WO1999034241A1

WO1999034241A1 - Retroreflective sheeting

Info

Publication number: WO1999034241A1
Application number: PCT/US1997/023873
Authority: WO
Inventors: Harald Bielak
Original assignee: Minnesota Mining And Manufacturing Company
Priority date: 1997-12-29
Filing date: 1997-12-29
Publication date: 1999-07-08
Also published as: AU5718798A; EP1058858A1

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, non-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

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). Perhaps the most common form of retroreflective article is retroreflective sheeting, which is often flexible, and which can be adhered to a substrate such as an aluminum sign panel or a vehicle surface. 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.

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, non- 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. 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 sufficient 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 non-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 "non-pressure-sensitive adhesive" means that the binder layer lacks 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 does not exhibit 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 Νostrand Reinhold, New York (1989)) at normal use temperatures, e.g., temperatures within the range of from about -15 to about +35 degrees centigrade. The effective meaning of the term "non-pressure-sensitive adhesive" thus is that the resultant sheeting is not self-adhesive or self-attaching (to a substrate) under normal use conditions, unless the binder layer comprises a heat- activatable or chemically-activatable adhesive and the requisite activating heat or chemical is supplied.

Preferably, the resultant binder layer will also 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 polyvinyl butyral, polyester resins, alkyd resins, acrylic resins, and the like, and mixtures thereof. Such materials can be heat-activatable or chemically-activatable adhesives, if a self-adhesive property, as described above, is desired. Preferably, the binder material comprises polyvinyl butyral. 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; 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 35 to about 85 weight percent (preferably, from about 50 to about 80 weight percent) or from about 15 to about 60 volume percent (preferably, from about 25 to about 57 volume percent) of the slurry, based upon the total weight or total volume 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, 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 to enable 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 • m²) (cd /(lx • m²)).

Cross-cut

The adhesion of the dispersion (of microparticles in binder layer) to the reflector was evaluated using DIN (Deutsches Institut fuer Normung) 53151 and ISO (International Organization for Standardization) 2409-1972.

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).

Brookfield Viscosity

Brookfield viscosity was measured according to DIN 53019.

Example 1 A dispersion of glass microspheres (having a refractive index of 2.26 and an average diameter of 71 microns) in a binder composition (20 weight percent solids) comprising BUTNAR B90 polyvinylbutyral resin (available from Monsanto) was prepared by combining 2 parts by weight of binder composition and 1 part by weight of solid glass microspheres at 23 °C and stirring with a high speed mixer until a homogeneous dispersion was obtained. The resulting dispersion of glass microspheres in binder composition (having a viscosity of 4.80 Pa-s) was coated onto the aluminized surface of an aluminum vapor-coated polyester film having a total thickness of 23 microns (available as RENAPAC MΝ from Tricon Neredlungs GmbH, Freiburg, Germany) using a knife coater. The dispersion was applied at a wet thickness of approximately 250 microns and then 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 final coating weight of 115 g/m² (118 microns thickness).

Samples of the resulting retroreflective sheeting were stored at 23 °C and ambient humidity. 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.

Example 2

Example 1 was repeated with the exception that an aluminum vapor-coated paper was used as the reflector backing. The dispersion of Example 1 was coated onto the aluminized side of an aluminum vapor-coated paper having a total thickness of 75 microns (83 g/m²; available as "ML-83 - smooth version" from Tricon Neredlungs GmbH, Freiburg, Germany) using a knife coater. The aluminum vapor coating had a thickness of between 1 and 3 microns. The thickness of the resulting dried coating was 104 microns (coating weight of 100 g/m²). The retroreflectivity properties of the resulting sheeting were measured essentially as in Example 1, and the results are summarized in Table 1.

Comparative Example 1

Retroreflectivity measurements were carried out using the aluminum vapor- coated surface of the aluminized film of Example 1 without a coating of dispersion. The results of the retroreflectivity measurements are shown in Table 1. Comparative Example 2

Example 1 was repeated, with the exception that a non-metallized 75 micron thick polyester film (available as 2600 RN 75 from Hoechst) was employed as the "reflector". The results of retroreflectivity measurements are shown in Table 1.

