WO1997037252A1

WO1997037252A1 - A color-fast retroreflective structure

Info

Publication number: WO1997037252A1
Application number: PCT/US1997/004890
Authority: WO
Inventors: David C. Martin
Original assignee: Reflexite Corporation
Priority date: 1996-04-01
Filing date: 1997-03-25
Publication date: 1997-10-09

Abstract

A retroreflective structure includes an outer layer having a first transparent plastic layer having a fluorescent colorant and a second transparent plastic layer having a non-fluorescent colorant which is significantly more stable to prolonged exposure to ultraviolet light than the fluorescent colorant. The second transparent plastic layer is bonded to the first transparent plastic layer. An array of retroreflective elements is bonded to the top layer. An inner layer is bonded to a second side of the array of retroreflective prism elements.

Description

A COLOR-FAST RETROREFLECTIVE STRUCTURE

Background of the Invention

Retroreflective materials are employed for various safety purposes, such as traffic signs. These materials are particularly useful at nighttime when visibility is critical under low light conditions. However, these materials are exposed during daylight hours to harsh ultraviolet light emitted by the sun, which over time can cause deterioration of the materials with a subsequent loss of brightness, color and contrast. Retroreflective materials are typically formed of a sheet of thermoplastic, which has a colorant mixed therein with the polymers. Attached to the sheet of thermoplastic is an array of cube-corner or prismatic retroreflectors as described in U.S. Patent 3,712,706, issued to Stamm on January 23, 1973. Generally, the prisms are made by forming a master die on a flat surface of a metal plate or other suitable material . To form the cube-corner, three series of parallel equidistant intersecting V-shaped grooves 60 degrees apart are inscribed in the plate. The die is then used to process the desired cube-corner array into a flat plastic surface. When the groove angle is 70 degrees, 31 minutes, 43.6 seconds, the angle formed by the intersection of two cube faces (dihedral angle) is 90 degrees and the incident light is retroreflected back to the source.

The efficiency of a retroreflective structure is the measure of the amount of incident light returned within a cone diverging from the axis of retroreflection. A distortion of the prismatic structure adversely affects the efficiency. Furthermore, cube-corner retroreflective elements have low angularity at some orientation angles, for instance, the elements will only brightly reflect light that impinges on it within a narrow angular range centering approximately on its optical axis. Low angularity arises from the inherent nature of these elements which are trihedral structures having three mutually perpendicular lateral faces. The elements are arranged so that the light to be retroreflected impinges into the internal space defined by the faces, and the retroreflection of the impinging light occurs by internal retroreflection of the light from face to face of the element . Impinging light that is inclined substantially away from the optical axis of the element (which is a trisection of the internal space defined by the faces of the element) strikes the face at an angle less than its critical angle, thereby passing through the face rather than being reflected. Further details concerning the structures and the operation of cube- corner microprisms can be found in U.S. Patent

3,684,348, issued to Rowland on August 15, 1972. A method for making retroreflective sheeting is also disclosed in U.S. Patent 3,689,346, issued to Rowland on September 5, 1972. The disclosed method is for forming cube-corner microprisms in a cooperatively configured mold. The prisms are bonded to sheeting which is applied thereover to provide a composite structure in which cube-corner microprisms project from one surface of the sheeting. A problem with fluorescent retroreflective sheeting, such as fluorescent orange roll-up signs that are made from a single layer of colored sheeting, is that the colorant of the transparent fluorescent vinyl window material is gradually consumed with lengthy exposure to ultraviolet light emitted by the sun. As the fluorescent colorant is consumed, the transparent orange vinyl fades to a yellowish clear film, but the retroreflective prism structure continues to function. However, the original color degrades within a short period of time. This time can be anywhere from about three to twelve months in some cases.

Summary of the Invention

A need exists for a fluorescent retroreflective structure that can be exposed to ultraviolet light for a prolonged period of time while not significantly diminishing its color.

The present invention relates to a fluorescent retroreflective structure formed of a layered structure comprised of a transparent fluorescent thermoplastic layer and a transparent non-fluorescent thermoplastic layer that is bonded to the transparent fluorescent thermoplastic layer. An array of retroreflective elements is bonded to the layered structure. A backing layer is bonded to the array of retroreflective elements . In a preferred embodiment, the transparent non- fluorescent layer includes a non-fluorescent colorant, and the transparent fluorescent layer includes a fluorescent colorant. Both colorants have essentially the same color coordinates and the non-fluorescent colorant is significantly more stable to degradation when exposed to ultraviolet light than the fluorescent colorant.

