "pressure sensitive adhesive" or "PSA" means an adhesive that, in a dry state, is relatively permanently tacky at room temperature to allow adherence to a variety of surfaces upon mere contact with slight hand pressure. Normally tacky PSA material has contained crude rubber and rosin or various hydrocarbon resins. The blend employed provides the four-fold balance of adhesion, cohesion, stretchiness, and elasticity. Certain vinyl polymers such as poly-n-butyl acrylate and polyvinyl n-butyl ether have provided the four-fold balance of properties in the absence of tackifying resins and other modifying agents. Examples of other vinyl polymers having inherent PSA properties are polymers of acrylate esters of acrylic acid with non-tertiary alkyl alcohols. Partici'arly preferred PSAs are copolymers consisting essentially of acrylic acid esters < f non-tertiary alkyl alcohols having one to 14 carbon atoms and acrylic monomers of acrylic acid, methacryiic acid, itaconic acid, acryiamide, methacrylamide, acrylonitrile, and methacrylonitrile; see U.S. Patent Re 24,906 to Ulrich, the disclosure of which is incorporated here by reference. The amounts of the monomers and proportions thereof being selected to provide a normally tacky and pressure-sensitive adhesive copolymer. The cover film preferably is highly transparent (that is, it has a high transmittance of visible light) to minimize loss of retroreflectivity from absorption or scattering of light. Typically, the cover films used in an encapsulated or an enclosed lens sheeting has a visible light transmittance of at least 75 percent and preferably at least 85 percent. Cover films may comprise polyethylene, polypropylene, or a copolymer comprising major portions of ethylene and propylene as disclosed in U.S. Patents 4,896,943, 4,897,136, and 5,064,272. Acrylic resins such as polymethyl methacrylate also may be used as cover films and can provide good durability and high transparency; see U.S. Patent 4,025,159 to McGrath, incorporated here by reference. FIG. 3 illustrates an example of a cube-comer retroreflective sheeting 30 having a retroreflective constituent 32 that contains cube-comer elements 34 and a reflective layer 36 (noted by a thick line) disposed behind the cube-comer elements. The cube-comer elements are united in the form of a sheeting by a body layer 38 (sometimes also referred to as a land). An optional overlay film 40 can be provided to protect the front of the cube-comer sheeting 30 from uit-a-violet rays and abrasion. A backing layer 42 can be placed in contact with the reflective layer 36, and an adhesive layer 44 can be applied to the backing layer 42. Alternatively, the adhesive layer 44 can be applied directly to the reflective layer 36. The adhesive layer may be a PS A layer as described above. A release liner 46 can be provided to cover the adhesive layer 44 until the retroreflective article 30 is adhered to a substrate (not shown) such as a panel for a sign. FIG. 4 illustrates an example of cube-comer elements 34 that may be used in a retroreflective sheeting of this invention. Each cube-comer element 34 has three planar faces 48 that meet at an apex 49. The apex 49 of each cube-comer element 34 may be vertically aligned with the center of the base of the cube-co er element - see, for example, U.S. Patent 3,684,348 - or the apex may be canted to the center of the base as disclosed in U.S. Patent 4,588,258. Of the many cube- comer configurations — see for example, U.S. Patents 4,775,219, 4,243,618, 4,202,600, and 3,712,706 — the cube-comer retroreflective elements described in U.S. Patent 4,588,258 are particularly preferred because they provide wide angle retroreflection among multiple viewing planes. The disclosures of the above patents are incorporated here by reference. A cube-comer retroreflective sheeting of this invention generally is about 75 micrometers (5 mils) to 600 micrometers (24 mils) thick. Each cube-comer element typically is about 60 micrometers (2.4 mils) to 200 micrometers (8 mils) high, and the body portion, typically, is about 25 micrometers (1 mil) to 250 micrometers (25 mils) thick. The body portion and the overlay film preferably interface at a substantially smooth surface to prevent the light from becoming dispersed. The overlay film, typically, is about 25 micrometers (1 mil) to 125 micrometers (5 mils) thick. The cube-comer sheeting for a retroreflective article of this invention can have a polymeric composition. Polymers