Formed ultra-flexible retroreflective cube-corner composite sheeting with target optical properties and method for making same

A retroreflective sheeting having a multiplicity of discrete, cube-corner elements cured in situ on a transparent, polymeric overlay film deformed into a three-dimensional structure so that base edges of a plurality of cube-corner elements are non-planar with respect to one another. The retroreflective article preferably has at least one target optical property. The present invention is also directed to a method of deforming the retroreflective sheeting to form a retroreflective article in which the base edges of a plurality of cube-corner elements are non-planar with respect to one another.

FIELD OF INVENTION 
The present invention relates to a flexible, retroreflective sheeting 
deformed to produce target optical properties and to a process of 
deforming a retroreflective sheeting into a three-dimensional article with 
such optical properties. 
BACKGROUND OF THE INVENTION 
Cube-corner retroreflectors typically comprise a sheeting having a 
generally planar front surface and an array of cube-corner elements 
protruding from the back surface. Cube-corner reflecting elements comprise 
generally trihedral structures that have three approximately mutually 
perpendicular lateral faces meeting in a single corner, i.e., cube-corner. 
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. The light rays are typically reflected at the cube faces due 
to either total internal reflection ("T.I.R."), or reflective coatings 
such as a vapor-deposited aluminum film. Use of metallized aluminum 
coating on the cube-corner elements tends to produce a gray coloration to 
an observer in ambient light or daylight conditions, and is thus 
considered aesthetically undesirable for some applications. 
A very common retroreflective sheeting uses an array of cube-corner 
elements to retroreflect light. FIGS. 1 and 2 illustrate an example of 
such a retroreflective sheeting, noted generally by numeral 10. The array 
of cube-corner elements 12 project from a first or rear side of a body 
portion 14 that includes a body layer 18 (also referred to in the art as 
an overlay) and may also include a land layer 16. Light illustrated as 
arrows 23 enters the cube-corner sheeting 10 through the front surface 21; 
it then passes through the body portion 14 and strikes the planar faces 22 
of the cube-corner elements 12 to return in the direction from which it 
came. 
FIG. 2 shows the back side of the cube-corner elements 12, where each 
cube-corner element 12 is in the shape of a trihedral prism that has three 
exposed planar faces 22. The cube-corner elements 12 in known arrays are 
typically defined by three sets of parallel v-shaped grooves 25, 26, and 
27. Adjacent planar faces 22 on adjacent cube-corner elements 12 in each 
groove form an external dihedral angle (a dihedral angle is the angle 
formed by two intersecting planes). This external dihedral angle is 
constant along each groove in the array. This has been the case for the 
variety of previously produced cube-corner arrays. 
The planar faces 22 that define each individual cube-corner element 12 
generally are substantially perpendicular to one another, as in the corner 
of a room. The internal dihedral angle--that is, the angle between the 
faces 22 on each individual cube-corner element in the array--typically is 
90.degree.. This internal angle, however, can deviate slightly from 
90.degree. as is well known in the art; see for example, U.S. Pat. No. 
4,775,219 to Appeldorn et al. Although the apex 24 of each cube-corner 
element 12 may be vertically aligned with the center of its base (see, for 
example, U.S. Pat. No. 3,684,348) the apex also may be offset or canted 
from the base center as disclosed in U.S. Pat. No. 4,588,258 to Hoopman. 
Other cube-corner configurations are disclosed in U.S. Pat. Nos. 
5,138,488, 4,066,331, 3,923,378, 3,541,606, and Re 29,396, 3,712,706 
(Stamm), 4,025,159 (McGrath), 4,202,600 (Burke et al.), 4,243,618 (Van 
Arnam), 4,349,598 (White), 4,576,850 (Martens), 4,588,258 (Hoopman), 
4,775,219 (Appeldorn et al.), and 4,895,428 (Nelson et al.). 
Where the cube-corner retroreflective sheeting is likely to be used in an 
environment where it could be exposed to moisture or other elements, e.g., 
outdoors or in high humidity, it may be preferred that cube-corner 
elements are encapsulated with a conformable sealing film. The 
aforementioned U.S. Pat. No. 4,025,159 discloses encapsulation of 
cube-corner elements using a sealing film. 
Basic cube-corner elements have a low angularity such that the element will 
only brightly retroreflect light that impinges on it within a narrow 
angular range centering approximately on its optical axis. The optical 
axis is the trisector of the internal space defined by the faces of the 
element. Impinging light that is inclined substantially away from the 
optical axis of the element strikes a face at an angle less than its 
critical angles, thereby passing through the face rather than being 
reflected. 
FIG. 3 is a graph in polar coordinates of the optical profile of a basic 
cube-corner retroreflective sheet, having six maxima and six minima at 
30.degree. azimuthal intervals. The intensity of the retroreflective beam 
from a cube-corner retroreflective sheeting is greatest when the incident 
beam has an angle of incidence of 0.degree. (normal to the plane of the 
sheeting). At higher angles of incidence (approximately greater than 
30.degree.) the brightness of the retroreflected beam is a function of the 
angle about an axis normal to the sheet called the azimuthal angle. When 
the angle of incidence of a light beam is held constant at a value of, for 
example 60.degree. from normal, and the azimuthal angle of the incident 
beam is varied from 0.degree. to 360.degree., the intensity of the 
retroreflected beam varies as illustrated in FIG. 3. 
There are a number of applications for cube-corner retroreflective sheeting 
with non-standard or customized optical profiles. For example, more 
uniform retroreflectivity or wider retroreflective angularity than shown 
in FIG. 3 is often required. For some applications it may be desirable to 
limit retroreflectivity to a narrow band of angularity and/or along a 
specific segment of the azimuthal angle. 
One method of changing the optical profile of cube-corner elements is to 
cut the master or mold formed thereon into pieces and reassembling the 
pieces in a pattern that produces differing zones of orientation on the 
retroreflective sheeting. For example, an optical profile with wide 
retroreflective angularity in multiple viewing planes can be achieved by 
rotating adjacent pieces of the mold or master 30.degree. or 90.degree. 
about an axis normal to the plane of the elements (rotating the pieces 
60.degree. or any multiple thereof effects no net change in orientation of 
the cube-corner elements). Reassembling the pieces of the mold or master 
with the necessary precision, however, is time consuming and expensive. A 
method of reassembling a master mold is disclosed in U.S. patent 
application Ser. No. 08/587,719 filed Jan. 19, 1996. 
Another method of changing the optical profile of cube-corner elements is 
to tilt or cant the optical axes of cube-corner elements with respect to 
one another. FIG. 4 illustrates a cube-corner element 30 with three 
mutually perpendicular faces 31a, 31b, and 31c that meet at the cube's 
apex 34. The cube's base edges 35 are generally linear and generally 1e in 
a single plane that defines the base plane 36 of the element 30. 
Cube-corner element 30 also has a central or optical axis 37, which is the 
trisector of the internal angles defined by lateral faces 31a, 31b, and 
31c. The optical axis may be disposed perpendicular to base plane 36, or 
it may be canted as described in U.S. Pat. No. 4,588,258 to Hoopman and 
U.S. Pat. No. 5,138,488 to Szczech. The cost of creating tooling necessary 
to practice the invention of Hoopman is relatively high. Moreover, this 
technique does not lend itself to rapid prototyping of customized optical 
profiles or angularity. 
Therefore, what is needed is a method of creating retroreflective articles 
with prototype or target optical properties without the need for expensive 
tooling. 
SUMMARY OF INVENTION 
The present invention relates to a flexible, retroreflective sheeting 
deformed to produce target optical properties. The present invention is 
also directed to a process of deforming a retroreflective sheeting into a 
three-dimensional article having such optical properties. 
The retroreflective sheeting includes a multiplicity of discrete, 
cube-corner elements cured in situ on a transparent, polymeric overlay 
film. The retroreflective sheeting is deformed into a three-dimensional 
structure so that base edges of a plurality of cube-corner elements are 
non-planar with respect to one another to produce at least one target 
optical property. The target optical properties may be a desired optical 
profile, angularity, three-dimensional appearance, whiteness, 
glitter-effect, or combinations thereof The retroreflective sheeting is 
preferably a single, unitary sheet. 
The base edges of a plurality of adjacent cube-corner elements may be 
non-planar or tilted with respect to one another. The base edges of one or 
more cube-corner elements are preferably not parallel to a front surface 
of the overlay film. The cube-corner elements may have a variable density 
across a portion of the retroreflective article. Adjacent cube-corner 
elements across a portion of the retroreflective article may have a 
variable spacing. The overlay film may have a thickness that varies across 
a portion of the retroreflective article. 
The present retroreflective article may be used as a master to produce 
tooling for forming additional retroreflective articles. 
The three-dimensional structure may have one or more embossed symbols. The 
retroreflective sheeting may optionally include a specular reflector 
coated on the cube-corner elements. The retroreflective sheeting may 
optionally include a sealing film extending substantially across the 
cube-corner elements opposite the overlay film. Metallized cube-corner 
elements may optionally be backfilled with a coating, such as a polymeric 
material, resin or adhesive. In one embodiment, the coating may be applied 
uniformly or in a pattern, such as printing symbols in one or more colors. 