The present invention also relates to a method for forming a fluorescent retroreflective structure. The steps include bonding a transparent fluorescent thermoplastic layer to a transparent non-fluorescent thermoplastic layer to form a layered structure. An array of retroreflective elements is bonded to the layered structure. A backing layer is bonded to the array of retroreflective elements, thereby forming a retroreflective structure.

This invention includes many advantages. One advantage is that a color-fast product or a sheeting can be formed that has one surface that can be truly fluorescent and a second surface that can be formed of a more ultraviolet light stable colorant. Another advantage is if a less fluorescent surface is required, the non-fluorescent color thermoplastic layer is used as the first surface and the array of retroreflective elements is placed against the fluorescent thermoplastic layer. The result of using this structure is the ability to obtain a daytime color that does not possess as a high degree of fluorescence but does fluoresce. The invention can be used in temporary road signs.

Brief Description of the Drawing

Figure 1 is a cross-sectional view of a first embodiment of a retroreflective structure of the present invention. Figure 2 is a cross-sectional view of a second embodiment of the retroreflective structure of the present invention.

Figure 3 is a chart of the results of outdoor weathering color tests of samples of the present invention having a fluorescent layer and non-fluorescent layer.

Figure 4 is a chart of the results of outdoor weathering color tests of samples of a single layer fluorescent sheet.

Detailed Description of the Invention

The features and other details of the method and article of manufacture of the invention will now be more particularly described with reference to the accompanying drawings and pointed out in the claims. The same numeral presented in different figures represent the same item. It will be understood that the particular embodiments of the invention are shown by way of illustration and not as limitations of the invention. The principal features of this invention can be employed in various embodiments without departing from the scope of the invention. All parts and percentages are by weight unless otherwise specified.

A first embodiment of the invention, as shown in Figure 1 in a cross-sectional view, is a retroreflective structure 10. Retroreflective structure 10 is formed of a top layer 12 comprised of an outer fluorescent layer 14 having a fluorescent colorant within it and a non- fluorescent layer 16 having a non-fluorescent colorant within it that is significantly more stable, for example, less susceptible to color deterioration, when exposed to ultraviolet light than the fluorescent colorant contained in the fluorescent layer 14. A retroreflective array 18, which has a window side 20 and facet side 22, is attached to non-fluorescent layer 16 of external sheet 12. The retroreflective array 18 can be formed by casting the prisms on the non-fluorescent layer 16 of top layer 12 and can retroreflect impinging light. A bottom layer 26 is attached by welding to top layer 12 through retroreflective array 18 to form a cover layer over the prism array. Welding can be by heat, infrared, ultrasonic or other suitable means known in the art. In another embodiment, fluorescent layer 14 and non-fluorescent layer 16 can be reversed with the non-fluorescent layer on the outside.

The fluorescent layer 14 is bonded to the non- fluorescent layer 16 by a suitable means, such as fusing, an adhesive, etc. Fluorescent layer 14 is composed of a polymer, which is transparent to visible light. The thermoplastic can be substantially flexible, and sheets of the thermoplastic can be rolled. Preferably, the polymer can comprise a vinyl thermoplastic film, such as polyvinyl chloride, polyvinylidene chloride, etc. The polymer of fluorescent layer 14 includes a colorant that has a color, such as orange, yellow, lime green, etc. In a preferred embodiment, the fluorescent layer is orange in color. Suitable colorants include dyes available that have a fluorescent color and are reasonably stable to prolonged exposure to ultraviolet light. Stability is considered maintaining color coordinates within a given color boundary, such as that described by the ASTM D4956-94 specification, for a period of six months to two years depending on the type of outdoor exposure. Prolonged exposure is considered a year or more under normal traffic sign usage. Fluorescence is considered the emission of radiation in the visible range as a result of absorption of radiation at a shorter wavelength than the wavelength emitted. A suitable transparent fluorescent film of polyvinyl chloride film is available from the American Renolit Corporation of

Whippany, New Jersey, under the name H-1W Renolit Clear, Tinted Fluorescent Orange #22006 with Cyasorb.

