Abstract:
A retroreflective sheet structure ( 10 ) comprising a transparent layer ( 20 ) having a front light-receiving surface ( 30 ) and a rear retroreflecting surface ( 32 ). Light incident on the front surface ( 30 ) will pass through the layer ( 20 ), impinge on the rear retroreflective surface ( 32 ) and reflect back out through the front surface ( 30 ) in a predetermined direction. An identifying indicia ( 44 ) is chosen and then formed on the retroreflecting surface ( 32 ). This indicia ( 44 ) can be used for identification purposes, even years after an end product incorporating the reflective sheet structure ( 10 ) has been out in the field.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation of U.S. application Ser. No. 13/439,133 filed Apr. 4, 2012, which is a division of U.S. application Ser. No. 11/915,930 filed Nov. 26, 2007, which is a 371 of International Application No. PCT/US2006/022173 filed Jun. 7, 2006, which claims priority to U.S. Provisional Application No. 60/691,338 filed Jun. 16, 2005, all of which are incorporated herein by reference in their entireties. 
    
    
     FIELD OF THE INVENTION 
     This invention relates generally, as indicated, to a retroreflective sheet structure and, more particularly, to a retroreflective sheet structure comprising a transparent thermoplastic layer with a front light-receiving surface and a rear retroreflecting surface. 
     BACKGROUND OF THE INVENTION 
     A retroreflective sheet structure comprises a transparent thermoplastic layer having a front light-receiving surface and a rear retroreflecting surface. Light incident on the front surface passes through the clear thermoplastic layer, impinges on the rear retroreflecting surface, and is reflected back out through the front surface in a predetermined direction (e.g., aligned with and/or parallel to the direction of incidence). In this manner, incident light can be used to illuminate markings, words, and other information in an otherwise dark environment. 
     The retroreflecting surface can be formed by a repeating array of retroreflective elements embossed in the thermoplastic layer. The retroreflective elements can comprise, for example, corner-cube elements which each have three flat faces arranged mutually at right angles and connected by edges which join at an apex. (See e.g., U.S. Pat. No. 1,906,655, U.S. Pat. No. 3,332,327, U.S. Pat. No. 3,541,606, U.S. Pat. No. 3,833,285, U.S. Pat. No. 3,873,184, and/or U.S. Pat. No. 3,923,378. See also, U.S. Pat. No. 6,767,102 which is assigned to the assignee of the present invention and the entire disclosure which is hereby incorporated by reference). 
     Over the years, retroreflective sheet structures have been incorporated into a wide range of end products including, for example, vehicle markings, highway signs, and construction barrels. In these and other applications, extended outdoor durability is important and the retroreflective sheet structure needs to withstand extended sun light exposure and other harsh environmental conditions. An expected useful life of twelve years is not considered an unreasonable requirement when a retroreflective structure is being used in a highway situation. Even in less demanding, more delicate settings, the retroreflective structure is expected to maintain its physical stability and optical reflectivity for a certain period of time (e.g., one to five years). 
     Typically, an end product manufacturer will receive a roll of retroreflective sheeting from an independent supplier. During the manufacture of the end product, the sheeting is unwound from the roll and separated into individual structures for integration into the end product. As such, a retroreflective sheeting supplier may be unaware of what end products its sheeting is being used in and/or where the end product is being used, especially after an extended period of time. Additionally or alternatively, an end product manufacturer having a plurality of sheeting suppliers (which is dictated by many companies&#39; purchasing policies) may find it difficult to track the identify of the supplier whose sheeting was used in a particular end product. 
     SUMMARY OF THE INVENTION 
     The present invention provides a retroreflective sheet structure including indicia that can be used to identify something about the sheeting used to form the structure. For example, the identifying indicia can allow a sheeting supplier to determine whether a specific retroreflective structure originated from its company and/or an end product manufacture to determine what suppliers&#39; sheeting was incorporated into a particular product. This may be important, for example, should a retroreflective structure not maintain its physical stability and/or optical reflectivity for an expected period of time. The present invention allows such a determination, even years after the end product incorporating the reflective sheet structure has been out in the field. 
     More particularly, the present invention provides a retroreflective sheet structure comprising identifying indicia formed on the rear retroreflecting surface of its transparent layer. A supplier can choose an identifying indicia that will not be used by another sheeting supplier. The identifying indicia is detectable during close inspection (e.g., within 20 cm or less), but does not interfere with the retroreflective qualities of the structure. 
     The rear retroreflecting surface can comprise a repeating array of retroreflective elements formed thereon, with some of the elements having disturbances arranged in a pattern corresponding to the identifying indicia. For example, if the retroreflective elements are microcubes, a small percentage of the cube faces can have a planar-disturbance thereon. Forming the identifying indicia in this manner allows an existing tool plate (having only undisturbed retroreflective elements) to be modified to practice the invention. Specifically, for example, the existing tool plate can be etched (e.g., laser etched) to create the inverse of the desired planar-disturbances. 
     These and other features of the invention are fully described and particularly pointed out in the claims. The following description and drawings set forth in detail a certain illustrative embodiment of the invention which is indicative of but one of the various ways in which the principles of the invention may be employed. 