The polymeric overlay film preferably has a first elastic modulus and the 
cube-corner elements preferably have a second elastic modulus greater than 
the first elastic modulus. The cube-corner elements preferably are 
constructed from a thermoset polymer. The polymeric overlay film is 
preferably constructed from a thermoformable polymer. The overlay film may 
be selected from the group consisting of the following: ionomeric ethylene 
copolymers, plasticized vinyl halide polymers, acid-functional ethylene 
copolymers, aliphatic polyurethanes, aromatic polyurethanes, other light 
transmissive elastomers, and combinations thereof The cube-corner elements 
may be selected from the group consisting of monofunctional, difunctional, 
and polyfunctional acrylates or combinations thereof. 
The present invention is also directed to a method of forming a 
retroreflective article having at least one target optical property. A 
cube-corner retroreflective sheeting is prepared having a multiplicity of 
discrete, cube-corner elements cured in situ on a transparent, polymeric 
overlay film. The flexible retroreflective sheeting is deformed into a 
three-dimensional configuration so that the base edges of a plurality of 
cube-corner elements are non-planar with respect to one another. 
The step of deforming may include tilting the base edges of the plurality 
of adjacent cube-corner elements with respect to one another. The step of 
deforming is preferably selected from the group consisting of 
thermo-forming, vacuum-forming, embossing, and combinations thereof The 
step of deforming may include forming a three-dimensional symbol in the 
retroreflective sheeting, altering the density and/or spacing of at least 
a portion of the cube-corner elements, or stretching the retroreflective 
sheeting in at least one direction. The step of stretching may include 
uniformly (or non-uniformly) stretching or biaxially stretching the 
retroreflective sheeting. The step of deforming may include altering the 
base edges of one or more cube-corner elements so that they are not 
parallel to a front surface of the overlay film. 
The cube-corner elements may optionally be coated with a spectral 
reflector. A sealing film may optionally be bonded substantially across an 
exposed surface of the cube-corner elements either before or after the 
step of deforming the retroreflective sheeting. 
In an alternate embodiment, a mold is formed from the cube-corner elements 
of the deformed retroreflective article. A polymeric material is applied 
to the mold and the polymeric material is at least partially cured. The 
polymeric material is then removed from the mold so that a second 
retroreflective article is produced. 
As used herein: 
Deforming refers to thermo-forming, vacuum-forming, embossing, molding, 
stamping, elastic or inelastic stretching, uniformly or non-uniformly 
stretching, or combinations thereof. 
Symbol refers to any alphanumeric character, logo, seal, geometric pattern 
or combinations thereof. 
Target Optical Properties refers to a desired optical profile, angularity, 
three-dimensional appearance, whiteness, glitter-effect, or combinations 
thereof.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
The present invention relates to a retroreflective article formed from a 
flexible, retroreflective sheeting to produce target optical properties 
and to a process of deforming a retroreflective sheeting into a 
three-dimensional article. The retroreflective sheeting has a multiplicity 
of discrete, cube-corner elements cured in situ on a transparent, 
polymeric overlay film. The retroreflective sheeting is deformed into a 
three-dimensional structure so that the base edges of a plurality of 
cube-corner elements are non-planar with respect to one another. 
The retroreflective article of the present invention has the ability to 
reflect substantial quantities of incident light back towards the light 
source while exhibiting target optical properties. The present 
retroreflective article is suitable for being incorporated into a variety 
of products, such as clothing, shoes, license plates, signs, vehicle 
markings, cone sleeves and barrel wraps. 
Methods of making a glittering retroreflective articles are disclosed in 
the following related applications filed on the same day herewith: "Method 
of Making Glittering Retroreflective Sheeting", Ser. No. 08/641,129; "Mold 
for Producing Glittering Cube-Corner Retroreflective Sheeting", Ser. No. 
08/640,383; and "Glittering Cube-Corner Retroreflective Sheeting", Ser. 
No. 08/640,326. 
FIG. 5 shows the backside of a unitary cube-corner sheeting 60 that has 
been deformed to produce at least one target optical property. Cube-corner 
elements 30 are similar to those depicted in FIG. 4. Each cube-corner 
element 30 meets, but is not necessarily connected to, an adjacent 
cube-corner element at a base edge 35. The array includes three sets of 
parallel grooves 45, 46, and 47. The external dihedral angles (designated 
as a in FIG. 6) between faces 31 of adjacent cube-corner elements 30 vary 
along the grooves 45-47 in the array. The base edges 35 of the cube-corner 
elements in the array are nonplanar. Consequently, the apex 34 of one 
cube, such as cube 30a may be relatively close to another apex such as 
cube 30b, but the apex of cube 30b may then be further away from another 
adjacent apex such as the apex of cube 30c. 
FIG. 6 is an exemplary illustration of distances the base edges 35 are 
offset or tilted with respect to one another, or with respect to the front 
surface 51. For cube-corner elements that are about 50 to 200 micrometers 
high, the variation in height between adjacent base edges typically is 
about 0 to 50 micrometers. It will be understood that the present 
retroreflective article may be deformed on a micro or macro level. As will 
be discussed in the Examples, the retroreflective sheeting may be deformed 
over coated abrasive paper containing abrasive grains with diameters of 
about 100 to 550 micrometers. Abrasive grains of this size have radii of 
curvatures of about 50 to 225 micrometers. The retroreflective sheeting 
may be deformed over smaller structures, in the range of about 10 to 50 
micrometers, although the change in the optical properties may be minimal. 
It is believed that the change in the optical properties of the 
retroreflective sheeting when deformed over micro structures in the range 
of about 250 to 10 microns is a function of the size of the cube-corner 
elements and the thickness of the overlay film. For example, smaller 
cube-corner elements and/or a thinner overlay film may be more susceptible 
to deformation over micro structures within this range. 
FIG. 6 is a sectional view of the cube-corner sheeting 60 of FIG. 5 showing 
the position of one cube apex relative to another. Additionally, FIG. 5 
shows tilting or canting of the base edges 35 relative to one another and 
relative to front surface 51. The base edge 35 of one cube may be disposed 
closer to or further away from the front surface 51 of overlay film 58 
than the base edges of other adjacent cube-corner elements due to 
deformation of the overlay film 58. If the unitary cube-corner sheeting 60 
possesses a land layer 56, it is also not uniformly spaced from the front 
surface 51. The cube-corner sheeting 60 preferably does not have a land 
area 56, such that each cube-corner element 30 is a discrete entity. When 
the cube-corner elements are tilted, the base edges 35 of many of the 
cube-corner elements 30 do not reside in the same plane as the front 
surface 51. Additionally, the edges 35 of one or more cube-corner elements 
30 are not parallel to the front surface 51. Either surface of the overlay 
film 58 may optionally contain symbols printed on or formed therein. 
FIG. 6 also shows the external dihedral angle, .alpha., that defines the 
angle between faces 31 of adjacent cube-corner elements 30. Angle .alpha. 
may vary along all grooves in a single parallel groove set, it may vary 
along all grooves in two parallel groove sets, or it may vary along 
grooves in all three groove sets in the array. In an array of randomly 
tilted cube-corner elements, angle .alpha. varies randomly amongst 
adjacent faces of adjacent cube-corner elements throughout essentially the 
whole array. 
The overlay film 58 in body portion 54 typically has an average thickness 
of approximately 20 to 1200 micrometers, and preferably is about 50 to 400 
micrometers. The cube-corner elements typically have an average height of 
about 20 to 500 micrometers, more typically of about 25 to 200 
micrometers. The optional land layer 56 preferably is kept to a minimal 
thickness of 0 to 150 micrometers, and is preferably as close to zero as 
possible so that the strain generated during deformation does not 
propagate laterally through the land area. A coating may optionally be 
applied to the exposed metallized cube-corner elements 30 to provide the 
deformations of the retroreflective article 60 with additional structural 
support. For some applications, it may be desirable for the 
retroreflective article to be a free-standing, self-supporting structure. 
In one embodiment, the coating is a polymeric material, resin or an 
adhesive. The coating may optionally contain a pigment or dye of one or 
more colors. Additionally, the coating may be applied uniformly or in a 
pattern containing symbols using a variety of printing techniques. 
Metallized retroreflective sheeting generally maintains higher brightness 
after deformation because T.I.R. tends to break down in the unsealed 
sheeting. 
FIG. 7 shows cube-corner elements intersected by a plane that is parallel 
to the front surface 51. As illustrated, the plane does not intersect each 
cube to produce a triangle 62 of the same cross-sectional area. One cube 
may be tilted or offset from the front surface 51 to such an extent that 
the intersecting plane only passes through a tip of the cube, resulting in 
a small triangular cross-section--whereas, a cube that stands upright may 
be intersected such that the triangle resulting from the cross-section is 
relatively large. Thus, even though the cube-corner elements in the array 
may be of similar size, they can produce triangles of random sizes when 
intersected as described because of the manner in which the cubes are 
tilted or offset with respect to a reference plane. It will be understood 
that the spacing between the cube-corner elements 30 can vary, as will be 
discussed below, although retroreflectivity tends to decrease as spacing 
increases. 
FIG. 8 shows a retroreflective article 61 that has a seal film 63 disposed 
over the backside of cube-corner elements 30, such as is disclosed in U.S. 
