Conformable cube corner retroreflective sheeting

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FIELD OF INVENTION

The present invention relates to retroreflective articles, particularly cube corner type retroreflective articles.

BACKGROUND

Retroreflective articles are made in a variety of forms, including sheetings such as are used on traffic signs and license plates, rigid safety reflectors mounted on motor vehicles and bikes, and patches and appliques such as are applied to garments and book bags, etc. One major use of retroreflective sheeting is in the field of highway markings and signs to improve the visibility and legibility of informational signs, traffic directions, barriers, etc. to drivers.

One common type of retroreflector employs transparent microspheres, typically with hemispheric reflectors thereon. Illustrative examples of such retroreflectors 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).

A second common type of retroreflector employs what are referred to as cube corner retroreflective elements. Such cube corner retroreflectors typically comprise a sheet having a generally planar front surface and an array of cube corner elements protruding from the back surface. In use, the retroreflector is arranged with the front surface disposed toward the anticipated location of intended observers. Light incident to the front surface enters the sheet, passes through the body of the sheet to be internally reflected by the faces of the elements so as to exit the front surface in a direction substantially toward the light source, i.e., retroreflection. Illustrative examples of cube corner type retroreflectors are disclosed in U.S. Pat. No. 3,712,706 (Stamm), U.S. Pat. No. 4,025,159 (McGrath), U.S. Pat. No. 4,202,600 (Burke et al. ), U.S. Pat. No. 4,243,618 (Van Arnam), U.S. Pat. No. 4,349,598 (White), U.S. Pat. No. 4,576,850 (Martens), U.S. Pat. No. 4,588,258 (Hoopman), U.S. Pat. No. 4,775,219 (Appeldorn et al.) and U.S. Pat. No. 4,895,428 (Nelson et al.). Cube corner retroreflectors have commonly been employed as safety devices on bicycles, automobiles, and other vehicles as well as on traffic signs.

Cube corner retroreflectors typically have a higher retroreflective efficiency than microsphere-based retroreflectors and are sometimes preferred for application to substrates for this reason. However, retroreflective posts, cones, barrels, safety helmets, and corrugations or rivets on truck trailer surfaces require that the sheeting bend and conform to curved substrates. The cubes of cube corner retroreflectors are typically made of resins having high glass transition temperatures so that the cubes maintain their dimensions, and thus are capable of providing bright retroreflection, upon being exposed to high temperatures or high levels of humidity over time. Such resins are typically rigid (i.e., have a high flexural modulus). Unlike microsphere-based sheetings where the microspheres are generally much higher in modulus than the binder resin in which the microspheres are embedded, the cube corner retroreflective elements of cube corner retroreflectors tend to undergo significant optically degrading deformation as the retroreflector is conformed to a non-planar substrate because the high modulus cubes are typically similar in modulus to the rest of the sheeting.

U.S. Pat. No. 3,684,348 (Rowland) discloses a retroreflective composite material which is adapted to be shaped and mounted to surfaces of various configurations. The composite material comprises a flexible body portion to which a multiplicity of minute cube corner elements are adhered. The cube corner elements have a side edge dimension of up to 25 mils (625 microns), but preferably less than 10 mils (250 microns) along the side edge.

In U.S. Pat. No. 3,992,080 (Rowland), it is noted that the cube corner elements of the retroreflective composite material disclosed in U.S. Pat. No. 3,684,348 are distorted when the material is stretched during application to a support surface, and such distortion renders the cube corner faces non-orthogonal to a degree, resulting in significant loss of brightness.

U.S. Pat. No. 4,555,161 (Rowland) discloses a retroreflective laminar sheet assembly comprising flexible base and cover sheets and an array of retroreflective film pieces seated within discrete adjacent cells formed by bonding of the base and cover sheets at selected areas. One retroreflective film piece is contained within each cell and typically is made of minute cube corner retroreflective elements. Typically, there is a gap of about 1/8 to 1/2 inch (0.3 to 1.3 cm) between the edge of each film piece and the adjacent bonding area. The retroreflective laminar sheet assembly can be formed into a collar and mounted upon a traffic cone as shown in FIG. 3 of the patent. However, it is believed that the sheet assembly is not particularly useful when mounted to non-planar substrates which have intricate shapes or very small dimensions such as truck trailer rivets and corrugations because of the shear size of the gaps and film pieces in the sheet assembly. These gaps are typically much smaller than the retroreflective film pieces, which are rigid. The gaps present areas of the sheet assembly which are not capable of retroreflecting light. It is believed that if more film pieces and gaps were provided in a given unit of area of the sheet assembly to achieve greater flexibility, retroreflective brightness would be greatly sacrificed because of the width of the gaps relative to the width of the film pieces. In other words, the gaps can likely be made only so small before bonding of the base and cover sheets in the bonding areas is not possible. In addition, the patent discloses that the base and cover sheets are flexible to provide for flexible constructions, but does not disclose conformable base and cover sheets to provide for conformable constructions. Lastly, the sheet assembly is typically difficult to manufacture because the retroreflective film pieces must generally be cut and arranged in stacks in the manufacturing process.

SUMMARY OF INVENTION

The present invention provides cube corner type retroreflective sheetings which are conformable to non-planar substrates and methods for making such sheetings. Such sheetings are particularly adapted to be applied to the corrugated surfaces of truck trailers and protruding rivets thereof. Other possible applications are in construction work zones, personal safety, safety at sea, and any other area where conformability of highly efficient retroreflective sheetings is needed.