    
    
     
       DETAILED DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A, 1B, and 1C  are schematic drawings of end products which incorporate a retroreflective sheet structure according to the present invention, the end products being a vehicle, a highway sign, and a construction barrel, respectively. 
         FIGS. 2A and 2B  are top and side views, respectively, of the retroreflective sheet structure isolated from the end product. 
         FIG. 2C  is a close-up bottom view of a retroreflective element. 
         FIG. 2D  is a close-up bottom view of a retroreflective element with a planar disturbance on one of its faces. 
         FIG. 3  is a perspective view of a roll of retroreflective sheeting which would be supplied to an end product manufacturer for fabrication into the retroreflective sheet structure, the sheeting including an embossed thermoplastic layer. 
         FIGS. 4A and 4B  are schematic views of a method of embossing the thermoplastic layer. 
         FIG. 4C  is a close-up side view of a pyramid projection of the tooling plate used in the method of embossing the thermoplastic layer. 
         FIGS. 5A and 5B  are schematic views of a method of modifying a tooling plate to emboss the thermoplastic layer. 
         FIG. 5C  is a close-up side view of a pyramid portion of the tooling prior to laser etching. 
         FIG. 5D  is a close-up side view of a pyramid portion of the tooling after laser etching. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Referring now to the drawings, and initially to  FIGS. 1A, 1B, and 1C , a retroreflective sheet structure  10  according to the present invention is shown incorporated into an end product  12 . In the illustrated embodiments, the end products  12  are a vehicle, a highway sign, and a construction barrel, respectively. With these end products  12 , extended outdoor durability is important and the sheet structure  10  needs to withstand extended sun light exposure and other harsh environmental conditions for an expected period of time. However, the present invention is not limited to outdoor situations, and the end product  12  can be any product which incorporates the retroreflective sheet structure  10  according to the present invention. In any event, the end product  12  includes a mounting surface  14  to which the sheet structure  10  is attached and, preferably, adhesively attached. 
     Referring now to  FIGS. 2A and 2B , the retroreflective sheet structure  10  is shown isolated from the end product  12 . The structure  10  comprises a transparent layer  20 , a reflection-aiding layer  22 , a backing layer  24 , and an adhesive layer  26 . A removable release layer  28  (shown in phantom) can be provided to cover the adhesive layer  26  during pre-mounting stages of end product fabrication. 
     The transparent layer  20  can comprise any suitable thermoplastic material which is compatible with desired manufacturing methods (e.g., acrylic, vinyl, polymethylacrylate, polycarbonate, polyurethane, polysulfone, polyarylate, polyether imide, polyetherimide, cyclo-olefinic copolymer, and/or acrylonitrile butadiene styrene). The reflection-aiding layer  22  can be a metallized film, granular silica particles, or any other acceptably reflective material. The backing layer  24  can serve as a space-filler behind the layers  20 / 22  and/or as a carrier for the adhesive layer  26  and, to this end, can comprise a paper, plastic, metal, or other sheet/substrate which performs these functions. The adhesive layer  26  is used to attach the reflective sheet structure  10  to the mounting surface  14  of the end product  12  and can comprise a pressure-sensitive or heat-activated adhesive. 
     The transparent layer  20  has a front light-receiving surface  30  and a rear retroreflective surface  32  on which a repeating array of retroreflective elements  34  are formed. Light incident on the smooth front surface  30  passes through the clear thermoplastic layer  20 , impinges on the retroreflective elements  34 , and is reflected back out through the front surface  30  in a predetermined direction (e.g., aligned with and/or parallel to the direction of light incidence). 
     As is best seen in  FIG. 2C , the retroreflective elements  34  are preferably microcubes which each comprise a three flat faces  36  arranged mutually at right angles and connected by edges  38  that meet an apex  40 . The size, shape and arrangement of the faces  36  determines, and can be varied to adjust, optical qualities. The cube area of each retroreflective element  34  (i.e., the area enclosed by the cube shape defined by pyramid of the perimeter of the three faces  36  in the direction of the principle refracted ray) can be about 1 mm.sup.2 or less. 
     In a large majority of the retroreflective elements  34  (e.g., more than 80%, more than 90%, more than 95%, and/or more than 98%), the three faces  36  are planar without any outthrusts or depressions. (See  FIG. 2D .) However, in a selected few of the retroreflective elements  34 , one face  36  includes a planar-disturbance  42  which disrupts the planar profile of the face  36 . (See  FIG. 2E .) The retroreflective elements  34  having a planar-disturbance  42  are arranged in a pattern corresponding to an identifying indicia  44 . For example, in the illustrated embodiment, the planar disturbances  42  collectively form an “ABC” logo which could correspond to the supplier of a retroreflective sheeting (namely retroreflective sheeting  48 , introduced below) used to fabricate the structure  10 . It is expected that a supplier will choose the identifying indicia  44  so that will not be another sheeting supplier. 