Pat. No. 4,025,159. The seal film 63 is bonded to the body portion of the 
sheeting through the cube-corner elements 30 by a plurality of seal lines 
64. The bonding pattern produces a plurality of hermetically sealed 
chambers 65 that prevent moisture and dirt from contacting the backside of 
the cube-corner elements. Chambers 65 enable the cube-air interface to be 
maintained to prevent loss of retroreflectivity. The cube-corner elements 
30 may optionally be coated with a reflective material on the surface 67, 
such as vapor depositing or chemically depositing a metal such as 
aluminum, silver, nickel, tin, copper, or dielectric materials as are 
known in the art of cube-corner retroreflective articles. It will be 
understood that the retroreflective sheeting 61 will typically have a 
metal layer on the surface 67 or a seal film 63, but not both. 
Although the glittering effect typically would not be noticeable, or 
significantly noticeable, within each seal line because the cube-corner 
elements typically become engulfed in the seal line, the glittering effect 
is very noticeable "substantially beyond" the seal line(s). That is, the 
glittering effect may be noticed at a distance beyond where heat and/or 
pressure from the sealing operation would affect the cube-corner elements 
in the array. Typically, a sealing operation that used heat and/or 
pressure would not affect the cube-corner elements at a distance greater 
than two millimeters (mm), and more typically at 5 mm or more from a seal 
line. 
Preferably, the sealing layer comprises a thermoplastic material with a 
similar low elastic modulus as the overlay film 68. Illustrative examples 
include ionomeric ethylene copolymers, plasticized vinyl halide polymers, 
acid functional polyethylene copolymers, aliphatic polyurethanes, aromatic 
polyurethanes, and combinations thereof In certain applications, the 
optional sealing layer 63 can provide significant protection for the 
cube-corner elements of the composite material from environmental effects, 
as well as maintaining a sealed air layer around the cube-corner elements 
which is essential for creating the refractive index differential needed 
for total internal reflection. As a result of the decoupling of 
cube-corner elements 30, the sealing layer 63 may optionally be adhered, 
at least in part, directly to the overlay film 68 between independent 
cube-corner elements. 
The seal film may be bonded to the cube-corner elements in the body portion 
of the sheeting using known techniques; see for example, U.S. Pat. No. 
4,025,159. Sealing technique examples include radio frequency welding, 
thermal fusion, conductive heat sealing, ultrasonic welding, and reactive 
welding. When applying a seal film to the backside of a retroreflective 
sheeting, considerable attention must be paid to the composition and 
physical properties of the seal film. The seal film must be able to 
securely bond to the backside of the cube-corner sheeting and should not 
contain components that could adversely affect retroreflectivity or the 
appearance of the retroreflective product. For example, the seal film 
should not contain components that could leach out (e.g., dyes) and 
contact the backside of the cube-corner elements. The sealing film 
typically comprises a thermoplastic material because such materials lend 
themselves well to fusing through relatively simple and commonly available 
thermo-bonding techniques. 
FIG. 9 is a schematic illustration of an apparatus 120 for casting and 
curing retroreflective sheeting suitable for use in the present invention. 
Overlay film 121 is drawn along guiding roller 122 or from a stock roll of 
material to nip roller 123, e.g., a rubber coated roller, where overlay 
film 121 contacts suitable resin formulations 124 previously applied to 
patterned tool roll 125 through coating die 126. The excess resin 
extending above the cube-corner element forming cavities 127 of tool 125 
is minimized by setting nip roller 123 to a gap setting that is 
effectively less than the height of the cube-corner forming elements of 
tool 125. It will be understood that the gap setting may be achieved by 
applying pressure to the nip roller 123. In this fashion, mechanical 
forces at the interface between nip roller 123 and tool 125 insure that a 
minimum amount of resin 124 extends above cavities 127 of tool 125. 
Depending on the flexibility of overlay film 121, film 121 may be 
optionally supported with suitable carrier film 128 that provides 
structural and mechanical durability to overlay film 121 during casting 
and curing. The carrier film 128 may be stripped from overlay film 121 
after the sheeting is removed from tool 125 or left intact for further 
processing of the retroreflective sheeting. Use of such a carrier film is 
particularly preferred for low modulus overlay films. 
The resin composition that forms the retroreflective array of cube-corner 
elements can be cured in one or more steps. Radiation sources 129 expose 
the resin to actinic radiation, e.g., ultraviolet light, visible light, 
etc. depending upon the nature of the resin in a primary curing step 
through the overlay film. As can be appreciated by one of skill in the 
art, the selected overlay film need not be completely or 100 percent 
transparent to all possible wavelengths of actinic radiation that may be 
used in curing the resin. Alternatively, curing can be performed by 
irradiation through a transparent tool 125, such as disclosed in U.S. Pat. 
No. 5,435,816. 
The tool 125 has a molding surface having a plurality of cavities opening 
thereon which have the shape and size suitable for forming desired 
cube-corner elements. The cavities, and thus resultant cube-corner 
elements, may be three sided pyramids having one cube-corner each, e.g., 
such as are disclosed in the U.S. Pat. No. 4,588,258, may have a 
rectangular base with two rectangular sides and two triangular sides such 
that each element has two cube-corners each, e.g., such as are disclosed 
in U.S. Pat. No. 4,938,563 Nelson et al.), or may be of other desired 
shape, having at least one cube-corner each, e.g., such as are disclosed 
in U.S. Pat. No. 4,895,428 (Nelson et al.). It will be understood by those 
skilled in the art that any cube-corner element may be used in accordance 
with the present invention. 
The tool 125 should be such that the cavities will not deform undesirably 
during fabrication of the composite article, and such that the array of 
cube-corner elements can be separated therefrom after curing. Materials 
useful in forming tooling 125 preferably machine cleanly without burr 
formation, exhibit low ductility and low graininess, and maintain 
dimensional accuracy after groove formation. The tool can be made from 
polymeric, metallic, composite, or ceramic materials. In some embodiments, 
curing of the resin will be performed by applying radiation through the 
tool. In such instances, the tool should be sufficiently transparent to 
permit irradiation of the resin therethrough. Illustrative examples of 
materials from which tools for such embodiments can be made to include 
polyolefins and polycarbonates. Metal tools are typically preferred, 
however, as they can be formed in desired shapes and provide excellent 
optical surfaces to maximize retroreflective performance of a given 
cube-corner element configuration. 
The primary curing can completely or partially cure the cube-corner 
elements. A second radiation source 130 can be provided to cure the resin 
after sheeting 131 has been removed from tool 125. The extent of the 
second curing step is dependent on a number of variables, among them the 
rate of feed-through of the materials, composition of the resin, nature of 
the crosslinking initiators used in the resin formulation, and the 
geometry of the tool. Illustrative examples include electron beam exposure 
and actinic radiation, e.g., ultraviolet radiation, visible light 
radiation, and infrared radiation. 
Removal of the retroreflective sheeting 131 from the tooling 125 typically 
generates sufficient mechanical stresses to fracture the minimal land area 
between the cube-corner elements, if any, that exists between the 
individual cube-corner elements of the sheeting. The decoupled, 
independent nature of the discrete cube-corner elements and strong bond of 
each independent element to the overlay film gives the retroreflective 
sheeting substantial flexibility, while retaining high levels of 
retroreflective performance after undergoing mechanical deformation 
stresses. 
Heat treatment of the sheeting 131 may optionally be performed after it is 
removed from the tool. Heating serves to relax stresses that might have 
developed in the overlay film or cube-corner elements, and to drive off 
unreacted moieties and reaction by-products. Typically, such treatment 
involves heating the sheeting to an elevated temperature, e.g., above the 
glass transition temperature of the subject resin. Typically a sheeting 
will exhibit an increase in retroreflective brightness after such 
treatment. 
FIG. 10 illustrates an alternate apparatus for casting and curing 
retroreflective sheeting suitable for making the present retroreflective 
article. Resin composition 124 is cast directly onto overlay film 121. The 
resin-film combination is then contacted with patterned tool roll 125 with 
pressure being applied through appropriate setting of nip roller 123. As 
in the configuration illustrated in FIG. 9, nip roller 123 serves to 
minimize the amount of resin extending above the cube-corner forming 
cavities 127 of tool 125. The resin can be cured by exposure to actinic 
radiation from a first radiation source 129, and optional second radiation 
source 130. The actinic radiation from first radiation source 129 must 
first pass through overlay film of the sheeting before impinging on the 
resin. 
The individual or discrete cube-corner elements are essentially totally 
decoupled from each other, providing the ultra-flexible character of the 
composite retroreflective sheeting. The decoupled cube-corner elements are 
no longer mechanically constrained by the effect of any land area, 
minimizing the mechanical stresses that might tend to deform them and lead 
to degradation of retroreflective performance. The discrete cube-corner 
elements of retroreflective sheeting retain a high degree of 
retroreflective brightness after being deformed. 