useful for forming a cube-comer sheeting include polymers that are transmissive to visible light. Examples of polymers that may be used in retroreflective articles of this invention include polycarbonates, polyalkylacrylates, polyalkylmethacrylates such as polymethylmethacrylate, polyesters, vinyl polymers, polyurethanes, cellulose esters, fluoropolymers, potyolefins, ionomeric copolymers and copolymers of ethylene or propylene with acrylic acid, methacrylic acid, or vinyl acetate. Combinations of such polymers also may be employed. For example, the cube-comer sheeting may be made from a blend of polymers, or the cube-comer elements and the body portion may be made from different polymers. Further, the body portion may comprise a plurality of layers of the same or different polymers. Polycarbonates are preferred polymers for a cube-comer sheeting of this invention because they possess tough, durable characteristics. Cube-comer sheetings can be made according to known techniques such as disclosed in U.S. Patents 5,122,902, 4,938,563, 4,895,428, 4,703,999, 4,601,861, 4,576,850, 4,588,258, 4,025,159, 3,935,359, 3,811,983, and 3,810,804. An overlay film for a retroreflective sheeting may be made from polymers similar to those of the cube-comer sheeting; typically, types which are stable to ultra-violet (UV) light and have good abrasion resistance such as acrylic polymers (e.g., polymethylmethacrylate) and copolymers. UV stabilizers also may be added to the overlay film. Whether the retroreflective sheeting is a beaded-lens sheeting or a cube- comer sheeting, or any other presently known or subsequently-developed retroreflective sheeting, in accordance with the present invention a phenolic resin is placed in chemical association with reflective layer that contains elemental aluminum. The term "phenolic resin" is used herein to mean an oligomeric or polymeric material containing units that have an aromatic nucleus that is substituted with one or more hydroxyl groups. The aromatic nucleus may be monocyclic or polycyclic and can be carbocyclic or heterocyclic. Typically, the aromatic nucleus is a six-membered carbocylic ring but may be, for example, a five-, nine-, or ten- membered heterocyclic ring, or a ten-membered carbocyclic ring. The aromatic nucleus typically contains 10 or less atoms in the ring(s) structure of the aromatic nucleus. The phenolic resin may be a resole or novalac type phenolic resin. Preferred phenolic resins used in the invention are the reaction products of an aldehyde and a phenol. The aldehyde may be, for example, formaldehyde, acetaldehyde, furaldehyde, and the like, and combinations thereof In lieu of an aldehyde, a terpene such as a-pinene or b-pinene, or pimaric acid, may be reacted with a phenol. Examples of phenols that may be reacted with the aldehydes to form a phenolic resin include: phenol; alkyl-substituted phenols such as p-tert butylphenol, p-octylphenol, p-nonylphenol, methylphenol and 2,3-xylenol; 1,3-benezenediol, 4,4'isopropylidenediphenyl (2,2-bis(4-hydroxylphenyl) propane; bisphenol-A). A preferred phenolic resin includes the reaction product(s) of formaldehyde with one or more of phenol, p-tert butyl phenol, p-octyl phenol, p-nonylphenol, and 1,3 benzenediol. The phenolic resin preferably has a total hydroxyl number of 90 to 1100, more preferably 100 to 500. Examples of suitable commercially available phenolic resins include: Varcum™ resins 29-008, 29-446, 29-445, and 29-401 from Occidental Chemical Corp, North Tonawanda, New York; CK 1634, CK 2400, CK 2500, and CK 2432 from Georgia Pacific Resins, Inc., Tacoma, Washington; Vinsol™ ester gum from Hercules Incorporated, Wilmington, Delaware. Although it is possible to incorporate the phenolic resin into essentially any portion of the retroreflective sheeting which allows the phenolic resin to chemically associate with the metal in the reflective layer, it is preferred that the phenolic resin be disposed in direct contact with the reflective layer or be disposed in a layer that is in direct contact with the reflective layer. In this regard, the phenolic resin preferably is disposed in the binder layer of a beaded-lens sheeting and the backing layer of a cube-comer sheeting. If a backing layer is not employed, the phenolic resin may be disposed in the adhesive layer of the cube-comer retrorefleαive sheeting. The phenolic resin also may be incorporated into the adhesive layer of a beaded lens sheeting when the binder layer permits the