Example 3

A dispersion of glass microspheres in binder composition was prepared by combining 1 part glass microspheres (having an average diameter of 59 microns and a refractive index of 2.25) and 2 parts of the binder composition described in Example 1. The dispersion (viscosity of 4.90 Pa-s) was coated onto aluminum vapor-coated polyester essentially as in Example 1, using a knife coater. The dry coating weight of the dispersion was 102 g/m² (106 microns thickness). The physical characteristics of the resulting sheeting are summarized in Table 2, and the results of retroreflectivity measurements are summarized in Table 1.

Example 4

A dispersion was prepared by combining 1 part glass microspheres (having a average diameter of 65 microns and a refractive index of 1.93) and 2 parts of the binder composition described in Example 1. The dispersion (viscosity of 4.50 Pa-s) was coated onto aluminum vapor-coated polyester essentially as in Example 1. The dry coating weight of the dispersion on the aluminized polyester was 102 g/m² (106 microns thickness). The physical characteristics of the resulting retroreflective sheeting are summarized in Table 2, and the results of retroreflectivity measurements are summarized in Table 1.

Example 5

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 of the binder composition described in Example 1 to form a dispersion. The dispersion was coated onto the aluminum vapor-coated surface of a polyester film and dried essentially as in Example 1. The dry coating weight of the dispersion was 103 g/m² (107 microns thickness). The physical properties of the resulting retroreflective sheeting are shown in Table 2, and the results of retroreflectivity measurements are summarized in Table 1.

Example 6 A dispersion was prepared by combining 3 parts of the microsphere mixture described in Example 5 and 9 parts of the binder composition described in Example 1. The dispersion was coated onto the aluminum vapor-coated surface of a polyester film and dried essentially as in Example 1. The dry coating weight of the dispersion was 96 g/m² (97 microns thickness). The physical properties of the resulting retroreflective sheeting are shown in Table 2, and the results of retroreflectivity measurements are summarized in Table 1.

Example 7

A dispersion was prepared by combining 3 parts of the microsphere mixture described in Example 5 and 12 parts of the binder composition described in

Example 1. The dispersion was coated onto the aluminum vapor-coated surface of a polyester film and dried essentially as in Example 1. The dry coating weight of the dispersion was 89 g/m² (88 microns thickness). The physical properties of the resulting retroreflective sheeting are shown in Table 2, and the results of retroreflectivity measurements are summarized in Table 1.

Examples 8-10

Example 1 was repeated with the exception that the dispersion was applied to aluminum vapor-coated polyester in three different thicknesses: one that was greater than the 250 micron thickness in Example 1 and two that were less.

Specifically, Examples 8-10 were prepared with coatings having wet thicknesses of 450 microns, 200 microns, and 150 microns, respectively. Dry thicknesses in Examples 8-10 were 135 microns, 100 microns, and 85 microns, respectively. The physical properties of the resulting retroreflective sheeting are shown in Table 2, and the results of retroreflectivity measurements are shown in Table 1.

Table 1.

Table 2.

20 weight percent solids Example 11

Example 1 was repeated with the exception that the coating of dispersion was allowed to rest undisturbed on the surface of the aluminized polyester for 15 min at room temperature before it was placed in an oven. Drying was then effected at 70°C for 1 min and 120°C for 2 min. The results of retroreflectivity measurements for the resulting sheeting are summarized in Table 1.

Example 12 A continuous primer layer consisting of a 50:50 weight to weight mixture of two polyurethane resins (commercially available as NeoRez™ 960 and NeoRez™ 963 from Zeneca) was applied to the aluminized polyester film of Example 1 by gravure printing. The primer was dried by placing the resulting primer-coated reflector in a forced-air oven for 1 min at 80°C. The dried primer coating had a thickness of 2-3 microns.

Transparent solvent-based red ink was applied to the primed side of the reflector using gravure printing in a pattern of circles of 1 cm diameter. The printed reflector was dried 2 min at 80°C in a forced air oven. The dispersion of Example 3 was coated onto the printed, primed side of the reflector and then dried essentially as in Example 1. Cross-cut tests showed that the adhesion of the dispersion to the aluminum vapor coating was improved by using a primer. The resulting retroreflective sheeting had an authentication mark provided by the transparent ink marking between the aluminum vapor coat and the dispersion. The marking was visible under normal illumination conditions. Under retroreflective conditions, however, the marking was not visible. Nisual examination of the sheeting under retroreflective conditions was performed using a hand-held viewer available as Security Laminate Verifier, product number 75-0299-7344-5, from 3M Company, St. Paul, MΝ. Example 13

The aluminized side of the polyester film of Example 1 was primed with a continuous primer layer essentially as described in Example 12. The primer was dried by placing the resulting primed reflector in a forced-air oven for 1 min at 80°C.