Retroreflective sheeting prepared according to the above method exhibits a 
retroreflective brightness, i.e., a coefficient of retroreflection, of 
greater than about 50, preferably greater than about 250, and more 
preferably greater than about 500, candela/lux/square meter, measured at 
an entrance angle of -4.degree. and an observation angle of -0.2.degree., 
when the sheeting is in a planar, non-deformed configuration. By planar it 
is meant that the sheeting is permitted to lay flat and by non-deformed it 
is meant that the sheeting has not been mechanically stressed after 
decoupling of the cube-corner elements. 
The resin composition and overlay film are preferably such that when the 
resin composition contacts the overlay film it penetrates the overlay film 
so that after the primary curing treatment an interpenetrating network 
between the material of the cube-corner elements and the material of the 
overlay film is formed such as disclosed in U.S. patent application Ser. 
No. 08/472,444 filed Jun. 7, 1995. The array of cube-corner elements 
preferably comprises a material that is thermoset or extensively 
crosslinked, and the overlay film preferably comprises a thermoplastic 
material. The superior chemical and mechanical properties of thermoset 
materials yield cube-corner elements optimally capable of maintaining 
desired retroreflectivity. 
A critical criterion in the selection of these components is the relative 
elastic modulus for each component. The term "elastic modulus" as used 
herein means the elastic modulus determined according to ASTM D882-75b 
using Static Weighing Method A with a 12.5 centimeter (5 inch) initial 
grip separation, a 2.5 centimeter (1 inch) sample width, and a 2.5 
centimeter/minute (1 one inch/minute) rate of grip separation. 
Alternatively, elastic modulus may be determined according to standardized 
test ASTM D882-75b using Static Weighing Method A with a five inch initial 
grip separation, a one inch sample width, and an inch per minute rate of 
grip separator. Under some circumstances, the polymer may be so hard and 
brittle that it is difficult to use this test to ascertain the modulus 
value precisely (although it would be readily known that it is greater 
than a certain value). If the ASTM method is not very suitable, another 
test, known as the "Nanoindentation Technique" may be employed. This test 
may be carried out using a microindentation device such as a UMIS 2000 
available from CSIRO Division of Applied Physics Institute of Industrial 
Technologies of Lindfield, New South Wales, Australia. Using this kind of 
device, penetration depth of a Berkovich pyramidal diamond indenter having 
a 65.degree. included cone angle is measured as function of the applied 
force up to the maximum load. After the maximum load has been applied, the 
material is allowed to relax in an elastic manner against the indenter. It 
is usually assumed that the gradient of the upper portion of the unloading 
data is found to be linearly proportional to force. Sneddon's analysis 
provides a relationship between the indenting force and plastic and 
elastic components of the penetration depth (Sneddon I.N. Int. J. Eng. 
Sci. 3, pp. 47-57 (1965)). From an examination of Sneddon's equation, the 
elastic modulus may be recovered in the form E/(1-v.sup.2). The 
calculation uses the equation: 
EQU E/(1-v.sup.2)=(dF/dh.sub.e)F.sub.max 1/(3.3h.sub.pmax tan(.theta.)) 
where: 
v is Poisson's ratio of the sample being tested; 
(dF/dh.sub.e) is the gradient of the upper part of the unloading curve; 
F.sub.max is the maximum applied force; 
h.sub.pmax is the maximum plastic penetration depth; 
.theta. is the half-included cone angle of the Berkovich pyramidal 
indenter; and 
E is the elastic modulus. 
Values obtained under the nanoindentation technique may have to be 
correlated back to ASTM D 882-75b. 
As discussed above in relation to the fundamental principles behind the 
optical properties of cube-corner elements, even slight distortion of the 
geometry of cube-corner elements can result in substantial degradation of 
optical properties of the cube-corner elements. Thus, higher elastic 
modulus materials are preferable for the cube-corner elements due to their 
increased resistance to distortion. The overlay film of the composite 
retroreflective sheeting is preferably a polymeric material of somewhat 
lower elastic modulus. 
During curing of the cube-corner component, depending on the composition of 
the cube-corner material, individual cube-corner elements may experience a 
certain degree of shrinkage. If the elastic modulus of the overlay film is 
too high, torsional stresses can be applied to the cube-corner elements if 
they shrink during curing. If the stresses are sufficiently high, then the 
cube-corner elements can become distorted with a resulting degradation in 
optical performance. When the elastic modulus of the overlay film is 
sufficiently lower than the modulus of the cube-corner element material, 
the overlay film can deform along with the shrinkage of cube-corner 
elements without exerting such deformational stresses on the cube-corner 
elements that would lead to undesirable degradation of the optical 
characteristics. The modulus differential between the overlay film and the 
cube-corner elements should be on the order of 1.0 to 1.5.times.10.sup.7 
pascals or more. 
As the height of the cube-corner elements diminishes, it is possible for 
this modulus differential to reach the low end of the range given 
immediately above. However, it should be kept in mind that there is a 
practical lower limit to the modulus of the cube-corner element material. 
Below a certain level, generally on the order of about 2.0 to 
2.5.times.10.sup.8 pascals for cube-corner elements about 175 microns 
(0.007 inches) in height, less for smaller cube-corner elements, the 
cube-corner elements become too flexible and do not possess sufficient 
mechanical rigidity to properly fracture upon application of stress. The 
cube-corner elements preferably have an elastic modulus of greater than 
about 25.times.10.sup.8 pascals. 
After curing, the thickness of the land area, i.e., the thickness of the 
cube-corner array material opposite the plane defined by the bases of the 
cube-corner elements, is preferably less than 10 percent of the height of 
the cube-corner elements, and more preferably less than 1 percent thereof. 
Preferably the resin will shrink at least 5 percent by volume when cured, 
more preferably between 5 and 20 percent by volume, when cured. It has 
been found that by using resin compositions of this type, cube-corner 
arrays with minimal or no land area thickness can be more easily formed, 
thereby achieving the high flexibility. For instance, resin compositions 
that shrink when cured will tend to retreat into the cube-corner-shaped 
cavity, tending to leave a land area that only connects adjacent cavities 
and therefore adjacent cube-corners with a narrow portion if applied to 
the tool in appropriate quantities. The narrow portion is readily broken 
resulting in decoupling of individual cube-corner elements as discussed 
below. Sheeting can in theory be formed with essentially no land area 
connecting adjacent cube-corner elements, however, in typical high volume 
manufacturing arrangements, a minimal land area having a thickness of up 
to 10 percent of the height of the cubes, preferably on the order of 1 to 
5 percent, will be formed. 
Resins selected for use in the array of cube-corner elements include 
cross-linked acrylate such as mono- or multi-functional acrylates or 
acrylated epoxies, acrylated polyesters, and acrylated urethanes blended 
with mono- and multi-functional monomers are typically preferred. These 
polymers are typically preferred for one or more of the following reasons: 
high thermal stability, environmental stability, and clarity, excellent 
release from the tooling or mold, and high receptivity for receiving a 
reflective coating. 
Examples of materials suitable for forming the array of cube-corner 
elements are reactive resin systems capable of being cross-linked by a 
free radical polymerization mechanism by exposure to actinic radiation, 
for example, electron beam, ultraviolet light, or visible light. 
Additionally, these materials may be polymerized by thermal means with the 
addition of a thermal initiator such as benzoyl peroxide. 
Radiation-initiated cationically polymerizable resins also may be used. 
Reactive resins suitable for forming the array of cube-corner elements may 
be blends of photoinitiator and at least one compound bearing an acrylate 
group. Preferably the resin blend contains a monofunctional, a 
difunctional, or a polyfunctional compound to ensure formation of a 
cross-linked polymeric network upon irradiation. 
Illustrative examples of resins that are capable of being polymerized by a 
free radical mechanism that can be used herein include acrylic-based 
resins derived from epoxies, polyesters, polyethers, and urethanes, 
ethylenically unsaturated compounds, aminoplast derivatives having at 
least one pendant acrylate group, isocyanate derivatives having at least 
one pendant acrylate group, epoxy resins other than acrylated epoxies, and 
mixtures and combinations thereof The term acrylate is used here to 
encompass both acrylates and methacrylates. U.S. Pat. No. 4,576,850 
(Martens) discloses examples of crosslinked resins that may be used in 
cube-corner element arrays. 
Ethylenically unsaturated resins include both monomeric and polymeric 
compounds that contain atoms of carbon, hydrogen and oxygen, and 
optionally nitrogen, sulfur, and the halogens may be used herein. Oxygen 
or nitrogen atoms, or both, are generally present in ether, ester, 
urethane, amide, and urea groups. Ethylenically unsaturated compounds 
preferably have a molecular weight of less than about 4,000 and preferably 
are esters made from the reaction of compounds containing aliphatic 
monohydroxy groups, aliphatic polyhydroxy groups, and unsaturated 
carboxylic acids, such as acrylic acid, methacrylic acid, itaconic acid, 
crotonic acid, iso-crotonic acid, maleic acid, and the like. Such 
materials are typically readily available commercially and can be readily 
cross linked. 
