SELF-ADHESIVE RETROREFLECTIVE SHEETING

Abstract: Retroreflective sheeting comprises (a) a flat reflector having at least one primed or unprimed specularly reflective surface, the reflector being selected from the group consisting of metallized paper and metallized polymer; and (b) a dispersion of substantially spherical, substantially transparent microparticles in a light-transmissive, pressure-sensitive adhesive binder layer, the dispersion being adjacent to and in direct contact with at least one specularly reflective surface; wherein the microparticles are embedded in the binder layer and are spaced from the reflector by an essentially uncontrolled distance. The sheeting exhibits good retroreflective properties while being cost-effective to manufacture.

Description: SELF-ADHESΓVΈ RETROREFLECTIVE SHEETING Field of the Invention This invention relates to retroreflective sheeting comprising a reflector and transparent particles and to methods for preparing such sheeting. Background of the Invention Transparent particle-based retroreflective articles are well known and have been widely used for safety purposes (e.g., as warning emblems on vehicles and hazard warning signs on roadways) and for information purposes (e.g., as traffic control signs and navigational signs). Retroreflective articles reflect incident light rays substantially back toward the light source as a cone of light. Thus, light emitted by the headlights of a motor vehicle toward a sign with a retroreflective face will be reflected back toward the vehicle so as to be visible to the occupants of the vehicle. In order for retroreflective articles to most effectively function in safety and informational applications, it is desirable that incident light be maximally reflected (high retroreflective brightness) over a wide range of entrance angles (wide entrance angularity). Numerous improvements in these properties have been achieved through the years. For example, spacer coatings and spacer films have been used to carefully control the spacing between a layer of transparent particles and an underlying specular reflector. This enables an optimization of retroreflective brightness by placing the reflector at the approximate location where light rays are focused by particles of a particular refractive index. Wide entrance angularity has been simultaneously achieved with, e.g., spacers of substantially uniform thickness. In addition to spacers, curved reflectors, e.g., in the form of pigmented binders and metal vapor coatings, have been used to obtain further improvements in optical properties. A major drawback of such approaches, however, is that the property improvements have generally been achieved through the development of more complex structures of necessarily higher cost. There is thus a need in the art for retroreflective articles that maintain good retroreflectivity properties while being less expensive to produce. In particular, there is a need for low cost retroreflective sheeting. Such sheeting, which is often flexible, is perhaps the most common form of retroreflective article and can be adhered to a variety of substrates such as aluminum sign panels or vehicle surfaces. Generally, adhesion to such substrates is achieved by coating an adhesive layer on the back of the sheeting or on its top coat or top sheet, thereby further increasing its cost. A means of avoiding the need for this additional layer is accordingly highly desirable. Summary of the Invention Briefly, in one aspect, this invention provides retroreflective sheeting comprising (a) a flat reflector having at least one primed or unprimed specularly reflective surface, the reflector being selected from the group consisting of metallized paper and metallized polymer; and (b) a dispersion of substantially spherical, substantially transparent microparticles in a light-transmissive, pressure- sensitive adhesive binder layer, the dispersion being adjacent to and in direct contact with at least one specularly reflective surface; wherein the microparticles are embedded in the binder layer and are spaced from the reflector by an essentially uncontrolled distance. Preferably, the reflector is metallized paper or metallized polymer film, and the substantially transparent microparticles are glass microspheres. As used herein, the term "metallized" means coated with a deposit of metal. The retroreflective sheeting of the invention exhibits good retroreflective properties while being easily and cost-effectively manufacturable using simple process steps. Surprisingly, the retroreflectivity properties of more complex sheeting structures are maintained to an unexpected degree, without the need for careful control of parameters such as reflector shape or the spacing between the microparticles and the reflector. The retroreflective sheeting is useful for applications that require good retroreflectivity properties at low cost, e.g., temporary applications such as temporary markings for vehicles and signs for short- term advertising, informational, and