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  Cube corner cavity based retroreflectors with transparent fill materialUSPTO Application #: 20060108062Title: Cube corner cavity based retroreflectors with transparent fill material Abstract: Retroreflective sheeting includes a body layer having a structured surface with recessed faces forming cube corner cavities. A reflective film is disposed at least on the recessed faces, and a fill material fills the cube corner cavities. The fill material comprises radiation-curable materials, adhesives, or both, and preferably transparent radiation-curable pressure-sensitive adhesives. The fill material preferably forms a continuous layer covering both the recessed faces and upper portions of the structured surface. A transparent cover layer preferably contacts the fill material layer. (end of abstract) Agent: 3m Innovative Properties Company - St. Paul, MN, US Inventors: Kenneth L. Smith, Gerald W. Benson, Michele A. Craton, Michael P. Daniels, Roger E. Luehrs USPTO Applicaton #: 20060108062 - Class: 156272200 (USPTO) The Patent Description & Claims data below is from USPTO Patent Application 20060108062. Brief Patent Description - Full Patent Description - Patent Application Claims
CROSS REFERENCE TO RELATED APPLICATION [0001] This patent application is a division of application Ser. No. 09/228,367, filed Jan. 11, 1999. The present invention relates to U.S. application Ser. No. 09/227,963, "Cube Corner Cavity Based Retroreflectors and Methods For Making Same", filed on Jan. 11, 1999 and incorporated by reference. BACKGROUND [0002] The present invention relates generally to retroreflective articles such as sheeting. More particularly, the invention relates to such articles or sheetings in which retroreflective elements comprise reflective faces arranged to form a cavity. [0003] The reader is directed to the glossary at the end of the specification for guidance on the meaning of certain terms used herein. [0004] Cube corner retroreflective sheetings can generally be categorized as those that use a rear-surface body layer and those that use a front-surface body layer. Commercially available cube corner retroreflective sheetings are of the former type, in which a thin transparent body layer has a substantially planar front surface and a rear structured surface comprising a plurality of geometric structures of pyramidal shape, some or all of which include three reflective faces configured as a cube corner element. Light is incident on the planar front surface, passes through the thickness of the body layer, and is retroreflected by the cube corner elements back through the front surface. In some known embodiments, a reflective coating such as aluminum is applied to the rear structured surface, followed by an adhesive layer that covers and conforms to some extent to the shape of the structured surface. However, in general no reflective coating is required so long as a clean air interface can be maintained at the structured surface, in which case reflections occur by total internal reflection. [0005] Some known cube corner retroreflective sheeting constructions use a front-surface body layer, in which the body layer has a front structured surface. See, e.g., U.S. Pat. No. 3,712,706 (Stamm), U.S. Pat. No. 4,127,693 (Lemelson), and U.S. Pat. No. 4,656,072 (Coburn, Jr. et al.), and PCT Publication WO 89/06811 (Johnson et al.). The front structured surface comprises a plurality of reflective faces arranged to form cube corner cavities. For this reason such retroreflective sheeting is referred to herein as cube corner cavity based retroreflective sheeting. A thin metal film can be applied to the structured surface to enhance reflectivity of the faces. Incident light does not penetrate through the body layer but rather is reflected by the faces forming the cube corner cavities. In some embodiments a cover layer that does transmit incident light is provided on top of the structured surface to protect the cavities from dirt or other degradation, with portions of the cover layer extending into and filling in the cube corner cavities of the structured surface. In other embodiments a cover layer is sealed or adhered to the structured surface by a colored pressure- or heat-sensitive adhesive that cancels, removes, or obliterates retroreflectivity of the structured surface. [0006] One advantage of cube corner cavity-based retroreflective sheeting is the ability to use a much wider variety of material compositions for the body layer than is otherwise possible, since it need not be optically clear. Another advantage is the ability to form certain types of structured surfaces in the body layer more rapidly than it takes to form a negative copy of such structured surfaces in rear-surface body layer constructions. This is because molds used to form the structured surface of a front-surface body layer can have grooves that are essentially unbounded in the direction of the groove. In contrast, molds used to form the structured surface of a rear-surface body layer typically have an array of closed (cube corner) cavities bounded by a plurality of inverted grooves, i.e., ridges. The unbounded grooves of the former molds are easier to fill with body layer material than the array of closed cavities provided on the latter molds. [0007] Unfortunately, this latter advantage can be essentially nullified in constructions where the cube corner cavities in the body layer are filled with a transparent substance. Filling the cavities with such a substance, referred to as a fill material, is desirable to increase the entrance angularity of the sheeting by refracting highly off-axis incident light closer to the symmetry axis of the cube corner element, as well as to keep dirt or other debris out of the cavities. But such filling is undesirable insofar as it requires forcing material into an array of closed cavities. Such filling is also undesirable to the extent it exposes the body layer to excessive heat, mechanical stress, or other process conditions that could compromise the fidelity of the structured surface. [0008] Constructions of the type described would benefit from fill materials having properties that make them easy to fill into the cube corner cavities of the body layer, preferably with minimal risk of damaging the fidelity of the structured surface. Preferred fill materials should be compatible with relatively low cost, high flexibility, and high visibility sheeting constructions. BRIEF SUMMARY [0009] Certain radiation-curable