Some illustrative examples of compounds having an acrylic or methacrylic 
group that are suitable for use in the invention are listed below: 
(1) Monofunctional compounds: 
ethylacrylate, n-butylacrylate, isobutylacrylate, 2-ethylhexylacrylate, 
n-hexylacrylate, n-octylacrylate, isooctyl acrylate, isobornyl acrylate, 
tetrahydrofurfuryl acrylate, 2-phenoxyethyl acrylate, and 
N,N-dimethylacrylamide; 
(2) Difunctional compounds: 
1,4-butanediol diacrylate, 1,6-hexanediol diacrylate, neopentylglycol 
diacrylate, ethylene glycol diacrylate, triethyleneglycol diacrylate, 
tetraethylene glycol diacrylate, and diethylene glycol diacrylate; and 
(3) Polyfunctional compounds: 
trimethylolpropane triacrylate, glyceroltriacrylate, pentaerythritol 
triacrylate, pentaerythritol tetraacrylate, and 
tris(2-acryloyloxyethyl)isocyanurate. 
Monofunctional compounds typically tend to provide faster penetration of 
the material of the overlay film and difunctional and polyfunctional 
compounds typically tend to provide more crosslinked, stronger bonds 
within and between the cube-corner elements and overlay film. Some 
representative examples of other ethylenically unsaturated compounds and 
resins include styrene, divinylbenzene, vinyl toluene, N-vinyl formamide, 
N-vinyl pyrrolidone, N-vinyl caprolactam, monoallyl, polyallyl, and 
polymethallyl esters such as diallyl phthalate and diallyl adipate, and 
amides of carboxylic acids such as N,N-diallyladipamide. 
Illustrative examples of photopolymerization initiators that can be blended 
with acrylic compounds in cube-corner arrays include the following: 
benzil, methyl o-benzoate, benzoin, benzoin ethyl ether, benzoin isopropyl 
ether, benzoin isobutyl ether, etc., benzophenone/tertiary amine, 
acetophenones such as 2,2-diethoxyacetophenone, benzyl methyl ketal, 
1-hydroxycyclohexylphenyl ketone, 2-hydroxy-2-methyl-1-phenylpropan-1-one, 
1-(4-isopropylphenyl)-2-hydroxy-2-methylpropan-1-one, 
2-benzyl-2-N,N-dimethylamino-1-(4-morpholinophenyl)-1-butanone, 
2,4,6-trimethylbenzoyldiphenylphosphine oxide, 2-methyl-1-4(methylthio), 
phenyl-2-morpholino-1-propanone, 
bis(2,6-dimethoxybenzoyl)(2,4,4-trimethylpentyl)phosphine oxide, etc. The 
compounds may be used individually or in combination. 
Cationically polymerizable materials including but are not limited to 
materials containing epoxy and vinyl ether functional groups may be used 
herein. These systems are photoinitiated by onium salt initiators, such as 
triarylsulfonium, and diaryliodonium salts. 
Preferably, the overlay film used is a polymeric material selected from the 
group consisting of ionomeric ethylene copolymers, plasticized vinyl 
halide polymers, acid functional polyethylene copolymers, aliphatic 
polyurethanes, aromatic polyurethanes, other light transmissive elastomer, 
and combinations thereof. Such materials typically provide overlay films 
that are imparted with the desired durability and flexibility to the 
resultant retroreflective sheeting while permitting desired preferred 
penetration by the cube-corner element resin composition. 
The overlay film preferably comprises a low elastic modulus polymer, e.g., 
less than about 13.times.10.sup.8 pascals, to impart easy bending, 
curling, flexing, conforming, or stretching to the resultant 
retroreflective composite. Generally, the overlay film comprises a polymer 
having a glass transition temperature less than about 50.degree. C. The 
polymer preferably is such that the overlay film retains its physical 
integrity under the conditions it is exposed to as the resultant composite 
retroreflective sheeting is formed. The polymer desirably has a Vicat 
softening temperature that is greater than 50.degree. C. The linear mold 
shrinkage of the polymer desirably is less than 1 percent, although 
certain combinations of polymeric materials for the cube-corner elements 
and the overlay will tolerate a greater extent of shrinkage of the overlay 
material. Preferred polymeric materials used in the overlay are resistant 
to degradation by UV light radiation so that the retroreflective sheeting 
can be used for long-term outdoor applications. The overlay film should be 
light transmissive and preferably is substantially transparent. 
The overlay film may be either a single layer or multilayer component as 
desired. Either surface of the overlay film may contain printed or formed 
(such as stamped or embossed) symbols. If multilayer, the layer to which 
the array of cube-corner elements is bonded should have the properties 
described herein as useful in that regard with other layers not in contact 
with the array of cube-corner elements having selected characteristics as 
necessary to impart desired characteristics to the resultant composite 
retroreflective sheeting. An alternate overlay is disclosed in U.S. patent 
application Ser. No. 08/516,165 filed Aug. 17, 1995. 
The overlay film should be sufficiently extensible to achieve decoupling of 
the cube-corner elements as discussed herein. It may be elastomeric, i.e., 
tend to recover to at least some degree after being elongated, or may have 
substantially no tendency to recover after being elongated, as desired. 
Illustrative examples of polymers that may be employed in overlay films 
herein include: 
(1) Fluorinated polymers such as: poly(chlorotrifluoroethylene), for 
example KEL-F800 Brand available from Minnesota Mining and Manufacturing, 
St. Paul, Minn.; poly(tetrafluoroethylene-cohexafluoropropylene), for 
example EXAC FEP Brand available from Norton Performance, Brampton, Mass.; 
poly(tetrafluoroethylene-co-perfluoro(alkyl)vinylether), for example, EXAC 
PEA Brand also available from Norton Performance; and poly(vinylidene 
fluoride-cohexafluoropropylene), for example, KYNAR FLEX-2800 Brand 
available from Pennwalt Corporation, Philadelphia, Pa.; 
(2) Ionomeric ethylene copolymers such as: poly(ethylene-co-methacrylic 
acid) with sodium or zinc ions such as SURLYN-8920 Brand and SURLYN-9910 
Brand available from E.I. duPont Nemours, Wilmington, Del.; 
(3) Low density polyethylenes such as: low density polyethylene; linear low 
density polyethylene; and very low density polyethylene; 
(4) Plasticized vinyl halide polymers such as plasticized 
poly(vinychloride); 
(5) Polyethylene copolymers including: acid functional polymers such as 
poly(ethylene-co-acrylic acid) and poly(ethylene-co-methacrylic acid) 
poly(ethylene-co-maleic acid), and poly(ethylene-co-fumaric acid); acrylic 
functional polymers such as poly(ethylene-co-alkylacrylates) where the 
alkyl group is methyl, ethyl, propyl, butyl, et cetera, or CH.sub.3 
(CH.sub.2)n-- where n is 0 to 12, and poly(ethylene-co-vinylacetate); and 
(6) Aliphatic and aromatic polyurethanes derived from the following 
monomers (1)-(3): (1) diisocyanates such as 
dicyclohexylmethane-4,4'-diisocyanate, isophorone diisocyanate, 
1,6-hexamethylene diisocyanate, cyclohexyl diisocyanate, diphenylmethane 
diisocyanate, and combinations of these diisocyanates, (2) polydiols such 
as polypentyleneadipate glycol, polytetramethylene ether gylcol, 
polycaprolactonediol, poly-1,2-butylene oxide glycol, and combinations of 
these polydiols, and (3) chain extenders such as butanediol and 
hexanediol. Commercially available urethane polymers include: PN-04, or 
3429 from Morton International Inc., Seabrook, N.H., or X-4107 from B. F. 
Goodrich Company, Cleveland, Ohio. 
Combinations of the above polymers also may be employed in the overlay 
film. Preferred polymers for the overlay film include: the ethylene 
copolymers that contain units that contain carboxyl groups or esters of 
carboxylic acids such as poly(ethylene-co-acrylic acid), 
poly(ethylene-co-methacrylic acid), poly(ethylene-co-vinylacetate); the 
ionomeric ethylene copolymers; plasticized poly(vinylchloride); and the 
aliphatic urethanes. These polymers are preferred for one or more of the 
following reasons: suitable mechanical properties, good adhesions to the 
cube-corner layer, clarity, and environmental stability. 
Colorants, ultraviolet ("UV") absorbers, light stabilizers, free radical 
scavengers or antioxidants, processing aids such as antiblocking agents, 
releasing agents, lubricants, and other additives may be added to one or 
both of the retroreflective layer and overlay film if desired, either 
uniformly in the configuration of a symbol. The particular colorant 
selected depends on the desired color; colorants typically are added at 
about 0.01 to 1.5 weight percent for a given layer. UV absorbers typically 
are added at about 0.5 to 2.0 weight percent. Illustrative examples of 
suitable UV absorbers include derivatives of benzotriazole such as TINUVIN 
Brand 327, 328, 900, 1130, TINUVIN-P Brand, available from Ciba-Geigy 
Corporation, Ardsley, N.Y.; chemical derivatives of benzophenone such as 
UVINUL Brand M40, 408, D-50, available from BASF Corporation, Clifton, 
N.J.; SYNTASE Brand 230, 800, 1200 available from Neville-Synthese 
Organics, Inc., Pittsburgh, Pa.; or chemical derivatives of 
diphenylacrylate such as UVINUL Brand N35, 539, also available from BASF 
Corporation of Clifton, N.J. Light stabilizers that may be used include 
hindered amines, which are typically used at about 0.5 to 2.0 weight 
percent. Examples of hindered amine light stabilizers include TINUVIN 
Brand 144, 292, 622, 770, and CHIMASSORB Brand 944 all available from the 
Ciba-Geigy Corp., Ardsley, N.Y. Alternate hindered amines are disclosed in 
U.S. Pat. No. 5,387,458. Free radical scavengers or antioxidants may be 
used, typically, at about 0.01 to 0.5 weight percent. Suitable 
antioxidants include hindered phenolic resins such as IRGANOX Brand 1010, 
1076, 1035, or MD-1024, or IRGAFOS Brand 168, available from the 
Ciba-Geigy Corp., Ardsley, N.Y. Small amounts of other processing aids, 
typically no more than one weight percent of the polymer resins, may be 
added to improve the resin's processability. Useful processing aids 
include fatty acid esters, or fatty acid amides available from Glyco Inc., 
Norwalk, Conn., metallic stearates available from Henkel Corp., Hoboken, 
N.J., or WAX E Brand available from Hoechst Celanese Corporation, 
Somerville, N.J. 