The fluorescent layer 14 can have a thickness in the range of between about 0.002 and 0.01 inches. In a preferred embodiment, the thickness is in the range of between about 0.005 and 0.007 inches. The selected thickness is dependent upon the method of fabrication, the thermoplastic selected and the characteristics desired for the retroreflective structure. The non-fluorescent layer 16 can be comprised of the same polymer material as the fluorescent layer 14 and bonded thereto. However, the non-fluorescent layer 16 has a non-fluorescent colorant instead of a fluorescent colorant . A suitable transparent non- fluorescent film of polyvinyl chloride film is available from the American Renolit Corporation under the name H- 1W Renolit Clear, Tinted Orange #22010 with Cyasorb. The non-fluorescent colorant is significantly more stable or not as easily decomposed or otherwise modified chemically upon prolonged exposure to ultraviolet light than a fluorescent colorant. For example, minimal movement in color coordinates shows color stability. The orange fluorescent colorant can fade to white (out of the color box) after approximately 48 hours in a carbon-arc weatherometer while the non-fluorescent orange colorant can still be in the color box after five hundred hours of exposure. An example of a color box is shown in Figure 3 as defined by with the orange color region boundary (Coordinate 1, x=0.550, y=0.360; Coordinate 2, x=0.630, y=0.370; Coordinate 3, x=0.581, y=0.418; Coordinate 4, x=0.516, y=0.394) . The color box coordinates are disclosed in ASTM D4956-95, Table 10 for Color Specification Limits (Daytime) for white, yellow, orange, green, red, blue and brown. The fluorescent colorant and the non-fluorescent colorant can have essentially the same color according to color meter tests. Spectrophotometer or colorimeter having 45°/0° or 0°/45° illumination and viewing geometry is suitable for measuring the color. Color coordinates are defined by tristimulus coordinates corresponding to the CIE 1931 Standard Colorimetric System by standard illuminant C. The retroreflective array 18 includes optical elements that are known in the art, such as cube-corner prisms, four-sided prisms, Fresnel lenses, glass beads, rounded lenses, etc. In one embodiment, retroreflective array 18 has a window side 20 and facet side 22. The retroreflective array 18 can be formed of a transparent polymer that can be selected from a wide variety of polymers that include the polymers of urethane, acrylic ethers, hard epoxy acrylates, etc. Other polymers include polycarbonates, polyesters and polyolefins, acrylated silanes and urethane acrylates. Other types of polymers that are flexible can include polyvinyl chloride, polyvinylidene chloride, polyethylene, polypropylene or any other type of flexible thermoplastic. Preferably, the polymer is cast in a mold with a monomer or oligomer, and the polymerization is initiated by ultraviolet radiation. Preferably, the retroreflective array 18 is formed of cube-corner prism elements having a length along each cube side edge in the range of between about 0.003 and 0.006 inches. In one embodiment, each cube-side edge has a length of about 0.0049 inches.

The top layer 12 provides a transparent substrate and a smooth surface upon which retroreflective array 18 is attached. Top layer 12 can be laminated by attaching the retroreflective array 18 with a transparent adhesive. However, the adhesive should be plasticizer resistant, because plasticizers can degrade many adhesives. Alternatively, top layer 12 can be fused to retroreflective array 18 by heating top layer 12 and retroreflective array 18 to a temperature near the melting points of the top layer and retroreflective array, thereby allowing the layers to fuse. In another embodiment, retroreflective array 18 can be directly cast onto top layer 12 in one step. On top layer 12, a sign message can be emblazoned, such as "Men Working", "Stop", "Caution", etc. In one embodiment, the top layer is orange in color and the lettering is black in color.

Attached to the facet side 22 of the retroreflective array 18 of prismatic elements is bottom layer 26. Bottom layer 26 is comprised of a thermoplastic which is similarly as flexible as the top layer 12. It can be formed of the same material, such as polyvinyl chloride, polyvinylidene chloride, etc. The bottom layer can be transparent to visible light. The bottom layer 26 is joined to at least a portion of the retroreflective array 18 by high frequency welding, heating or other suitable means at surface 28. The portion of the retroreflective array 18 not joined to the prism array is air backed (not metalized to achieve specular reflection) . The prisms operate according to the principle of total internal reflection when air backed. The weld pattern can be in the form of squares having a side length of between about 0.5 and 1.25 inches.

As shown in Figure 2, if an adhesive 30 is employed on the retroreflective array facets, the adhesive can wet the surface of the prisms, thereby destroying the air interface and eliminating the ability of the prism to retroreflect. As a result, a retroreflective coating can be deposited on the surface of the dihedral facets. Typically, the retroreflective coating 32 is formed by sputtering aluminum, silver or gold or by vacuum metalization. Alternatively, metal lacquers, dielectric coatings and other specular reflecting coating materials can be employed.