phenolic resin to chemically associate with the aluminum metal in the reflective layer. In a similar vein, the phenolic resin may be incorporated into the adhesive layer of a cube-comer sheeting when a backing layer is present between the aluminum metal and adhesive, provided the backing layer allows the phenolic resin to chemically associate with the aluminum metal in the reflective layer. When the phenolic resin is incorporated into the binder layer of a beaded-lens sheeting, the phenolic resin preferably is disposed therein at about 0.5 to 25 weight percent, more preferably at about 1 to 10 weight percent. When the phenolic resin is incorporated into the adhesive layer of a beaded-lens sheeting, the phenolic resin preferably is disposed therein at about 1 to 25 weight percent, more preferably at about 5 to 20 weight percent. When the phenolic resin is incorporated into the backing layer of a cube-comer retroreflective sheeting, the phenolic resin preferably is disposed therein at about 0.5 to 25 weight percent, more preferably about 1 to 10 weight percent. When the phenolic resin is incorporated into an adhesive of a cube-comer sheeting that contains a backing layer, the phenolic resin is incorporated therein at about 1 to 25 weight percent, more preferably at about 5 to 20 weight percent, and when the phenolic resin is incorporated into an adhesive layer that is in direct contact with the reflective layer, the phenolic resin is incorporated therein at about 0.5 to 25 weight percent, preferably about 1 to 10 weight percent. The phenolic resin also may be placed on a surface onto which the retroreflective article becomes secured. As illustrated above, when the phenolic resin is disposed in a layer that is in direct contact with the reflective layer, the phenolic resin preferably is disposed therein at about 1 to 10 weight percent, whereas if the phenolic resin disposed in a layer that is spaced from the reflective layer, then the content of the phenolic resin preferably is increased to about 5 to 20 weight percent. Features and advantages of this invention are further illustrated in the following examples. It is to be expressly understood, however, that while the examples serve this purpose, the particular ingredients and amounts used as well as other conditions and details are not to be construed in a manner that would unduly limit the scope of this invention. EXAMPLES Examples 1-3 Superbeckacite™ 1003, a resole phenolic resin with a total hydroxyl number of about 476 — previously available from Reichold Chemical Company, now available from Occidental Chemical Corporation as Varcum™ 29446 — was dissolved in methyl ethyl ketone (MEK) to prepare a 50% (w/w) solution. Vinsol™ ester gum, a phenolic resin with a total hydroxyl number of about 186 was dissolved in MEK to prepare a separate 50% (w/w) solution. To separate 100 gram portions of a solution — for a terpolymer PSA containing 57.5 weight percent iso-octylacrylate, 35 weight percent methyl acrylate, 7 weight percent acrylic acid (27 weight percent in 50/50 ethyl acetate toluene)) — were added 5 grams of the Superbeckacite™ 1003/MEK or Vinsol Ester Gum MEK solution. A separate 100 gram portion of the isooctylacrylate/methyl acrylate/acrylic acid terpolymer without any added phenolic resin also was prepared. A knife coater with an orifice set to give a dry film thickness of 0.254 mm (1 mil) was used to coat these phenolic resin/psa solutions onto a release liner. After coating, the adhesives were dried in an oven for 10 minutes at 200 °F to remove solvent. The adhesive films then were applied to the backside of the binder layer (support layer) of an encapsulated lens retroreflective sheeting prepared according to example 12 of U.S. patent 4,025,159. After removing the release liner, the adhesive coated retroreflective sheeting specimens were applied to aluminum panels (5052 H33 alloy, with an Alodine™ 1200 chromate conversion coating (Parker- Amchem, Madison Heights, Michigan). Retroreflectance was measured on each sample in accordance with ASTM E 810-91 using a -4° entrance angle and a 0.2° observation angle. The samples were directly exposed outdoors to sunlight and other elements at an angle of 45° to the horizontal in Miami, Florida according to ASTM G 7-89. Further reference to "ASTM E-810-91" herein means ASTM E 810-91 wherein the entrance angle, observation angle, outdoor exposure are as just described. Table 1 shows the retroreflective retention results