RETROREFLECTIVE ARTICLES AND RETROREFLECTIVE ELEMENTS COMPRISING A SPHERICAL CORE AND TWO CONCENTRIC OPTICAL INTERFERENCE LAYERS

• US 20100302639A1
• Filed: 12/16/2008
• Published: 12/02/2010
• Est. Priority Date: 12/21/2007
• Status: Active Grant

• Current Assignee: 3M Innovative Properties Company (3M Company)
• Original Assignee: 3M Innovative Properties Company (3M Company)
• Inventors: Haas, Christopher K., Frey, Matthew H., Krishnan, Vivek, Budd, Kenton D.
• Granted Patent: US 8,496,340 B2
• Time in Patent Office: Days
• US Class Current: 359/542
• CPC Class Codes: G02B 5/128 transparent spheres being embedded in matrix
• CPC Class Codes: G02B 5/286 having four or fewer layers, e.g. for achieving a colour effect

Abstract

Retroreflective elements and articles that include such elements. The retroreflective elements (100) include a solid spherical core (110) having an outer surface. A first complete concentric optical interference layer (112) overlays the outer surface of the core providing a first interface between the core and the first optical interference layer, and a second complete concentric optical interference layer (122) overlays the first optical interference layer to provide a second interface between the first optical interference layer and the second optical interference layer. In some embodiments, a third complete concentric optical interference layer overlays the second optical interference layer to provide a third interface between the second optical interference layer and the third optical interference layer. The retroreflective articles include a substrate having a first major surface and a second major surface with a plurality of the retroreflective elements affixed along the first major surface of the substrate.

Claims

1. A retroreflective element comprising:

a solid spherical core comprising an outer core surface, the outer core surface providing a first interface;

a first complete concentric optical interference layer having an inner surface overlying core surface and an outer surface, the outer surface of the first complete concentric optical interference layer providing a second interface;

a second complete concentric optical interference layer having an inner surface overlying the outer surface of the first complete concentric optical interference layer and an outer surface, the outer surface of the second complete concentric optical interference layer providing a third interface.

2. The retroreflective element according to claim 1, wherein the first complete concentric optical interference layer and the second complete concentric optical interference layer each comprise material selected from the group consisting of TiO₂, SiO₂, ZnS, CdS, CeO₂, ZrO₂, Bi₂O₃, ZnSe, WO₃, PbO, ZnO, Ta₂O₅, Al₂O₃, B₂O₃, MgO, AlF₃, CaF₂, CeF₃, LiF, MgF₂, Na₃AlF₆, and combinations of two or more of the foregoing.

3. The retroreflective element according to claim 1, wherein the difference between the refractive index of the spherical core and the refractive index of the first complete concentric optical interference layer is greater than 0.1;

and wherein the difference between the refractive index of the first complete concentric optical interference layer and the refractive index of the second complete concentric optical interference layer is greater than 0.1.

4. The retroreflective element according to claim 3 wherein the core has a refractive index within the range from 1.55 to 2.5, the first complete concentric optical interference layer comprises a first material and the second complete concentric optical interference layer comprises a second material that is different than the first material.

5. The retroreflective element according to claim 4, wherein the core is spherical having a diameter between 30 microns and 90 microns.

6. The retroreflective element according to claim 5 wherein the first material is silica and the second material is titania.

7. The retroreflective element according to claim 5 wherein the first material is titania and the second material is silica.

8. The retroreflective element according to claim 1 further comprising a third complete concentric optical interference layer having an inner surface overlying the outer surface of the second complete concentric optical interference layer and an outer surface, the outer surface of the third complete concentric optical interference layer providing a fourth interface.

9. A retroreflective article comprising:

A substrate having a first major surface and a second major surface;

A plurality of retroreflective elements according to claim 1 affixed along the first major surface of the substrate, the article being retroreflective.

10. The retroreflective article according to claim 9, having a coefficient of retroreflection, measured at −

4 degrees entrance angle and 0.2 degrees observation angle, greater than 50 Cd/lux/m², the article comprising no auxiliary reflector, the coefficient of retroreflection value being at least 2.5 times greater than that of an article comprising retroreflective elements consisting of the solid spherical core having no complete concentric optical interference layers thereon.