The present retroreflective article can be made in accordance with two 
different techniques. In the first technique, a retroreflective article is 
made by providing a first cube-corner sheeting that has the cubes arranged 
in a conventional configuration, namely, a non-random orientation, and 
deforming this sheeting under heat and/or pressure. In the second 
technique, the deformed retroreflective article can be used to create 
tooling. The tooling may be used as a mold to cast or form additional 
retroreflective articles. 
In one embodiment, the retroreflective article of the present invention is 
made by thermoforming the cube-corner retroreflective sheeting over a 
structured three-dimensional surface of a mold, such as illustrated in 
FIGS. 11 and 12. In FIG. 11, the cube-corner elements 150 are placed over 
the structured surface of a mold 152. The overlay film 154 is located 
opposite an isolation web 156 to prevent the overlay film 154 from melting 
or adhering to diaphragm 158. Alternatively, the diaphragm 158 may have 
release properties that perform the function of the isolation web 156. 
Heat and/or pressure are applied to the retroreflective sheeting 160 
through the thermoforming diaphragm 158. The three-dimensional shape of 
the mold 152 may also include a variety of embossed symbols. 
In an alternate embodiment illustrated in FIG. 12, overlay film 170 is 
placed on the structured surface of a mold 172. The cube-corner elements 
174 is located opposite an isolation web 176. Heat and/or pressure are 
applied to the retroreflective sheeting 180 through the diaphragm 178. An 
apparatus suitable for thermoforming the retroreflective sheeting to form 
the present retroreflective article is available under the trade 
designation Scotchlite.TM. Heat Lamp Vacuum Applicator available from 
Dayco Industries, Inc. of Niles, Mich. or P.M Black Co. of Stillwater, 
Minn. 
Important thermoforming processing variables that may determine the nature 
of the retroreflective article created include temperature, pressure, 
duration of each, thickness and thermal characteristics of the 
thermoforming diaphragm and the nature of the structured surface on the 
mold. The size, uniformity and rigidity of the mold may also alter the 
processing specifications of the thermoforming process as well as whether 
the mold has an optical or a non-optical pattern. The construction of the 
retroreflective sheeting, such as the thickness, softening temperature and 
extensibility of the overlay film, size of the cube-corner elements, the 
presence or absence of a vapor coat, whether a sealing film is present and 
the optical design of the retroreflective sheeting may also determine 
thermoforming processing variables. 
Vacuum forming yields a retroreflective article in which the overlay film 
becomes thinner in proportion to the distance the sheet travels to contact 
the mold surface. Consequently, the spacing gradient between adjacent 
cube-corner elements increases from the top of a protrusion on the mold 
toward the bottom of the depression. The increased spacing generally 
produces lower retroreflectivity. Additionally, if the retroreflective 
sheeting includes a sealing film, the film is visible through the gap 
between the cube-corner elements. The sealing film may be applied either 
before or after deformation of the cube-corner sheeting. The sealing film 
may include one or more colors that would be visible during daytime 
viewing. 
In an embodiment in which the cube-corner elements of the retroreflective 
sheeting are coated with a specular reflector, a colored back coating may 
be visible through separations between the cube-corner elements. A colored 
back coating or adhesive serves to soften or alter the color and reduce 
the "grayness" of the specular reflector layer. Alternatively, the 
specular reflector may be a "non-silver" color, such as copper. 
In an alternate embodiment, the retroreflective sheeting may be deformed by 
drape forming. The thickness distribution of the overlay film using drape 
forming is opposite that of vacuum forming, so that the spacing gradient 
between the cube-corner elements increases along the top of a protrusions 
during formation, while the spacing between cube-corner elements along the 
bottom of a depression remains generally the same. The retroreflective 
sheeting may also be stretched in one or more directions prior to or 
during deformation. Stretching increases the gap between adjacent 
cube-corner elements and thereby reduces retroreflectivity. Reduced 
retroreflectivity may be desirable for some applications. 
In an alternate embodiment of the present invention, the retroreflective 
article of the present invention may be used to prepare a master tooling 
which can in turn be used to prepare additional retroreflective articles. 
Retroreflective sheeting may be prepared directly from the tooling. Use of 
such masters produces sheeting that is capable of retroreflecting light 
and displays the target optical properties of the original retroreflective 
article from which the tooling was prepared. Images printed, deposited, or 
formed directly on the exposed back side of the cube-corner elements by 
various techniques may also be replicated in the mold making process. 
Angularity 
Angularity refers to the concept of how retroreflectivity varies as the 
entrance angle varies. Retroreflectivity varies according to the entrance 
angle and the observation angle. The entrance angle is the angle between 
an illumination axis from a light source and a retroreflector axis normal 
to the surface of the retroreflective article. Entrance angle is usually 
no larger than 90.degree.. Angularity is typically described in terms of a 
plot of retroreflectivity on the vertical axis versus entrance angle on 
the horizontal axis. When the illumination axis, observation axis and 
retroreflector axis are in the same plane, the entrance angle can be 
considered negative when the retroreflector axis and observational axis 
are on opposite sides of the illuminator axis. 
The observation angle is the angle between the illumination axis from the 
light source and the observation axis. The observation angle is always 
positive and is typically a small acute angle. 
Optical Profile 
Optical profile refers to the concept of rotational and orientational 
symmetry of a retroreflective article. Rotational and orientational 
symmetry refers to how the retroreflected light varies as the 
retroreflective article is rotated about a normal perpendicular to the 
retroreflective surface. Plots of symmetry of rotation indicate how the 
retroreflective performance of an article will vary when oriented in 
varying directions about this axis. FIG. 3 is an example of a plot of an 
optical profile. 
EXAMPLES 
Features and advantages of this invention are further explained in the 
following illustrative Examples. For purposes of these Examples, the 
retroreflective sheeting included cube-corner elements with optical axes 
tilted or canted with respect to one another, such as generally shown in 
U.S. Pat. No. 4,588,258 to Hoopman. 
Retroreflective Brightness Test 
The coefficient of retroreflection, R.sub.A, was measured in accordance 
with standardized test ASTM E 810-93b. RA values are expressed in candelas 
per lux per square meter (cd.cndot.1x.sup.-1 .cndot.m.sup.-2). 
For observation angle scans, the other test parameters were held constant 
at: 
entrance angle=-4.0 degrees 
orientation angle=0.0 degrees 
presentation angle=0.0 
For entrance angle scans, the other test parameters were held constant at: 
orientation angle=0.0 degrees 
observation angle=0.2 degrees 
presentation angle=0.0 degrees. 
Example 1 
Preparation of a flexible retroreflective sheet 
One percent by weight of Darocur Brand 4265 (50:50 blend of 
2-hydroxy-2-methyl-1-phenylpropan-1-one and 
2,4,6-trimethylbenzoyldiphenylphosphine oxide, available from Ciba-Geigy 
Corp., Hawthorne, N.Y.) was added to a resin blend of 40 percent by weight 
Photomer Brand 4035 (phenoxyethyl acrylate available from Henkel Corp. of 
Ambler, Pa.) and 60 percent by weight Photomer Brand 3016 (bis-phenol A 
epoxy diacrylate available from Henkel Corp. of Ambler, Pa.), and 1 
percent by weight Darocur 1173 (2-hydroxy-2-methyl-1-phenylpropan-1-one, 
available from Ciba-Geigy Corp., Hawthorne, N.Y.). The resulting solution 
was used as a resin composition for forming cube-corner elements. 
The resin composition was cast onto a 0.152 mm (0.006 inches) thick 
aliphatic polyurethane overlay film (MORTHANE Brand 3429 urethane from 
Morton International, Inc., Seabrook, N.H.) on a polyethylene 
terephthalate (PET) carrier film. The coated film was passed between a 
polyurethane nip roll and the nickel electroformed tool to create 62.5 
microns (0.0025 inches) tall cube-corner elements at 57.degree. C. 
(135.degree. F.). The gap between the 90 durometer polyurethane rubber nip 
roll and the nickel tool was set to minimize the resin in the cavities. 