after 78 months of outdoor exposure in Miami, Florida. TABLE 1 78 Month Exposure TϋM Percent Percent Example Phen licResin OH PhenoBc Retained Number Resin Retro¬ reflectance 1 Superbeckacite™ 476 10 79 1003 2 Vinsol™ Ester 186 10 78 Gum 3* none — — 49 'Comparative Example The data in Table 1 demonstrate that samples of the invention (Examples 1 and 2) showed dramatically better retention of retroreflectance, as compared to a sample (Example 3) that contained no phenolic resin. Although the data is not presented in Table 1, Examples 1 and 2 also showed no lifting of the top film at the test panel edges, while the sample of Example 3 had noticeable lifting of top film at the test panel edges. Examples 4-6 The procedures of Examples 1-3 were followed to prepare retroreflective sheeting samples using an 94% isooctylacrylate/6% acrylic acid PSA (36% w/w solution polymer in ethyl acetate) tackified with Foral™ 85, a hydrogenated rosin ester having a total OH number of about 60, (available from Reichold Chemicals), a terpene phenolic having a total OH number of about 70-80, (Nirez™ 2019 from Reichold Chemicals), or a resole phenolic resin (Superbeckacite™ 1003 from Reichold Chemicals) having a total OH number of about 476. After preparing the retroreflective sheeting specimens, retroreflectance was measured according to ASTM E 810-91 after being exposed outdoors in Miami, Florida for 48 months Results of the tests are set forth below in Table 2. TABLE 2 48 Month Exposure Total Percent Percent Example Phenolic Resin Off PkenoBc Retained Number Ram reflectance 4* Foral™ 85 60 33 70 5* Nirez™ 2019 70-80 5 70 6 Superbeckacite™ 476 5 91 1003 'Comparative examples The data in Table 2 demonstrate that the sample of the invention has improved durability to long-term exposures to sunlight. Examples 7-8 The procedures of Examples 1-3 were followed to prepare and test retroreflective sheeting specimens except a PSA was used that contained ninety-five weight percent of a copolymer comprising 90 weight percent 2-methylbutylacrylate, 10 weight percent acrylic acid, and 5 weight percent Superbeckacite™ 1003. Table 3 shows the retained retroreflectance after 12 months of outdoor exposure in Miami, Florida. TABLE 3 12 Month Exposure Total OH Percent Percent Example PhetwUc Resin Number Phenolic Retained Resin Retro¬ reflectance 1* none — none 31 8 Superbeckacite™ 476 5 94 1003 "Comparative Example The data in Table 3 show how samples of the invention demonstrate substantially improved durability when containing relatively low amounts of phenolic resin in chemical association with the adhesive layer. Examples 9-16 These Examples illustrate how treatment of a sign substrate with a dilute solution of a phenolic resin will enhance the durability of retroreflective sheetings. Each resin evaluated was dissolved in methyl ethyl ketone (5% w/w). Aluminum panels (5052H33 alloy) - treated either with an Alodine™ 1200 chromate conversion coat, or with an etch and desmut surface created by caustic etch and a dilute nitric acid wash - were then dipped for 10 seconds in the dilute MEK/resin solution, air dried for approximately 10 minutes, and dried in an oven for 10 minutes at 150°F. Reichold 24-003 is believed to be the same as Superbeckacite™ 1003, Reichold 29-008 (now believed to be Varcum™ 29008) is a non-heat reactive novolac phenolic resin, and Reichold 29-402 (now believed to be Varcum™ 921) and Reichold 29-503 (now believed to be Varcum™ 1198) are heat reactive alkyl phenolic resins. The aluminum panels were allowed to cool before applying Scotchlite™ 3870 High Intensity Grade Sheeting, 3M Company, St. Paul, Minnesota. After the Scotchlite™ 3870 High Intensity Retroreflective Sheeting was applied to the aluminum panels, retroreflectance was measured according to ASTM E 810-91 according to the conditions described above. The test results are shown in Table 4. TABLE 4 60 Month Exposure Example Aluminum Phenolic Total on Percent Panel " ' Resin-' Number Retained Retro- reflectance 9* Alodine™ 1200 none — 35 1 10 Alodine™ 1200 Reichold™ 24-003 476 84 11* etch and desmut none — 66 12* etch and desmut Foral™ 85 60 35 13 etch and desmut Reichold™ 24-003 476 82 14 etch and desmut Reichold™ 29-008 455 85 15 etch and desmut Reichold™ 29-402 350 70 16 etch and desmut Reichold™ 29-530 350 77 'Comparative Results 1 Results after 36 months, when panels were retired The data in Table 4 shows the percent retained retroreflectance from 10 measurements (two replicate