directional purposes. If desired, a top coat or a top sheet (e.g., of polymer film) can be added to the retroreflective sheeting for increased durability and/or improved retroreflective performance under exposure to environmental conditions. The retroreflective sheeting of the invention is particularly useful, however, for applications where the pressure-sensitive adhesive binder layer of the sheeting can also function as the means for attachment to a substrate, e.g., windshield sticker applications. It has been discovered that light-transmissive, pressure-sensitive adhesive binder layers can simultaneously function as an optical component and as an attachment means. In other aspects, this invention also provides a process for preparing the retroreflective sheeting of the invention and an article comprising the sheeting. Brief Description of the Drawings These and other features, aspects, and advantages of the present invention will become better understood with regard to the following description, appended claims, and accompanying drawings, wherein: Figures 1 and 2 show sectional views of portions of two embodiments of the retroreflective sheeting of the invention. These figures, which are idealized, are not drawn to scale and are intended to be merely illustrative and nonlimiting. Detailed Description of the Invention Flat reflectors suitable for use in preparing the retroreflective sheeting of the invention comprise paper or similarly flat polymeric backings bearing a coating of deposited metal. The metal can be vacuum-deposited, chemically-deposited, vapor- coated, or deposited by other means so as to provide a specularly reflective surface. Aluminum or silver vapor-coatings are typically preferred because they tend to provide the highest retroreflective brightness, but other metals (e.g., gold) or mixtures of metals can be utilized, if desired. The reflective color of silver coatings is generally preferred to that of aluminum coatings. However, aluminum degrades less in response to outdoor exposure, and aluminum is also preferable from the standpoint of cost. Suitable paper and polymeric backings are those which exhibit sufficient surface smoothness to permit the formation of a specularly reflective surface (through metallization) and suflBcient strength and stability to enable sheeting manufacture and use. Representative examples of suitable papers include coated papers, e.g., clay- or polymer-coated papers, and highly calendered papers. Representative examples of suitable polymeric backings include polyesters, e.g., polyethylene terephthalate (PET), polyolefins, e.g., polyethylene and polypropylene, cellulose acetate, and the like. Copolymers and polymer blends can also be utilized. Preferred backings are those that exhibit at least some flexibility. Thus, paper and polymeric film are preferred, with paper being more preferred in view of cost considerations. If desired, the paper or polymeric backing can be treated or its composition modified so as to enhance the adhesion of the metal coating to the backing. For example, the backing can be corona- or electron beam- treated or coated with an adhesion-promoting composition, or, alternatively, the backing can contain added adhesion-promoting compounds or chemical groups. The backing can also contain other types of additives, e.g., fillers or plasticizers. Microparticles suitable for use in preparing the retroreflective sheeting of the invention are generally substantially spherical in shape in order to provide the most uniform and efficient retroreflection. Suitable microparticles are also substantially transparent, at least to the wavelengths of light for which retroreflection is desired, so as to minimize light absorption and thereby maximize the amount of light that can potentially be retroreflected. The microparticles are preferably substantially free of bubbles and other internal discontinuities that may negatively impact the desired retroreflection. The microparticles are also preferably colorless, although they may be colored to produce special effects if desired. Accordingly, suitable microparticles can be of ceramic, e.g., glass, or synthetic resin having the optical properties and physical characteristics described herein. Glass microspheres are generally preferred, due to their generally lower cost, their hardness, and their high durability. Useful glass compositions include barium titanates and lead silicates, both typically with modifiers, but other compositions can also be utilized. The microparticles can be coated, e.g., with a fluorochemical or with an adhesion promoter, if desired. The microparticles can generally have an average diameter of between about 30 and about 850 microns. Microparticles that are smaller than this range may tend to provide lower levels of retroreflection because of diffraction effects, whereas microparticles larger than this range may tend to impart undesirably high thicknesses to the resultant sheeting or to render the sheeting more susceptible to cracking when