The resin was cured through both the overlay film and the carrier film 
with one AETEK medium pressure mercury lamp (available from AETEK 
International of Plainfield, Ill.) set at 160 watts/cm (400 W/in). The 
feed rate of material through the cure station was 1.524 meters/min. (5 
fpm). Upon completion of the microreplication process and removal from the 
tool, the side of the composite with the cube-corner elements was 
post-cured by irradiating it with a medium pressure mercury lamp (AETEK 
International) operating at 80 watts/cm (200 w/in). 
Example 2 
Vacuum-Formed Retroreflective Articles 
The retroreflective sheeting of Example 1 was placed into a clamping frame 
with the plano-side (overlay film) of the film facing upward on a 
vacuum-former Type Comet, Jr., Model 10X10 from Comet Industries, Inc. of 
Sanford, Fla. After heating the film to approximately 150.degree. C. using 
the resistance heater on the vacuum former, the film started to sag 
(approximately 20 seconds). The softened composite film was rapidly 
lowered onto a porous mold bearing a rectangular array of 90 (9.times.10) 
hemi-spherical .about.1.59 cm (0.625 inch) diameter depressions while a 
vacuum was being applied to the mold. The softened film formed a 
reflective sheet with retroreflective hemi-spherical cavities or 
depressions, shown in both a plan view and a perspective view in FIGS. 13. 
FIG. 16 illustrates an alternate retroreflective article with 
hemi-spherical protrusions formed using the process of the present 
Example. 
FIG. 14 is a photomicrograph (50X) taken from the cube side of the deformed 
retroreflective sheeting at the bottom of a vacuum-formed depression of 
FIG. 13. FIG. 15 is a photomicrograph (50X) taken of a vacuum-formed 
depression from the overlay side. The cube-corner elements are shown in 
dark and the separations between them is in white. The photomicrograph 
illustrates a ratio of the base edge of the cube-corner elements to the 
separations therebetween is in the range of about 0.5:1 to 2:1. The 
cube-corner elements are nominally adjacent to one another prior to 
deformation. As is clear from FIGS. 14 and 15, however, the vacuum forming 
process stretches and thins the overlay film and increases the separation 
of the cube-corner elements at the bottom of a depression. The generally 
uniform separation between the cube-corner elements is enhanced by heating 
the retroreflective sheeting to soften the overlay film prior to vacuum 
forming. 
Example 3 
The retroreflective sheeting of Example 1 was placed into a clamping frame 
with the plano-side of the film facing downward. The film was heated using 
the method of Example 2 until the film started to sag (approximately 10-15 
seconds). The softened composite film was rapidly lowered onto a porous 
mold bearing a rectangular array of 90 (9.times.10) hemi-spherical 
depressions (.about.0.75 inch diameter), such as illustrated in FIG. 13, 
while a vacuum was being applied to the mold. The softened film formed a 
reflective sheet with retroreflective hemi-spherical protrusions. 
FIG. 17 is a photomicrograph (50X) taken from the cube side of the deformed 
retroreflective sheeting at the top of a vacuum-formed protrusion. FIG. 18 
is a photomicrograph (50X) taken of a vacuum-formed protrusion from the 
overlay side. The cube-corner elements are shown in dark and the 
separations between them is in white. The cube-corner elements are 
nominally adjacent to one another. As is clear from FIGS. 17 and 18, 
however, the vacuum forming process stretches and thins the overlay film 
and increases the separation of the cube-corner elements at the top of a 
protrusion. The separations between the cube-corner elements are random 
due to non-uniform heating and draw, primarily a function of the shortened 
heating cycle. Some cube-corner elements are grouped together, others are 
isolated. The random separation of the cube-corner elements created a 
glittery visual appearance. It will be understood that the separation 
between the cube-corner elements can be further altered by controlling the 
draw ratio of the overlay film over the mold. 
The present photomicrographs of the retroreflective sheeting with enhanced 
glittering showed a substantially greater degree of cube-corner element 
reorientation and separation, than is present on undeformed 
retroreflective sheeting. It is believed that the enhanced glittering 
effect is related to the additional reflective paths available to light 
incident on the adjacent cube-corner elements. Accordingly, there is a 
general range of glittering image forming abilities of the retroreflective 
article of the invention which can be achieved by changing the processing 
variables. 
Example 4 
Backfilled Formed Retroreflective Article 
The retroreflective sheeting of Example 1 was metallized by vapor 
deposition of aluminum metal on the cube-corner elements. The metallized 
retroreflective sheeting was vacuum-formed with the plano-side of the film 
in contact with a mold to form a series of letters that spelled the word 
"VIPER" as shown in FIG. 19. While the formed film was still in the mold, 
a two-part polyurethane was poured into the cavity to backfill the 
cube-corner elements and thermally cured. The individual letters were cut 
out and adhered to a steel plate with a gloss black coating. The 
retroreflective sheeting is generally planar, except along the transition 
edges of the letters. The retroreflective article exhibited standard 
retroreflectivity along the planar surface. Some localized glitter-effect 
was noted along the transition edges of the letters. 
Example 5 
Preparation of a flexible retroreflective sheeting 
A mixture of 1 percent by weight of Darocur Brand 4265 (50:50 blend of 
2-hydroxy-2-methyl-1-phenylpropan-1-one and 
2,4,6-trimethylbenzoyldiphenylphosphine oxide, available from Ciba-Geigy 
Corp., Hawthorne, N.Y.) was added to a resin mixture of 19 percent by 
weight PHOTOMER Brand 3016 (a bisphenol A epoxy diacrylate, available from 
Henkel Corp., Ambler, Pa.), 49.5 percent by weight TMPTA 
(trimethylolpropane triacrylate) and 30.5% Sartomer 285 (THFA is 
tetrahydrofurfiryl acrylate, available from Sartomer Corp.). This resin 
composition was cast at 57.degree. C. (135.degree. F.) between a tool with 
85 microns (0.0034 inches) tall cube-corner elements and an aliphatic 
polyurethane overlay film 0.114 mm (0.0045 inches) thick (MORTHANE Brand 
3429 urethane from Morton International, Inc., Seabrook, N.H.) on a 
polyethylene terephthalate (PET) carrier film 0.51 mm (0.002 inches) 
thick. The rubber nip roll gap was set to minimize the amount of resin 
composition over the cavities of the tool. The resin was cured through 
both the overlay film and carrier film with one AETEK medium pressure 
mercury lamp (available from AETEK International of Plainfield, Ill.) set 
at 160 watts/cm (400 watts/in). The feed rate of material through the cure 
station was controlled to attain the desired degree of curing (exposure to 
100 to 1000 millijoules/cm.sup.2). After the microreplication process was 
completed, the cube-corner side of the composite post-cured by irradiating 
it with a medium-pressure mercury lamp (AETEK International) operated at 
80 watts/cm (200 W/in). 
Example 6 
Seated Retroreflective Sheeting 
The retroreflective sheeting of Example 5 was thermally sealed to a white 
polyurethane sealing film as follows. A laminate sample of retroreflective 
sheeting and sealing film was prepared by first protecting it with a 0.025 
mm (0.001 inch) polyester terephthalate film. This construction was then 
fed into a nip between a heated steel embossing roll and a 85 durometer 
rubber roll. The sealing film was a 0.05 mm (0.002 inches) thick white 
(TiO.sub.2) pigmented aliphatic polyester urethane (MORTHANE Brand PNO3 
supplied by Morton International, Seabrook, N.H.). The embossing pattern 
was of a chain link configuration and the embossing roll surface was 
220.degree. C. (410.degree. F.). The rubber roll surface temperature was 
63.degree. C. (145.degree. F.). The rolls were turning at a surface speed 
of 6.09 meters/minute (20 feet/minute), and the force on the nip was held 
at 114 Newtons/centimeter (65 pounds/inch). The polyester terephthalate 
protective layers were removed from the samples prior to further use. 
Example 7 
Preparation of license plate 
A 152.4.times.304.8 mm (6".times.12") piece of the retroreflective sheeting 
with a sealing film was prepared as described in Example 6. The sealed 
cube sheeting was then laminated to a pressure sensitive adhesive with a 
liner, product number 467 MP available from Minnesota Mining and 
Manufacturing Company of St. Paul, Minn. The liner was removed and the 
sheeting was laminated to a flat, white license plate blank. The resulting 
article was embossed using conventional license plate embossing 
techniques. The sample embossed very well and did not tent over the 
letters. In the view box, the sample was noticeably brighter and whiter 
than conventional beaded license plate sheeting. The candela/lux/square 
meter was 200 in the horizontal direction and 300 in the vertical 
direction. 
Example 8 
Flexible retroreflective sheeting embossed over netting 
The retroreflective sheeting of Example 6 using a pressure sensitive 
adhesive was embossed over five samples of small mesh industrial netting, 
as shown in FIG. 20. Heat lamination of the retroreflective sheeting is 
preferable, because it helps the retroreflective sheeting conform to the 
underlying netting. The industrial netting of FIG. 20, viewed from left to 
right, is sold under the product designations: NO 888 Regent--nylon 6.35 
mm (0.25 inch) square; NO 916 nylon delta 1.3 cm (0.5 inch) hex; 504-nylon 
1.3 cm (0.5 inch) square; PE-101 polyester 1.59 cm (0.625 inch) hex; and 
the horizontally orient specimen--NO 61339 polyester 3.175 mm (0.125 inch) 
hex, all available from Sterling Net Co. of Montclair, N.J. 