specimens, five measurements each) after 60 months of exposure in Miami, Florida at 45° to the horizontal In addition to showing higher retained retroreflectance, test specimens using the phenolic resins (Examples 10, 13-16) showed less lifting of the top film at panel edges than the test specimens using no phenolic resin (Example 11) or with a resin having a low total hydroxyl number (Example 12). Examples 17-21 Examples 17-21 show how phenolic resins can provide enhanced durability relative to that obtained when other glassy polymers such as poly-alphamethyl styrene (Kristalex™ 3085, available from Hercules Chemical Inc.) or a coumarone- indene resin (Cumar™ R-13, available from Neville Chemical Co, Pittsburgh, Pennsylvania). Union Carbide CK1634 is believed to be a heat reactive resole phenolic resin now available from Georgia Pacific. The procedure described in Examples 9-17 was used to treat 5052H33 aluminum panels that had an etch and desmut surface. Scotchlite™ 870 High Intensity Grade retroreflective sheeting was applied to each panel after they had been treated, dried, and allowed to cool. Retroreflectance was tested as described above. Each sample was visually observed for adherence of the top film to the remainder of the retroreflective structure. The portion of the top film that remained well adhered to the sheeting was determined by measuring the area of top film remaining attached and expressing it as a percent of the total applied to the test panel. The results are set forth in Table 5. TABLE 5 84 Month Exposure Example Aluminum Phenolic Total Percent Percent Panel Resin OH Retained Well Surface Number Retro¬ Adhered reflectance Tøp FUm 17* etch and none - 21 27 desmut 18 etch and Superbeckacite™ 476 86 90 desmut 1003 19* etch and Kπstalex™ 3085 0 52 36 desmut 20* etch and Cumar™ R13 30 32 45 desmut 21 etch and Union Carbide™ 420 82 88 desmut CK1634 •Comparative Examples Table 5 shows the percent retained retroreflectance from 6 measurements (two replicate test panels, three measurements per panel) after 84 months exposure in Miami, Florida at 45°. The samples of the invention, examples 18 and 21 demonstrated improved retained retroreflectance and also showed significantly less lifting of the top film at panel edges when compared to Examples 17, 19, and 20. Examples 22-24 Examples 22-24 were prepared and tested using the procedure described in Examples 9-16 and illustrate that durability can be enhanced when the substrate onto which the retroreflective sheeting is mounted is first treated with a solution ∞ntaming a phenolic resin (5% w/w - Example 23; 1% w/w - Example 24) Scotchlite™ 3870 High Intensity Grade retroreflective sheeting was applied to aluminum panels after they had been treated, dried, and allowed to cool. TABLE 6 48 Month Exposure Example Aluminum Pon Phenolic Resin Total Retained Surface ON Retro¬ Number reflectance 22* etch and desmut none - 74 23 etch and desmut Superbeckacite™ 476 85 1003 24 etch and desmut Superbeckacite™ 476 84 1003 •Comparative Example The data in Table 6 shows that samples of the invention demonstrated superior retroreflectance after being exposed to the sun and other elements for 48 months. Further, Examples 23 and 24 also showed significantly less lifting of the top film at test panel edges when compared to Example 22, which used no phenolic resin. Examples 25-29 Examples 26-29 show how adding small amounts of phenolic resin to the binder layer of a retroreflective sheeting can enhance durability. The procedures described in Example 12 of U.S. 4,025,159 were followed to prepare an encapsulated lens retroreflective sheeting. Small amounts (1% w/w) of several resins were added to the xylene solution of a methylmethacrylate/ethyl acrylate/iso-octylacrylate terpolymer, DuPont™ TiPure™ R960 Titanium Dioxide, and tetra-ethyleneglycol diacrylate. After the sheeting samples were prepared, they were coated with a 90/102-methylbutyl acrylate acrylic acid which contained 5% (w/w) Emery Plastolein™ 9754U plasticizer previously available from Emery Corporation (now believed to be available from Henkel Corporation, Gulph Mills or Ambler, Pennsylvania, or Cincinnati, Ohio) Nirez™ 5120 is a terpene resin formerly available from Reichold Chemical Table 7 shows the retention of retroreflectance and appearance observations for Examples 25-29 after being exposed outdoors for 24 months in Miami, Florida and being tested in accordance with ASTM E 810-91. TABLE 7 •Comparative Examples ••Exhibited visual darkening indicative of impending large loss of rclroreflectivity The data in Table 7 demonstrate that non-phenolic tackifiers show little or no improvement in durability as compared to the control sample (Example 25), whereas, the inventive samples demonstrated improved durability as shown by the higher percent retained retroreflectance values. This invention may take on various modifications and alterations without departing from the spirit and scope thereof. Accordingly, it is to be understood that this invention is not to be limited to the above-described, but is to be controlled by the limitations set forth in the following claims and any equivalents thereof. It is also to be understood that this invention may be suitably practiced in the absence of any element not specifically disclosed herein

Claims: CLAIMS: 1. A retroreflective article that comprises: a retroreflective constituent that includes optical elements and a reflective layer that contains elemental aluminum, the reflective layer that contains elemental aluminum being disposed behind the optical elements; and a phenolic resin in chemical association with the elemental aluminum, the phenolic resin having a total hydroxyl number at least 90; with the proviso that if the retroreflective article possesses an adhesive layer for mounting the article to a substrate, the phenolic resin is incorporated in the adhesive layer at less than 25 percent by weight. 2. The retroreflective article of claim 1, further comprising a layer of pressure-sensitive adhesive for mounting the retroreflective article on a substrate, the pressure-sensitive adhesive containing 0.5 to 25 weight percent of the phenolic resin. 3. The retroreflective article of claim 2, wherein the pressure-sensitive adhesive consists essentially of (1) acrylic esters of non-tertiary alkyl alcohols having 1 to 14 carbon atoms, and (2) at least one monomer copolymerizable with the acrylic acid ester and selected from the group consisting of acrylic acid, methacrylic acid, itaconic acid, acrylamide, methacrylamide, acrylonitrile, and methacrylonitrile. 4. The retroreflective article of claims 1-3, wherein the phenolic resin includes the reaction products of an aldehyde and a phenol. 5. The retroreflective article of claim 4, wherein the aldehyde is selected from the group consisting of formaldehyde, acetaldehyde, anisaldehyde, cinnamaldehyde, furaldehyde, proprionaldehyde, oxalaldehyde, and salicylaldehyde and combinations thereof, and the phenol is selected from the group consisting of phenol, p-tert butylphenol, p-tert octylphenol, p-nonylphenol, methylphenol, 1,3-benezenediol, 4,4 , -isopropylidenediphenyl, and 2,3-xylenol, and combinations thereof. 6. The retroreflective article of claims 1-5, wherein the phenolic resin is the reaction product of formaldehyde and one or more of the following: phenol, p-tert butylphenol, p-tert octylphenol, p-nonylphenol, and 1,3-benezenediol. 7. The retroreflective article of claims 1-6, wherein the phenolic resin has a total hydroxyl number of 90 to 1, 100. 8. The retroreflective article of claims 1-7, wherein the phenolic resin has a total hydroxyl number of 100 to 500. 9. The retroreflective article of claims 1-9, wherein the retroreflective article is a beaded lens sheeting, and the phenolic resin is disposed in the binder layer of the beaded lens sheeting at 0.5 to 25 weight percent. 10. The retroreflective article of claims 1-8, wherein the retroreflective article is a cube-comer sheeting, and the phenolic resin is disposed in a backing layer at 0.5 to 25 weight percent. 11. The retroreflective article of claims 1-10, being an encapsulated lens sheeting. 12. The retroreflective article of claim 11, wherein the encapsulated lens sheeting includes a layer of a pressure sensitive adhesive to secure the encapsulated lens sheeting to a substrate, the phenolic resin being disposed in the layer of pressure sensitive adhesive at 5 to 20 weight percent. 13. The retroreflective article of claims 1-12, wherein the article is capable of demonstrating at least an 80 percent retained retroreflectance after 48 months when tested in accordance with ASTM E 810-91.

Inventors: DANIELS MICHAEL P, KETOLA WARREN D, PAVELKA LEE A, DANIELS, MICHAEL, P., KETOLA, WARREN, D., PAVELKA, LEE, A.
Applicants: MINNESOTA MINING & MFG, MINNESOTA MINING AND MANUFACTURING COMPANY
Application year: 1995
Main classification: G02B5
IPC codes: B32B7, B32B15, B32B27, E01F9, G02B5