flexed. Preferably, the microparticles have an average diameter of between about 40 and about 200 microns (more preferably, between about 50 and about 120 microns). The refractive index of the microparticles used in the retroreflective sheeting can generally be between about 1.50 and about 2.60 (preferably, between about 1.70 and about 2.30). However, microparticles having refractive indices outside this range can also be utilized. Suitable binder materials for use in binder compositions for preparing the retroreflective sheeting of the invention are those that will form, e.g., upon drying or curing, a pressure-sensitive adhesive binder layer that will bind to the microparticles and to the reflector (and to a top coat or top sheet, if utilized). As used herein, the term "pressure-sensitive adhesive" means that the binder layer exhibits the balance of the properties of adhesion, cohesion, stretchiness, and elasticity that is recognized in the art as being characteristic of pressure-sensitive adhesives, and that the binder layer is normally tacky and exhibits pressure sensitive tack (as determined, e.g., by the thumb test described by D. Satas in the Handbook of Pressure Sensitive Adhesive Technology, Second Edition, page 39, Nan Nostrand Reinhold, New York (1989)) at normal use temperatures, e.g., at least some temperatures within the range of from about -15 to about +35 degrees centigrade. The effective meaning of the term "pressure-sensitive adhesive" thus is that the resultant sheeting is self-adhesive or self-attaching (to a substrate) under normal use conditions. Surprisingly, the balance of pressure-sensitive adhesive (PSA) properties can be sufficiently maintained to enable bonding, in spite of the microparticle loading that is essential to obtaining retroreflectivity. Preferred binder materials exhibit, upon drying or curing to form the binder layer, sufficient adhesive and cohesive strength to enable the sheeting to adhere to a substrate surface and to remain adhered until removal is desired. In addition, the binder layer will preferably be flexible and dimensionally stable (so that the sheeting will maintain its structural integrity when exposed to environmental conditions). Suitable binder materials are light-transmissive, so as to form a binder layer having a suitable refractive index (preferably, between about 1.4 and about 1.6) that will enable the sheeting to provide the desired retroreflection. Representative examples of suitable binder materials include acrylic PSAs, silicone PSAs, rubber resin PSAs, poly(alpha olefin) PSAs, other PSAs that possess the above-described requisite optical characteristics, and mixtures thereof. Preferred binder materials include acrylic PSAs and silicone PSAs (more preferably, acrylic PSAs). In addition to binder material, the binder composition can further comprise additives (at levels that do not impair the retroreflectivity of the sheeting to a degree that is unacceptable for a particular application) such as whitening agents, e.g., titanium dioxide pigment, to increase the overall whiteness of the resultant sheeting; coloring agents, e.g., dyes or pigments; adhesion promoters, e.g., silane coupling agents, to enhance the adhesion of the resulting binder layer to the reflector and/or to the microparticles; crosslinkers; curatives; heat- or ultraviolet-stabilizers; plasticizers; tackifiers; and the like. The retroreflective sheeting of the invention can be prepared by coating at least a portion of at least one primed or unprimed specularly reflective surface of a chosen reflector with a dispersion or slurry of selected microparticles in binder composition. The microparticles can generally constitute from about 4 to about 80 weight percent (preferably, from about 10 to about 50 weight percent; more preferably, from about 15 to about 40 weight percent) of the slurry, based upon the total weight of microparticles and binder material. The microparticles can be of a single average diameter and/or a single refractive index. Alternatively, a mixture of microparticles of two or more average diameters and/or two or more refractive indices can be utilized. Application of the slurry to the reflector can be carried out by known techniques (e.g., knife coating, bar coating, roll coating, spraying, extrusion coating, and screen printing) that can provide a coating wherein the microparticles become embedded in the resulting binder layer upon drying or curing. Solvents (e.g., n- butanol, water, or other binder-compatible solvents, and mixtures thereof) can be utilized, if desired, to adjust the viscosity of the binder composition and/or the resulting dispersion or slurry to levels suitable for a particular application technique. The retroreflective sheeting of the invention can comprise other components. For example, if desirable for a particular purpose or use, a top sheet or top coat can optionally be added to the retroreflective sheeting (on top of the dispersion of microparticles in binder layer). Selection of a suitable top or