The retroreflective structure can be formed by various apparatuses and methods that are known in the art . An example of an apparatus and method is disclosed in U.S. Patent 4,244,683, issued to Rowland on January 13, 1981.

The retroreflective structure 10 is formed by bonding fluorescent layer 14, such as a polyvinyl chloride, to a non-fluorescent layer 16, thereby forming a top layer 12. The fluorescent layer 14 can be bonded to non-fluorescent layer 12 by fusing, an adhesive or other suitable means. Retroreflective array 18 is formed by casting or other suitable means and is attached on the window side to top layer 12. Retroreflective array 18 can be attached by an adhesive or fusing. The bottom layer 26, formed of a suitable thermoplastic, such as a polyvinyl chloride, is attached to portions of top layer 12 through retroreflective layer 18, such as by radio frequency welding. The remaining portion of the facet side of the retroreflective layer is air backed.

Alternatively, bottom layer 26 can be attached to the retroreflective array 18 with an adhesive. However, prior to attaching the bottom layer 26, a reflective coating 32 is applied to form a specular reflective layer.

Example

A series of outdoor weathering tests were conducted under ASTM D4956-94 to compare an embodiment of the present invention (a two-ply product) , a two-ply 0.012 inch laminate of vinyl comprising an orange non- fluorescing outer layer, fused to an orange fluorescent second layer which was tie-coated and upon which cube- corner prisms were cast. This structure was radio frequency welded to a fabric reinforced vinyl backing and compared against a one-ply commercially available orange fluorescent sign product, Reflexite Classic Fluorescent Orange RUS, available from Reflexite Corporation of Avon, Connecticut, and which included a single-ply, 0.010 inch vinyl film comprising an orange fluorescent layer onto which cube-corner prisms were cast. This structure was radio frequency welded onto a fabric reinforced vinyl backing. The tests were conducted in Florida and Arizona where the samples, nine by nine inches in size, were directly exposed to sunlight and oriented to face a southerly direction at a 45° angle to the ground. The samples were exposed for periods of three and six months. Such exposure to sunlight is representative of more than two years of normal use of roll up signs, because the signs typically are not outside all of the time and also are not always facing south. A control not exposed to sunlight was maintained for each sample type.

The results of the tests on the present invention are shown in Figure 3 and in Table 1. After three and six months of direct sunlight exposure, the two-ply orange signs maintained their orange color as defined by the region bordered by the solid squares in Figure 3 of the CIE 1931 (X,Y) - Chromaticity Diagram.

Table I

Two-Ply Product Weathering Tests

Luminance Chromaticity Coordinates

Sample Factor x y_

Control 59 . 2 0 . 599 0 . 3 89

Arizona 3 mon. 42 . 6 0 . 328 0 . 342 Arizona 6 mon. 38 . 8 0 . 315 0 . 322

Florida 3 mon. 51 . 1 0 . 428 0 . 515

Florida 6 mon. 55 . 4 0 . 366 0 . 466

The luminance factor is the ratio the luminance, also known as brightness, of a body when illuminated and observed under certain conditions to that of a perfect diffuser under the same conditions. Luminance is the ratio of the luminous intensity in a given direction of an infinitesimal element of a surface containing the point under consideration, to the orthogonally projected area of the element on a plane perpendicular to the given direction. The brightness of the signs remained reasonably constant in the Florida two-ply test samples. The results of the comparative samples are shown in Figure 4 and in Table 2. After three and six months of direct sunlight exposure, the orange one-ply signs significantly lost their orange color as defined by CIE 1931. The one-ply comparative fluorescent samples turned to a white, non-fluorescent color after six months.

Table 2

One-Ply Product Weathering Tests

Luminance Chromaticity Coordinates

Sample Factor x y_

Control 24.5 0.610 0.373

Arizona 3 mon. 15.1 0.573 0.392

Arizona 6 mon. 14.6 0.555 0.394

Florida 3 mon. 16.3 0.573 0.396

Florida 6 mon. 14.3 0.560 0.405

The brightness of the one-ply signs declined by about 40% for each of the test samples as compared to the control sample.

Equivalents

Those skilled in the art will recognize, or be able to ascertain using no more than routine experimentation, many equivalents to specific embodiments of the invention described specifically herein. Such equivalents are intended to be encompassed in the scope of the following claims.