In brief summary, the invention provides in one of its aspects a conformable cube corner retroreflective sheeting comprising a plurality of discrete cube corner segments which are conformably bonded together, each cube corner segment comprising a plastic body portion or land having a substantially planar front major surface and Side walls and at least one minute cube corner retroreflective element projecting rearwardly from the body portion and defining a cube corner point side of the cube corner segment. The word "conformable" is used herein to describe a material which is capable of being shaped or formed. In particular, the term "conformable" is used herein to describe materials such as carrier layers and sheetings which are omni-directionally extensible at some ambient application temperature or elevated temperature and can take essentially the same shape as non-planar substrates to which the materials are conformed. The word "discrete" is used herein to indicate that the cube corner segments are not rigidly connected together. The phrase "conformably bonded together" and close variants of this phrase are used herein to indicate that adjacent cube corner segments are at least one of the following: (1) separated by a gap of less than about 1 millimeter and bonded together through a conformable carrier layer; or (2) separated by a gap which is substantially filled with a conformable resin that bonds the side walls of adjacent cube corner segments together. Each cube corner retroreflective element typically has a plurality of facets or faces and a base adjacent the body portion. Typically, substantially all of the cube corner retroreflective elements located closest to the side walls of the body portions are intact and capable of retroreflecting light.

The peripheries of the cube corner segments can be defined by a plurality of separations extending from the cube corner point sides to the front major surfaces of the cube corner segments, the separations being disposed between adjacent cube corner segments. The term "separations" is used interchangeably herein with the term "gaps," and is intended to denote any separations in the continuity of a sheeting, whether the separations are caused by:

a) cutting the sheeting with a cutting device utilizing a laser beam or a sharp edge;

b) stretching or flexing the sheeting with the sheeting optionally being scored in the areas where separations are desired;

c) molding the sheeting to form such separations;

d) propagating a discontinuity in the sheeting, the discontinuity typically being initiated by thermally shocking the sheeting, mechanically or ultrasonically vibrating the sheeting, impacting the sheeting for a short duration, Or mechanically stressing the sheeting; or

e) any other suitable process,

Typically, the cube corner segments are defined by a pattern of the separations. Such a pattern can comprise a plurality of contiguous polygons selected from the group consisting of parallelograms, triangles, and hexagons.

As noted above, adjacent cube corner segments can be separated by a gap of less than about 1 millimeter and bonded together through a conformable carrier layer. The conformable carrier layer can comprise a continuous, transparent film which is bonded to the front major surfaces of the cube corner segments, typically through a transparent adhesive.

Also, as noted above, the cube corner segments can be bonded together through a conformable resin disposed in the gaps between adjacent cube corner segments. The conformable resin bonds the side walls of adjacent cube corner segments together. The gap between adjacent cube corner segments can range between about 0.5 and about 3 millimeters. Further, a back sealing film can be disposed adjacent the cube corner retroreflective elements and bonded to the cube corner segments through the conformable resin. Also, a continuous, transparent film can be bonded to the front major surfaces through the conformable resin, the conformable resin typically being transparent.

In another of its aspects, the invention relates to a method for making a conformable cube corner retroreflective sheeting, comprising:

a) providing a tool having a molding surface which comprises a plurality of raised protrusions and retroreflective element-forming cavities adapted for molding a cube corner retroreflective sheeting comprising a plurality of the above-described cube corner segments;

b) depositing a hardenable molding material on the molding surface of the tool, the molding material being sufficient in amount and fluidity to essentially completely fill the cavities;

c) applying a conformable carrier layer to the molding material on the molding surface under sufficient pressure to effect intimate surface contact between the carrier layer, the raised protrusions of the tool, and the molding material;

d) effecting substantial solidification of the molding material and bonding of the molding material to the carrier layer to form the conformable sheeting; and

e) removing the conformable sheeting from the molding surface.

In another of its aspects, the invention relates to a method for making a conformable cube corner retroreflective sheeting, comprising:

a) providing the tool described in the above method;

b) placing a thermoplastic sheet on the tool;

c) heating the resin of the sheet to a temperature at least as high as its softening temperature;

d) pressing the sheet onto the molding surface of the tool to thereby form a plurality of cube corner segments;

e) conformably bonding the cube corner segments together with a conformable carrier layer to form the conformable sheeting; and

f) removing the conformable sheeting from the tool.

The term "softening temperature" is a well known term of art. It is used herein to denote the temperature at which a material first softens and is capable of being pressed into a desired shape upon heating the material. U.S. Pat. No. 5,117,304 (Huang et al.) discloses a suitable method for measuring the softening temperature of a polymer sample and is incorporated herein by reference.

The invention also relates to a method for making a conformable cube corner retroreflective sheeting, comprising the steps of:

a) providing an initial cube corner retroreflective sheeting comprising a plastic body portion having a substantially planar front major surface and a multiplicity of minute cube corner retroreflective elements projecting rearwardly from the body portion and defining a cube corner point side of the initial sheeting;

b) dividing the body portion into a plurality of the above-described cube corner segments so that they are discrete and so that the peripheries of the cube corner segments are defined by a plurality of separations extending from the cube corner point sides of the cube corne