cover sheet (or coat) will be determined in part by the conditions to which the sheeting will be exposed, by its desired properties (e.g., color, outdoor durability, flexibility, glossy appearance, etc.), and by the characteristics of the binder layer. An optional adhesive layer (e.g., a layer of heat-activatable, pressure-sensitive, or chemically- activatable adhesive) can also be applied to the backside of the reflector for use in attachment of the sheeting to a substrate surface. Other optional components are possible, including primers and printing materials. For example, the specularly reflective surface of the reflector can be primed by applying a coating of a primer composition or by other treatment to improve the adhesion of the dispersion to the reflector. A patterned prime coat can be utilized to provide differential adhesion and, when printed thereon, a security feature that can indicate tampering. Printing inks (or other printing materials) can be applied either prior to or subsequent to the application of the dispersion (provided that retroreflectivity and adhesion are not impaired to a degree that is unacceptable for a particular application). The sheeting can also further comprise metallized or non-metallized holographic film, water marks, or other types of security marks generated by various means, e.g., by a laser imaging process such as those described in U.S. Patent Nos. 4,688,894 (Hockert) and 4,200,875 (Galanos). These and other features of the retroreflective sheeting of the invention can be better understood by reference to the accompanying drawings, where Figure 1 shows an embodiment 10 of the retroreflective sheeting of the invention wherein a flat metallized polymer reflector consisting of a polymer sheet 12 bearing an aluminum vapor coat 14 has been coated with a dispersion of glass microspheres 18 in binder layer 16. The microspheres 18 are a mixture of microspheres of two different diameters. Figure 2 shows a second embodiment 10 wherein a flat metallized paper reflector consisting of a paper sheet 12 bearing an aluminum vapor coat 14 has been coated with a dispersion of glass microspheres 18 in binder layer 16. In this embodiment, the microspheres 18 are of a single size. In these and other embodiments of the invention, the microspheres are embedded in the binder layer, and the spacing of the individual microspheres from the reflector is essentially uncontrolled and can vary (so as to not be uniform) throughout the dispersion. Objects and advantages of this invention are further illustrated by the following examples, but the particular materials and amounts thereof recited in these examples, as well as other conditions and details, should not be construed to unduly limit this invention. In the following Examples, all parts are by weight unless otherwise specified. Examples Test Methods: Retroreflectivity The retroreflectivity of the sheeting of the invention was measured using ASTM (American Society for Testing and Materials) Method E-810. Results were recorded in units of candelas / (lux • m 2 ) (cd /(lx - m 2 )). 90° Peel Adhesion This property was measured according to ISO (International Organization for Standardization) 8510-1. Results were recorded in units of N / 2.54 cm. Thickness The thickness of the retroreflective sheeting of the invention was measured with an electronic caliper gauge (commercially available as Model 51D2 from Lorenzen & Wettre, Stockholm, Sweden). Example 1 Three parts of a mixture of glass microspheres of three different sizes and three different refractive indices (consisting of a first set having a refractive index of 2.26 and an average diameter of 71 microns, a second set having a refractive index of 2.25 and an average diameter of 59 microns, and a third set having a refractive index of 1.93 and an average diameter of 65 microns) were combined with 6 parts by weight of an acrylate-based pressure-sensitive adhesive (a copolymer of isooctyl acrylate and acrylic acid in a weight ratio of 93/7 dissolved in an ethyl acetate and heptane mixture at approx. 25% solids). The resulting dispersion of glass microspheres in solvent-borne PSA was knife-coated onto aluminum vapor-coated polyester film (having a total thickness of 23 microns; available as RENAPAC MΝ from Tricon Neredlungs GmbH, Freiburg, Germany) at a wet thickness of 250 microns and was dried 1 min at room temperature, 1 min at 70°C in a forced air oven, and 1 min at 120°C in a forced air oven to give a dry coating thickness of 77 microns. The retroreflectivity properties of the sheeting were evaluated using the above-cited test method. Retroreflectivity values for a variety of standard combinations of observation angle and entrance angle are summarized in Table 1. The pressure-sensitive adhesive character of the retroreflective surface of the sheeting was evaluated by measuring 90° peel adhesion. The normally tacky pressure-sensitive adhesive surface was adhered to aluminum, to painted stainless steel, and to glass surfaces, respectively. Adhesion measurements were made after dwell times of 30 min and 100 hrs. The adhesion results are summarized in Table 2. Example 2 Example 1 was repeated using 3 parts of the microsphere mixture and 12 parts of the adhesive solution. The resulting dispersion was again coated at 250 microns wet thickness and dried essentially as in Example 1 to give a dry thickness of 78 microns. The results of retroreflectivity measurements for the resulting sheeting are summarized in Table 1. 