Claims

1. A retroreflective structure comprising: a) a planar body comprised of a transparent fluorescent layer and a transparent non- fluorescent layer bonded to said transparent fluorescent layer; b) an array of retroreflective elements bonded to said planar body; and c) a backing layer bonded to said array of retroreflective elements.

2. The retroreflective structure of Claim 1 wherein the array of retroreflective elements are comprised of prisms having a facet side and a window side.

3. The retroreflective structure of Claim 2 wherein the transparent fluorescent layer of the layered structure is bonded to the window side of the array of retroreflective prism elements.

4. The retroreflective structure of Claim 2 wherein the transparent non-fluorescent layer of the layered structure is bonded to the window side of the array of retroreflective prism elements.

5. The retroreflective structure of Claim 3 wherein the transparent non-fluorescent layer includes a non-fluorescent colorant and the transparent fluorescent layer includes a fluorescent colorant, wherein said non-fluorescent colorant and fluorescent colorant have essentially the same color coordinates and said non-fluorescent colorant is significantly more stable to degradation after prolonged exposure to ultraviolet light than the fluorescent colorant.

6. The retroreflective structures of Claim 7 wherein the transparent fluorescent layer and the transparent non-fluorescent layer are both orange in color.

7. The retroreflective structure of Claim 8 wherein the transparent fluorescent layer is a thermoplastic polymer.

8. The retroreflective structure of Claim 9 wherein the thermoplastic polymer includes polyvinyl chloride.

9. The retroreflective structure of Claim 10 wherein the transparent non-fluorescent layer is a thermoplastic polymer.

10. The retroreflective structure of Claim 9 wherein the thermoplastic polymer includes polyvinyl chloride.

11. The retroreflective structure of Claim 10 wherein the backing layer is comprised of a thermoplastic polymer which includes polyvinyl chloride.

12. The retroreflective structure of Claim 11 wherein the polyvinyl chloride is opaque.

13. The retroreflective structure of Claim 2 wherein a metalization layer is deposited on the facet side of the array of retroreflective elements.

14. The retroreflective structure of Claim 2 wherein the array of retroreflective elements includes a polymer which is selected from the group consisting of polyvinyl chloride, urethane acrylic ethers, and epoxy acrylates .

15. A retroreflective structure comprising: a) a transparent fluorescent layer; b) a transparent non-fluorescent layer bonded to said transparent fluorescent layer; c) an array of retroreflective elements bonded to said transparent non-fluorescent layer; and d) a backing layer bonded to said array of retroreflective elements.

16. The retroreflective structure of Claim 15 wherein the transparent non-fluorescent layer includes a non-fluorescent colorant and the transparent fluorescent layer includes a fluorescent colorant, wherein said non-fluorescent colorant is significantly more stable to degradation after prolonged exposure to ultraviolet light than the fluorescent colorant and said colorants are essentially the same color.

17. The retroreflective structure of Claim 16 wherein the fluorescent colorant and the non-fluorescent colorant are orange in color.

18. A retroreflective structure comprising: a) a transparent non-fluorescent layer; b) a transparent fluorescent layer bonded to said transparent non-fluorescent layer; c) an array of retroreflective elements bonded to said transparent fluorescent layer; and d) a backing layer bonded to said array of retroreflective elements.

19. The retroreflective structure of Claim 18 wherein the fluorescent colorant and the non-fluorescent colorant are orange in color.

20. A traffic control sign which includes the retroreflective structure of Claim 15.

21. A traffic control sign which includes the retroreflective structure of Claim 18.

22. A retroreflective structure having an array of retroreflective prism elements that has a facet side and a window side, said facet side is attached to an inner layer and said window side is attached to an outer structure: the improvement being that the outer structure has a transparent fluorescent layer with a fluorescent colorant and a non-fluorescent transparent layer with a non-fluorescent colorant, wherein the fluorescent colorant and the non-fluorescent colorant have essentially the same color coordinates and that the non- fluorescent colorant is significantly more stable to degradation than the fluorescent colorant after prolonged exposure to ultraviolet light.

23. A method for forming a retroreflective structure, comprising the steps of : a) bonding a transparent fluorescent layer to a transparent non-fluorescent layer to form a layered structure; b) bonding an array of retroreflective elements to said layered structure; and bonding a third layer to said array of retroreflective elements, thereby forming the retroreflective structure.