materials, particularly radiation-curable pressure-sensitive adhesives, have been found to exhibit significant manufacturing and/or construction advantages when used as fill materials for cube corner cavity based retroreflective sheeting. [0010] Retroreflective articles are disclosed having a body layer with a structured surface in which recessed faces define cube corner cavities. A transparent adhesive material fills the cube corner cavities. The adhesive material is preferably a pressure-sensitive adhesive. In one embodiment, a release liner covers the fill material. In another embodiment, a transparent cover layer takes the place of the release liner. The cover layer adds durability to the article, and can also incorporate dyes, colorants, or the like to affect the appearance of the sheeting or to convey information. [0011] Methods are disclosed in which a film of reflective material is applied at least to recessed faces of a body layer structured surface, such recessed faces forming cube corner cavities. A flowable composition such as a resin is applied to the structured surface. The composition is one suitable for forming a transparent PSA, or one that is radiation curable and suitable for bonding to the film of reflective material, or, preferably, both. After the composition has substantially completely filled the cube corner cavities, the composition is crosslinked or otherwise cured by exposure to radiation such as UV light. After the exposure step, the crosslinked composition bonds to the reflective film and preferably also to a transparent cover layer. [0012] To reduce cost while maintaining functionality and durability, the constructions preferably utilize thermoplastic materials for the body layer and the cover layer. Good flexibility of sheeting articles can be aided by the use of fill materials whose elastic modulus after crosslinking is less than about 50,000 psi (345.times.10.sup.6 Pascals), and preferably less than about 25,000 psi (172 MPa). BRIEF DESCRIPTION OF THE DRAWINGS [0013] FIG. 1 is a perspective view of a retroreflector where a top cover layer thereof is shown only partially laminated to a body layer to reveal cube corner cavities formed in the body layer; [0014] FIG. 2 is a cross-sectional view of a portion of the retroreflector of FIG. 1 taken along line 2-2, and additionally showing a fill material filling the cube corner cavities and bonding the cover layer to the body layer; [0015] FIG. 3 depicts a process for fabricating cube corner cavity-based retroreflective sheeting; and [0016] FIGS. 4A-C demonstrate a self-replicating phenomenon observed with some types of fill materials. [0017] In the drawings, the same reference symbol is used for convenience to indicate elements that are the same or that perform the same or a similar function. DETAILED DESCRIPTION OF THE ILLUSTRATIVE EMBODIMENTS [0018] In FIG. 1, a portion of a retroreflective sheeting 10 is shown enlarged. Sheeting 10 comprises a body layer 12 having a structured surface 14, and a transparent cover layer 16. Structured surface 14 includes recessed faces 18 and top surfaces 20, the recessed faces 18 forming cube corner cavities 22. The recessed faces 18 are shown shaded for visual effect. In a preferred construction, a vapor-coated film of reflective material such as aluminum, silver, or the like is exposed on the recessed faces but masked on top surfaces 20, whether by the absence of such reflective material or the presence of a masking material on the top surfaces. Alternatively, the film of reflective material can be exposed on both faces 18 and surfaces 20, but the surfaces 20 are physically roughened to impart a diffuse reflectivity to the film. In still another alternative, top surfaces 20 can be eliminated by allowing the recessed faces to converge or intersect along sharp edges. [0019] FIG. 2 shows a sectional view of a portion of the sheeting 10, additionally showing a discontinuous film 24 of reflective material on the recessed faces 18, and a fill material 26 that fills cube corner cavities 22. Fill material 26 is preferably sufficiently transparent to allow light rays to propagate through it with minimal degradation of retroreflective efficiency. In contrast to known constructions, fill material 26 forms a strong bond not only with transparent cover layer 16 but also with film 24 and with any exposed portions of body layer 12. Thus, fill material 26 is preferably coextensive with the structured surface 14 and, for ease of construction, forms a substantially continuous layer that covers both the recessed faces and the top surfaces of the structured surface. In an alternative embodiment the fill material can be coextensive with structured surface 14 but discontinuous, encapsulated by a network of bonds directly between cover layer 16 and top surfaces 20. This may be advantageous where, for example, a direct bond between the cover layer and the body layer can be made stronger than one in which the fill material and/or the reflective material are interposed. However, embodiments having a continuous fill material layer are preferred in part because the construction process is more robust by avoiding the stringent requirement of having to apply a precise amount of fill material to the structured surface--just enough to substantially fill the cavities, but not so much that the fill material covers the upper portions of the structured surface in a way that interferes with the network of bonds between the cover layer and upper portions of the body layer. The continuous fill material layer embodiments also allow the fill material to flow from one cube corner cavity to another before the fill material is solidified by cross-linking. Finally, in constructions where the fill material functions as a bonding agent between the body layer and the cover layer, a continuous fill material layer improves bond strength by increasing the surface area of the bond. Continue reading... Full patent description for Cube corner cavity based retroreflectors with transparent fill material Brief Patent Description - Full Patent Description - Patent Application Claims Click on the above for other options relating to this Cube corner cavity based retroreflectors with transparent fill material patent application. ###  How KEYWORD MONITOR works... a FREE service from FreshPatents1. Sign up (takes 30 seconds). 2. Fill in the keywords to be monitored. 3. Each week you receive an email with patent applications related to your keywords. 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