90° peel adhesion was measured on aluminum, painted stainless steel, and glass after dwell times of 30 min and 100 hrs. The adhesion results are summarized in Table 2. Table 1. Table 2. Comparative Example 1 An acrylate-based, tackified, uncrosslinked pressure-sensitive adhesive comprising an acrylic copolymer of 93 parts by weight isooctyl acrylate and 7 parts by weight acrylic acid having an inherent viscosity in the range of 1.5 - 1.75 and NIREZ 2019 tackifier (available from Arizona Chemical, Panama City, Florida; a terpene phenol resin), as generally described in U.S. Patent No. 4,418,120, was coated using a knife coater onto the metallized side of a 23 micron thick polyester film which had been vapor coated on one side with aluminum (available as REVAPAC MN from Tricon Veredlungs GmbH, Freiburg, Germany). The adhesive-coated film was dried first at room temperature for 2 minutes and then in a forced air oven (80°C) for 2 minutes. 90° Peel adhesion measurements were made using glass as a substrate, and the mechanism of bond failure was recorded. Retroreflectivity was also evaluated. The results are shown in Tables 3 and 4 below. Examples 3-9 The adhesive of Comparative Example 1 was combined with varying amounts of solid glass beads. The beads had a refractive index of 2.26 and an average diameter of 59 microns. The resulting mixtures were coated using a knife coater onto the metallized side of a 23 micron thick polyester film which had been vapor coated on one side with aluminum (available as REVAPAC MN from Tricon Veredlungs GmbH, Freiburg, Germany). The coated film was dried first at room temperatμre for 2 minutes and then in a forced air oven (80°C) for 2 minutes. 90° Peel adhesion measurements were made using glass as a substrate, and the mechanism of bond failure was recorded. Retroreflectivity was also evaluated. The mixture compositions and test results are summarized in Tables 3 and 4 below. Examples 10-13 Examples were prepared essentially as in Examples 3-9, with the exception that the glass beads utilized had a refractive index of 1.93 and an average diameter of 65 microns. 90° Peel adhesion from glass was measured, as well as retroreflectivity. The mixture compositions and test results are summarized in Tables 3 and 4 below. Table 3. Failure Modes: adh-film = the adhesive coating separated from the film and remained adhered to the substrate mixed = the adhesive coating remained partly on the film and partly on the substrate adh-sub = the adhesive coating remained completely on the film and separated cleanly from the substrate cohes = the adhesive coating showed cohesive failure Table 4. Comparative Example 2 A solvent-based, silicone-based pressure-sensitive adhesive (available as Q2 7735 from Dow Corning) was coated onto the aluminized surface of the aluminum vapor-coated polyester film described in Examples 3-13 using a knife coater at a wet coating thickness of approximately 250 microns and dried 2 minutes in a forced air oven at 70°C, followed by an additional 5 minutes at 150°C. 90°Peel adhesion measurements were made using glass as a substrate, and the mechanism of bond failure was recorded. Retroreflectivity was also evaluated. The results are shown in Tables 5 and 6 below. Examples 14-17 A solvent-based, silicone-based pressure-sensitive adhesive (available as Q2 7735 from Dow Corning) was combined with varying amounts of solid glass beads. The beads had a refractive index of 2.26 and had an average diameter of 59 microns. The resulting mixtures were coated using a knife coater onto the metallized side of a 23 micron thick polyester film which had been vapor coated on one side with aluminum (available as RENAPAC MΝ from Tricon Neredlungs GmbH, Freiburg, Germany). The thickness of the wet adhesive coating was approximately 250 microns. The adhesive-coated film was dried 2 minutes in a forced air oven at 70°C, followed by an additional 5 minutes at 150°C. 90° Peel adhesion measurements were made using glass as a substrate, and the mechanism of bond failure was recorded. Retroreflectivity was also evaluated. The mixture compositions and test results are summarized in Tables 5 and 6 below. Table 5 . Failure Modes: mixed = the adhesive coating remained partly on the film and partly on the substrate adh-sub = the adhesive coating remained completely on the film and separated cleanly from the substrate Table 6. Various modifications and alterations of this invention will become apparent to those skilled in the art without departing from the scope and spirit of this invention.

