Prefabricated retroreflective sign

#The present invention relates generally to retroreflective signs. The invention has particular application to signs that utilize cube corner retroreflective sheeting operating on principles of total internal reflection (TIR).

#The term “sign” as used herein refers to a stand-alone article that conveys information, usually by means of alphanumeric characters, symbols, graphics, or other indicia, and that in use is mounted to an object such as a post, bracket, wall, or similar body. Specific examples include signs used for traffic control purposes (STOP, YIELD, speed limit, informational, roadside markers, etc.), street signs, and vehicle license plates. The term “retroreflective” as used herein refers to the attribute of reflecting an obliquely incident light ray in a direction antiparallel to its incident direction, or nearly so, such that it returns to the light source or the immediate vicinity thereof.

#Two known types of retroreflective sheeting are microsphere-based sheeting and cube corner sheeting. Microsphere-based sheeting, sometimes called “beaded” sheeting, employs a multitude of microspheres typically at least partially imbedded in a binder layer and having associated specular or diffuse reflecting materials (e.g., pigment particles, metal flakes, vapor coats) to retroreflect incident light. Illustrative examples are disclosed in U.S. Pat. No. 3,190,178 (McKenzie), U.S. Pat. No. 4,025,159 (McGrath), and U.S. Pat. No. 5,066,098 (Kult). In contrast, cube corner retroreflective sheeting comprises a body portion typically having a substantially planar front surface and a rear structured surface comprising a plurality of cube corner elements. Each cube corner element comprises three approximately mutually perpendicular optical faces that cooperate to retroreflect incident light. Examples include U.S. Pat. No. 1,591,572 (Stimson), U.S. Pat. No. 4,588,258 (Hoopman), U.S. Pat. No. 4,775,219 (Appledorn et al.), U.S. Pat. No. 5,138,488 (Szczech), U.S. Pat. No. 5,213,872 (pricone et al.) U.S. Pat. No. 5,691,846 (Benson, Jr. et al.), and U.S. Pat. No. 5,696,627 (Benson et al.).

#FIG. 1 is a fragmentary sectional view of a PRIOR ART cube corner retroreflective sheeting.

#FIG. 2 is a fragmentary sectional view of a PRIOR ART cube corner retroreflective sheeting applied to a substrate such as a sign board.

#FIG. 3 shows in fragmentary plan view a portion of the structured surface of the retroreflective sheeting of FIG. 2.

#In the drawings, the same reference symbol is used for convenience to indicate elements which are the same or which perform the same or a similar function.

#Turning to FIG. 4, a retroreflective sign 40 is provided which eliminates the need for a dedicated seal film. Sign 40 has a retroreflective sheet 42, composed of a body layer 44 and a top layer 46, and a sign board backing 48. Top layer 46 can include conventional UV absorbing materials, patterned ink layers or other patterned layers forming indicia such as alphanumeric characters, symbols, or graphics, and combinations thereof. Top layer 46 can also comprise Electrocut™ film sold by 3M Company and like films capable of forming indicia. Body layer 44 can also be of conventional design, so long as it includes a rear structured surface that defines cube corner elements 44a and raised sections 44b. The cube corner elements 44a have reflective faces operating on principles of TIR which cooperate to convert incident light 50a into retroreflected light 50b. The cube corner faces are smooth portions of the body layer 44 exposed to air and substantially free of vapor-deposited metal films or the like. The raised sections 44b are interconnected to define cells that surround groups of cube corner elements. Sections 44b can be integrally formed in the body layer 44, as shown in FIG. 4, or they can be formed as a distinct layer which is then combined with body layer 44. Notably, sign 40 does not utilize a seal film such as used in the prior art. Rather, the sign board itself is used to help seal the cells bounded by raised sections 44b. The sealed cells keep moisture and dirt away from the reflective faces of the cube corner elements. The sign board 48 has a smooth front surface 48a to which the raised sections can be readily bonded. Various types of bonds between sections 44b and surface 48a can be used, with some being more desirable than others depending on the composition and other properties of the sign board 48 and sections 44b.

#For example, if raised structures 44b and sign board 48 are both made from thermoplastic polymers with melting points of sufficient similarity and with required morphologies known to those skilled in the art, ultrasonic energy applied by a vibrating source and with sufficient pressure can cause these members to melt and upon cooling bond to one another. In one such embodiment, raised structures 44b are made of polycarbonate and sign board 48 is made of polycarbonate or acrylonitrile/butadiene/styrene polymers. In another embodiment, heat is used instead of ultrasonic energy to form the bond. For example, the sign board surface 48a can carry a thin adhesive layer of a relatively low melting point material relative to the remainder of the sign board 48 and to the raised structures themselves. Such adhesives are sometimes referred to as hot melt adhesives. The sign board/adhesive combination is then heated to a temperature above the melting point of the adhesive layer but below the melting point of the sign board. Raised sections 44b of a sheeting are then brought into contact with the molten adhesive and subsequently allowed to cool below the adhesive layer melting point, thus forming a bond. As an example, raised sections 44b can comprise polycarbonate and the adhesive layer can comprise polyurethanes, or Bynel™ brand resins sold by E.I. du Pont de Nemours and Company (“DuPont”) of Wilmington, Del. Copolyesters synthesized from terephthalic and isophthalic acids can also be used for the adhesive layer.