The netting changed both the angularity of the cube-corner elements and 
acted as a filler or cushion for the embossed retroreflective sheeting. 
The portion of the retroreflective sheeting deformed by the netting is 
shown in white and the space between the netting is shown in black. A 
localized glitter-effect was visible along the sharp transition regions in 
the retroreflective sheeting deformed over the netting. It will be 
understood that a metallized retroreflective sheeting with a suitable 
adhesive may alternately be embossed over the netting. One possible use 
could be in temporary pavement markings, which need a different angularity 
from standard retroreflective sheeting, as well as cushioning when run 
over by a car. 
Example 9 
The retroreflective sheeting of Example 1 was vacuum formed on a mold 
bearing a .RTM. symbol approximately 6.35 mm in diameter. FIG. 21 is a 
photomicrograph (50X) taken from the overlay side of the retroreflective 
sheeting. The cube-corner elements are shown in black and the separations 
in white. The asymmetry of the .RTM. symbol prevented a uniform draw, 
resulting in substantial randomization of the cube-corner elements. 
Example 10 
An unsealed retroreflective sheeting according to Example 5 with 
cube-corner elements 0.086 mm (0.0034 inches) high was thermo-formed over 
60, 100, 150 and 220 grit coated abrasive paper available from Minnesota 
Mining and Manufacturing Company of St. Paul, Minn. using the 
Scotchlite.TM. Heat Lamp Vacuum Applicator discussed above. The 
cube-corner elements were positioned opposite the coated abrasive paper. 
The bake cycle included warming the applicator to approximately 
118.degree. C. and baking for about 1.5-2.5 minutes. The lamp bank was 
raised at the end of the bake cycle to cool the retroreflective articles. 
FIG. 22A is a plot of the relative brightness versus entrance angle for the 
resulting retroreflective articles. FIG. 22B is a plot of the relative 
brightness versus the observation angle. The control plot is the 
undeformed retroreflective sheeting. The retroreflective article had a 
glittery appearance presumably due to the high level of randomization of 
the base edges of the cube-corner elements. 
Example 11 
An unsealed retroreflective sheeting according to Example 5 with 
cube-corner elements 0.086 mm (0.0034 inches) high was metallized by vapor 
deposition of aluminum metal on the cube-corner elements. The metallized 
retroreflective sheeting was thermo-formed over 60, 100, 150 and 220 grit 
coated abrasive paper according to the method of Example 10. The grit 
designations refer to abrasive particles with diameters no larger than 551 
microns, 336 microns, 169 microns and 100 microns, respectively. The 
cube-corner elements were positioned opposite the coated abrasive paper. 
FIG. 23A is a plot of the relative brightness versus entrance angle for 
the resulting retroreflective articles. FIG. 23B is a plot of the relative 
brightness versus the observation angle. The control plot is the 
undeformed metallized retroreflective sheeting. 
The retroreflective article had a glittery appearance presumably due to the 
high level of randomization of the base edges of the cube-corner elements. 
The retroreflective sheeting was also thermo-formed over a beaded pavement 
marker available under product designation 5160 Scotchlane.TM. foil backed 
tape from Minnesota Mining and Manufacturing Company of St. Paul, Minn. 
according to the method of Example 10. FIG. 23C is a bar graph showing the 
increase in whiteness of the retroreflective sheeting after the 
thermoforming process for the four coated abrasive paper specimens and the 
beaded pavement marker. Whiteness is measured using a spectrophotometer 
with a bidirectional optical measuring system according to ASTM E 1349-90. 
Whiteness is believed to be an approximate measure of the glittery 
appearance of retroreflective sheeting. The level of whiteness for the 
retroreflective article thermo-formed over the 100 grit coated abrasive 
paper is believe to be a function of the size of the cube-corner elements 
relative to the grit of the coated abrasive paper. That is, the 100 grit 
coated abrasive paper provided the greatest level of randomization of the 
base edges of cube-corner elements 0.086 mm high. 
Example 12 
An unsealed retroreflective sheeting according to Example 5 with 
cube-corner elements 0.086 mm (0.0034 inches) high was thermo-formed over 
a series of specimens using the method of Example 10. The specimen 
included a beaded pavement marker available under product designation 5160 
Scotchlane.TM. foil backed tape and a raised pavement marker available 
under product designation A381 Stamark.TM. high performance tape, both 
from Minnesota Mining and Manufacturing Company of St. Paul, Minn.; a tool 
for manufacturing retroreflective sheeting with cube-corner elements 0.178 
mm (0.007 inches) high; and a light diffuser available under the product 
designation Clear Prismatic from Plaskolite, Inc. of Columbus, Ohio. The 
cube-corner elements were positioned opposite the specimens. 
FIG. 24A is a plot of the relative brightness versus entrance angle for the 
resulting retroreflective articles. FIG. 24B is a plot of the relative 
brightness versus the observation angle. The control plot is undeformed 
retroreflective sheeting. Variation in the glittery appearance of the 
retroreflective articles was presumably due to the various levels of 
randomization of the base edges of the cube-corner elements. 
Example 13 
An unsealed retroreflective sheeting according to Example 5 with 
cube-corner elements 0.086 mm (0.0034 inches) high was metallized by vapor 
deposition of aluminum metal on the cube-corner elements. The metallized 
retroreflective sheeting was thermo-formed over the beaded pavement 
marker, raised pavement marker and light diffuser of Example 12 using the 
method of Example 10. The cube-corner elements were positioned opposite 
the specimens. 
FIG. 25A is a plot of the relative brightness versus entrance angle for the 
resulting retroreflective articles. FIG. 25B is a plot of the relative 
brightness versus the observation angle. The control plot is the 
undeformed retroreflective sheeting. 
Example 14 
A retroreflective sheeting according to Example 5 with cube-corner elements 
0.086 mm (0.0034 inches) high was metallized by vapor deposition of 
aluminum metal on the cube-corner elements. The metallized retroreflective 
sheeting was thermo-formed using the method of Example 10 over a 
polypropylene industrial mesh netting with a 1.27 cm (0.5 inch) hex 
pattern, sold under the product designation NO916 by Sterling Net Co. of 
Montclair, N.J. The netting softened during the thermo-forming process and 
thus remained bonded to the retroreflective sheeting. The cube-corner 
elements were positioned opposite the specimens. 
FIG. 26A is a plot of the relative brightness versus entrance angle for the 
resulting retroreflective articles. FIG. 26B is a plot of the relative 
brightness versus the observation angle. The control plot is the 
undeformed retroreflective sheeting. 
Example 15 
Three samples of the unsealed retroreflective sheeting according to Example 
5 with cube-corner elements of different sizes were thermo-formed over a 
beaded pavement marker available under product designation 5160 
Scotchlane.TM. foil backed tape from Minnesota Mining and Manufacturing 
Company of St. Paul, Minn. The cube-corner elements were 0.0625 mm (0.0025 
inches); 0.086 mm (0.0034 inches) and 0.178 mm (0.007 inches) high, 
respectively. The greatest glitter-effect was visible on the 
retroreflective sheeting thermo-formed over the 0.178 mm cubes. The least 
amount of glitter-effect was visible on the retroreflective sheeting 
thermo-formed over the 0.0625 cubes. 
Example 16 
An unsealed retroreflective sheeting according to Example 5 with 
cube-corner elements 0.086 mm (0.0034 inches) high was metallized by vapor 
deposition of aluminum metal on the cube-corner elements. The metallized 
retroreflective sheeting was thermo-formed over the cube-corner side of 
three commercial reflectors. Reflector A was a 7.62 cm (3 inch) circular 
reflector divided into 6 pie-shaped wedges of cube corners, sold as Model 
V472R from Peterson Manufacturing of Grandview, Miss. Reflector B was a 
7.62 cm (3 inch) circular reflector having about 20 diamond shaped 
patterns 1.27.times.2.54 cm (0.5.times.1.0 inch) containing cube corner 
elements, sold as Model Sate-lite-30 from KyKu Products of Bedford 
Heights, Ohio. The Rectangular reflector 6.35.times.7.62 cm (2.5.times.3.0 
inches) had vertical rows of cube corner elements offset from each other, 
sold as Model PEC 4200C from The Refractory of Newburgh, N.Y. 
FIG. 27A is a plot of the relative brightness versus entrance angle for the 
resulting retroreflective articles. FIG. 27B is a plot of the relative 
brightness versus the observation angle. The control plot is the 
undeformed metallized retroreflective sheeting. FIGS. 27C is a plot of the 
relative brightness versus entrance angle for the commercial reflectors 
illustrated in FIGS. 27A and 27B. FIG. 27D is a plot of the relative 
brightness versus the observation angle for the commercial reflectors. 
All patents and patent applications cited above are incorporated by 
reference in their entirety into this document. 
The present invention has now been described with reference to several 
embodiments thereof It will be apparent to those skilled in the art that 
many changes can be made in the embodiments described without departing 
from the scope of the invention. Thus, the scope of the present invention 
should not be limited to the structures described herein, but rather by 
the structures described by the language of the claims, and the 
equivalents of those structures.