An opaque solvent-based black ink was applied to the primed side of the reflector using gravure printing in a pattern of circles of 1 cm diameter. The printed reflector was dried 2 min at 80° in a forced air oven. The dispersion of Example 3 was coated onto the printed, primed side of the reflector and dried essentially as in Example 1. Under normal illumination, the printing between the aluminum coating and the dispersion was visible. Under retroreflective viewing conditions, both retroreflective and non-retroreflective areas (corresponding to areas where opaque ink was absent and present, respectively) could be observed.

Example 14

Retroreflective sheeting was prepared essentially as in Example 12, with the exception that the primer was applied to the aluminized side of the polyester film in a checkerboard pattern with squares having a dimension of 1 cm, rather than in a continuous layer as in Example 12. The above-described cross-cut test was performed, and, upon removal of the adhesive tape, portions of the ink and of the dispersion remained on the aluminized film and portions of each remained with the adhesive tape. The portions remaining on both the film and the tape were in the form of a checkerboard pattern. The retroreflective layer or dispersion was irreversibly destroyed during the separation process.

Example 15

Retroreflective sheeting was prepared essentially as in Example 12 with a full-surface coating of primer under the dispersion. A checkerboard pattern of squares having a dimension of 2 cm was gravure printed on top of the dispersion using a transparent solvent-based red ink. The ink was dried for 5 min at room temperature. Upon viewing the surface of the resulting sheeting under normal illumination conditions, the checkerboard pattern of ink was clearly visible. Under retroreflective conditions, the ink pattern was not visible, and the sheeting had a uniform retroreflectivity.

Example 16

Example 15 was repeated with the exception that a higher viscosity of red ink was employed. Under retroreflective conditions, the ink pattern was clearly visible, and the sheeting had a non-uniform retroreflectivity in the checkerboard pattern of the applied ink. Areas where ink was present had reduced or no retroreflectivity, while areas not coated with ink were retroreflective.

Example 17

Example 1 was repeated, with the additional step of lamination of a layer of acrylic pressure-sensitive adhesive (Scotch™ Adhesive Transfer Tape 467, available from 3M Company, St. Paul, MN) borne on a siliconized release liner to the non- aluminized surface of the polyester film.

Example 18

The aluminum vapor coat of the polyester film of Example 1 was etched away with a laser in certain areas to form an emblem or logo. The polyester film was then used essentially as in Example 1 to produce retroreflective sheeting. The laser marked emblem or logo of the sheeting was visible under both normal illumination and retroreflective viewing conditions.

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, non-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 a binder material selected from the group consisting of polyvinyl butyral, polyester resins, alkyd resins, acrylic resins, and mixtures thereof.

10. The sheeting of Claim 1 further comprising a top coat or top sheet.

11. The sheeting of Claim 1 further comprising printed indicia.

12. 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, non-pressure-sensitive adhesive binder layer comprising polyvinyl butyral, 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.

13. A retroreflective article comprising the sheeting of Claim 1.

14. 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, non-pressure-sensitive adhesive binder layer, said coating being carried out at a coating thickness that is sufficient for said microparticles to become embedded in said non-pressure-sensitive adhesive binder layer upon drying or curing of said binder composition.

15. The process of Claim 14 wherein said microparticles constitute from about 50 to about 85 weight percent of said dispersion.

16. The process of Claim 14 wherein said microparticles have an average diameter of from about 30 to about 850 microns.

17. The process of Claim 14 wherein said microparticles have a refractive index in the range of from about 1.50 to about 2.60.

18. The process of Claim 14 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.

19. 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, non-pressure-sensitive adhesive binder layer comprising polyvinyl butyral, said coating being carried out at a coating thickness that is sufficient for said microspheres to become embedded in said binder layer upon drying or curing of said binder composition.