     In the illustrated embodiment, the planar-disturbance  42  is a protrusion and, more particularly, a protuberance having a knob-like shape. However, other protrusion geometries are certainly possible with, and contemplated by, the present invention. Moreover, other disturbances in the planar profile of the selected faces  36  could be used instead of, or in addition to, the protuberances  42 . For example, indentations, notches, pits, depressions or other recesses could be used to form the identifying indicia  44 . The planar-disturbances  42  can be same, similar, or different among the “disturbed” retroreflective elements  34 . 
     The planar-disturbance  42  will occupy only a small percentage (e.g., less than 30%, less than 20%, less than 10%, and/or less than 5%) of the surface area of the disturbed face  36  of the respective retroreflective element  24 . Thus, most the reflective regions of the disturbed faces  36  are left intact. Moreover, the identifying indicia  44  collectively formed by the disturbances  42  will preferably occupy an area of less than 16 cm.sup.2 on the surface  32  of the thermoplastic layer  20 . In this manner, the identifying indicia  44  will be detectable during close inspection (i.e., within 20 cm or less of the structure  10 ), but will not interfere with, or detract from, the retroreflective qualities of the structure  10 . That being said, larger identifying indicia  44  could be used if such interference and/or detraction is acceptable or desired in a particular situation. 
     Referring now to  FIG. 3 , retroreflective sheeting  48  is shown which can be used to create a plurality of the structures  10 . Typically, a manufacturer of the end product  12  (or of subassemblies therefor) will receive a roll of the retroreflective sheeting  48  from an independent supplier. During the manufacture of the end product  12 , the sheeting  48  is unwound from the roll and separated into individual structures  10  for integration into the end product  12 . To this end, the sheeting  48  can comprise a transparent thermoplastic layer  50 , a reflection-aiding layer  52 , a backing layer  54 , an adhesive layer  56 , and a removable release layer  58 . The transparent layer  50  includes the repeating array of retroreflective elements  34  and includes the identifying indicia  44  at predetermined positions and/or intervals to insure that each structure  10  includes at least one such indicia  44 . 
     Referring now to  FIGS. 4A and 4B , a method of making the thermoplastic layer  50  is schematically shown. In this method, a thermoplastic film  60  is embossed by a tool plate  62 , and then cooled to solidify the embossed microstructure. The tooling plate  62  used in this method can be a variety of sizes with widths/lengths ranging from, for example, five inches to sixty inches. For example, the tooling plate  62  can be thirty inches wide and sixty inches long, or it can be five inches wide and five inches long. 
     As is best seen by referring to  FIG. 4C , the tool plate  62  has a topography corresponding to the inverse of the retroreflective elements  34 . Specifically, the tool plate  62  has a series of pyramid (or other polyhedron) projections  64  comprising three faces  66  arranged at mutually right angles and connected by edges  68  which join at an apex  70 . The majority of the faces  66  of the tool plate  62  have flat planar profiles to produce retroreflective elements  34  with three planar faces  36  without any out-thrusts or depressions. However, the faces  66 ′ of the tool plate  62  intended to collectively form the identifying indicia  44  include a disturbance  72  corresponding to the inverse of the intended planar-disturbance  42 . Thus, if the intended geometry of the disturbance  42  is a protrusion, as shown, the disturbance  72  will be a depression. (Likewise, if the intended geometry of the disturbance  42  was a recess, the disturbance  72  would be a projection.) 
     Referring now to  FIG. 5 , a method of making the tooling plate  62  is schematically shown. In this method, a tooling plate  92  is provided in which has a topography corresponding to the retroreflective elements  34  all having three flat planar faces  36 . ( FIG. 5A .) Specifically, for example, the tooling plate  92  could comprise a series of pyramid projections  64  comprising three flat face sections  66  arranged at mutually right angles and connected by edges  68  which all join at an apex  70 . After identifying indicia  44  has been chosen, and the location of the indicia  44  relative to the overall thermoplastic layer is determined, the face sections  66 ′ that correspond to the location of the planar-disturbances  72  can be charted. 
     The planar-disturbance  72  is then formed on each charted face section  66 ′ by a suitable technique such as the application of energy, chemicals, or machining. For example, a laser beam (e.g., a YAG pulse laser or a CO.sub.2 laser) can be focused on the face section  66 ′ to melt the surface and form the disturbance  72 . Chemicals can be particularly useful with a plastic material, in that the application of a drop of solvent on the charted face section can cause the surface to pucker to form the disturbance  72 . Machining methods can include, for example, micro-drilling. 
     Advantageously, the present invention does not require the construction of a new tooling plate  62 , but rather allows the retrofitting an existing plate  92 . 
     One may now appreciate that the present invention provides a retroreflective sheet structure  10  wherein the identifying indicia  44  allows a sheeting supplier to determine whether a particular structure originated from its company and/or an end product manufacture to determine what suppliers&#39; sheeting was incorporated into a particular end product. Although the invention has been shown and described with respect to certain preferred embodiments, it is obvious that equivalent and obvious alterations and modifications will occur to others skilled in the art upon the reading and understanding of this specification. The present invention includes all such alterations and modifications and is limited only by the scope of the following claims.