Claims: What Is Claimed Is: 1. Retroreflective sheeting comprising (a) a flat reflector having at least one primed or unprimed specularly reflective surface, said reflector being selected from the group consisting of metallized paper and metallized polymer; and (b) a dispersion of substantially spherical, substantially transparent microparticles in a light-transmissive, pressure-sensitive adhesive binder layer, said dispersion being adjacent to and in direct contact with at least one said specularly reflective surface; wherein said microparticles are embedded in said binder layer and are spaced from said reflector by an essentially uncontrolled distance. 2. The sheeting of Claim 1 wherein said reflector is selected from the group consisting of metallized paper and metallized polymer film. 3. The sheeting of Claim 2 wherein said polymer is selected from the group consisting of polyester, polyolefin, cellulose acetate, and blends thereof. 4. The sheeting of Claim 1 wherein the metal used for metallizing said metallized paper and metallized polymer is selected from the group consisting of aluminum and silver. 5. The sheeting of Claim 1 wherein said microparticles are made of glass. 6. The sheeting of Claim 5 wherein said microparticles have an average diameter of from about 30 to about 850 microns. 7. The sheeting of Claim 5 wherein said microparticles have a refractive index in the range of from about 1.50 to about 2.60. 8. The sheeting of Claim 5 wherein said microparticles comprise a blend of at least two sets of microparticles, each set having a different average microparticle diameter and/or a different refractive index. 9. The sheeting of Claim 1 wherein said binder layer comprises pressure-sensitive adhesive selected from the group consisting of acrylic pressure- sensitive adhesives, silicone pressure-sensitive adhesives, rubber resin pressure- sensitive adhesives, poly(alpha-olefin) pressure- sensitive adhesives, and mixtures thereof. 10. The sheeting of Claim 1 wherein said microparticles constitute from about 4 to about 80 weight percent of said dispersion. 11. The sheeting of Claim 1 further comprising a top coat or top sheet. 12. The sheeting of Claim 1 further comprising printed indicia. 13. Retroreflective sheeting comprising (a) a flat reflector having at least one primed or unprimed specularly reflective surface, said reflector being selected from the group consisting of aluminized paper and aluminized polyester film; and (b) a dispersion of glass microspheres in a light-transmissive, pressure-sensitive adhesive binder layer comprising acrylic pressure-sensitive adhesive, said dispersion being adjacent to and in direct contact with at least one said specularly reflective surface; wherein said microparticles are embedded in said binder layer and are spaced from said reflector by an essentially uncontrolled distance. 14. The sheeting of Claim 13 wherein said specularly reflective surface bears a security mark generated by a laser imaging process. 15. A retroreflective article comprising the sheeting of Claim 1. 16. A process for preparing retroreflective sheeting comprising coating at least a portion of at least one primed or unprimed specularly reflective surface of metallized paper or metallized polymer with a dispersion of substantially spherical, substantially transparent microparticles in a binder composition that upon curing or drying forms a light-transmissive, pressure-sensitive adhesive binder layer, said coating being carried out such that said microparticles become embedded in said binder layer upon drying or curing of said binder composition. 17. The process of Claim 16 wherein said microparticles constitute from about 4 to about 80 weight percent of said dispersion. 18. The process of Claim 16 wherein said microparticles have an average diameter of from about 30 to about 850 microns. 19. The process of Claim 16 wherein said microparticles have a refractive index in the range of from about 1.50 to about 2.60. 20. The process of Claim 16 wherein said microparticles comprise a blend of at least two sets of microparticles, each set having a different average microparticle diameter and/or a different refractive index. 21. The process of Claim 16 wherein said binder layer comprises pressure-sensitive adhesive selected from the group consisting of acrylic pressure- sensitive adhesives, silicone pressure-sensitive adhesives, rubber resin pressure- sensitive adhesives, poly(alpha-olefin) pressure-sensitive adhesives, and mixtures thereof. 22. A process for preparing retroreflective sheeting comprising coating at least a portion of at least one primed or unprimed specularly reflective surface of aluminized paper or aluminized polyester film with a dispersion of glass microspheres in a binder composition that upon curing or drying forms a light- transmissive, pressure-sensitive adhesive binder layer comprising acrylic pressure- sensitive adhesive, said coating being carried out such that said microspheres become embedded in said binder layer upon drying or curing of said binder composition.

Inventors: WEBER GABRIELE I, WEBER, GABRIELE, I.
Applicants: MINNESOTA MINING & MFG, MINNESOTA MINING AND MANUFACTURING COMPANY
Application year: 1997
Main classification: G02B5
IPC codes: G02B5