11. The retroreflective article according to claim 9, the article comprising no auxiliary reflector, having a coefficient of retroreflection, measured at −

4 degrees entrance angle and 0.2 degrees observation angle, greater than 50 Cd/lux/m² and a retroreflective color with chromaticity coordinates defining a point on the CIE chromaticity diagram (1931 version) that lies within 0.01 of the line that describes black body emission between 4800K and 7500K.

12. The retroreflective article according to claim 9, further comprising an auxiliary reflector.

13. The retroreflective article according to claim 9, wherein the article, when viewed in a retroreflective mode, exhibits enhanced retroreflective brightness.

14. The retroreflective article according to claim 9, wherein the article, when viewed in a retroreflective mode, exhibits interference-enhanced retroreflective brightness, wherein the article exhibits a coefficient of retroreflection value at least 2 times greater than that of an otherwise identical article comprising retroreflective elements consisting of the solid spherical core having no complete concentric optical interference layers thereon.

15. The retroreflective article according to claim 14, wherein the article exhibits a coefficient of retroreflection value at least 4 times greater than that of an otherwise identical article comprising retroreflective elements consisting of the solid spherical core having no complete concentric optical interference layers thereon.

16. The retroreflective article according to claim 8, wherein the article, when viewed in a retroreflective mode, exhibits retroreflective color.

17. The retroreflective article according to claim 9, wherein the article, when viewed in a retroreflective mode, exhibits enhanced retroreflective brightness and does not exhibit retroreflective color.

18. The retroreflective article according to claim 17, further comprising an auxiliary reflector.

19. The retroreflective article according to claim 9, wherein the retroreflective elements are affixed to the first major surface of the substrate with an adhesive, a portion of each retroreflective element embedded into the adhesive.

20. The retroreflective article according to claim 9, wherein the substrate is a thermoplastic polymer and the retroreflective elements are embedded into the substrate.

21. The retroreflective article according to claim 9 wherein the first substrate comprises a polymer.

22. The retroreflective article according to claim 9, wherein the retroreflective elements further each comprise a third complete concentric optical interference layer having an inner surface overlying the outer surface of the second complete concentric optical interference layer and an outer surface, the outer surface of the third complete concentric optical interference layer providing a fourth interface.

23. The retroreflective article according to claim 22, having a coefficient of retroreflection, measured at −

4 degrees entrance angle and 0.2 degrees observation angle, greater than 100 Cd/lux/m², the article comprising no auxiliary reflector.

24. The retroreflective article according to claim 9, wherein the first complete concentric optical interference layer and, the second complete concentric optical interference layer and the third complete concentric optical interference layer each comprise material selected from the group consisting of TiO₂, SiO₂, ZnS, CdS, CeO₂, ZrO₂, Bi₂O₃, ZnSe, WO₃, PbO, ZnO, Ta₂O₅, Al₂O₃, B₂O₃, MgO, AlF₃, CaF₂, CeF₃, LiF, MgF₂, Na₃AlF₆, and combinations of two or more of the foregoing.

25. The retroreflective article according to claim 22, wherein the difference between the refractive index of the spherical core and the refractive index of the first complete concentric optical interference layer is greater than about 0.1;

and wherein the difference between the refractive index of the first complete concentric optical interference layer and the refractive index of the second complete concentric optical interference layer is greater than about 0.1; and

wherein the difference between the refractive index of the second complete concentric optical interference layer and the refractive index of the third complete concentric optical interference layer is greater than about 0.1.

26. The retroreflective article according to claim 22 wherein the core has a refractive index within the range from about 1.55 to about 2.5, the first complete concentric optical interference layer comprises silica, the second complete concentric optical interference layer comprises titania, and the third complete concentric optical interference layer comprises silica.

27. The retroreflective article according to claim 26, wherein the core is spherical having a diameter between about 30 microns and about 90 microns, wherein the first complete concentric optical interference layer has a thickness between about 95 nm and about 120 nm, the second complete concentric optical interference layer has a thickness between about 45 nm and about 80 nm and the third complete concentric optical interference layer has a thickness between about 70 nm and about 115 nm.

28. The retroreflective article according to claim 22 wherein the spherical core has a refractive index within the range from about 1.55 to about 2.5, the first complete concentric optical interference layer comprises titania, the second complete concentric optical interference layer comprises silica, and the third complete concentric optical interference layer comprises titania.