Optical: systems and elements – Signal reflector – Including a curved refracting surface
Reexamination Certificate
2000-04-11
2002-07-09
Phan, James (Department: 2872)
Optical: systems and elements
Signal reflector
Including a curved refracting surface
C359S538000, C359S540000, C428S204000, C428S207000
Reexamination Certificate
active
06416188
ABSTRACT:
BACKGROUND
The present invention relates generally to launderable retroreflective products for use in personal articles such as articles of clothing. More particularly, the invention relates to retroreflective appliques that use a dielectric mirror to assist in retroreflection and to permit the applique to exhibit a variety of daytime colors.
The reader is directed to the glossary at the end of the specification for guidance on the meaning of certain terms used herein.
The use of retroreflective materials (sometimes referred to in the literature as reflective materials) such as retroreflective appliques to increase the visibility of pedestrians has long been known. Such materials have the property of reflecting incident light, such as light from a vehicle headlamp, back in the general direction from which the light originated, regardless of the angle at which the incident light impinges on the surface of the material. Thus, a person wearing such a material can be highly visible to drivers of such vehicles at night, depending on (i) the amount of retroreflective material used, and (ii) the reflectivity of the material.
The retroreflectivity is provided by a multitude of reflective facets arranged as cube corner elements, or, more commonly, by a layer of tiny glass beads or microspheres that cooperate with a specularly reflective material that is referred to herein as a mirror. In the latter case, the beads are partially embedded in a binder layer that holds the beads to a fabric or other substrate material, and partially exposed to the atmosphere. Incident light enters the exposed portion of a bead and is focused by the bead onto the mirror, which is disposed at the back of the bead embedded in the binder layer, whereupon the light is reflected back through the bead, exiting through the exposed portion in a direction opposite to the incident direction. This type of construction is referred to as “exposed lens”, because it uses microspheres with portions that are exposed to the atmosphere.
In exposed lens retroreflective appliques, a simple aluminum coating is the most commonly used mirror. Aluminum provides the article with good reflectivity for densely packed beads, and has good durability, but the aluminum is opaque to visible light and renders a “gray-cast” daytime appearance. In applications where it is important that the applique exhibit other daytime colors, it is known to provide a dielectric layer or layers as the mirror. See, e.g., U.S. Pat. No. 3,700,305 (Bingham) or U.S. Pat. No. 4,763,985 (Bingham). The dielectric layer is partially reflective and partially transmissive to visible light. This characteristic permits the article to exhibit color by use of a colored binder layer or, if the binder layer is colorless, by use of a colored barrier coat, fabric, or other substrate. The color of the binder layer, fabric, or substrate is visible not only in the small spaces that may exist between neighboring beads, but also through the beads themselves.
Examples of known exposed lens dielectric mirror appliques will now be described with the aid of
FIGS. 1
a-d
. In
FIG. 1
a
, a monolayer of beads
10
, also commonly referred to as microspheres, has been formed on a temporary carrier layer
12
. The beads
10
are typically made of glass or ceramic, and have a refractive index of nominally about 1.9 but that can range from about 1.7 to 2.0. The beads typically have diameters of about 30 to 200 &mgr;m, but the size of the beads is not considered critical. The beads
10
have been cascaded onto the carrier to form a monolayer of beads partially embedded in the carrier. In
FIG. 1
a
, the carrier
12
is shown as a two-layer construction that consists of an upper layer
12
a
, in which the beads are partially embedded, and a lower layer 12
b
. The upper layer
12
a
is made of a heat-softenable composition to permit easy removal of the carrier
12
after application. In one known embodiment, layer
12
a
consists of polyethylene, and layer
12
b
consists polyethylene teraphthalate (PET). In another known embodiment, layer
12
a
consists of polyethylene and layer
12
b
consists of paper. A dielectric mirror is then formed on the exposed portions of the beads
10
by first depositing a layer
14
of sodium aluminum fluoride (Na
3
AlF
6
, also known as cryolite), followed by a layer
16
of zinc sulfide (ZnS). Cryolite has a typical refractive index of about 1.34 in the visible spectrum, which is low relative to the refractive index of ZnS, which is typically about 2.35. The optical thickness (i.e., the physical thickness multiplied by the refractive index) of each layer
14
,
16
is approximately one-quarter wavelength of visible light for optimal reflective performance.
Next, as shown in
FIG. 1
b
, a layer of bead bond material
18
is applied to the dielectric mirror formed by the layers
14
,
16
. The bead bond layer maintains the integrity of the monolayer of beads
10
when the carrier
12
is later removed. An example of a known bead bond material
18
is a fluorescent colored polyester urethane.
To produce a finished transfer sheet, a polyurethane barrier coat layer
20
and a transfer adhesive layer
22
are applied in sequence as shown in
FIG. 1
c
. The barrier coat layer
20
includes certain pigments to help provide the desired daytime color. The transfer adhesive layer
22
is a heat activated polyester adhesive so that the applique can easily be applied to a substrate of interest.
In
FIG. 1
d
, the transfer sheet of
FIG. 1
c
has been applied to a fabric
24
by application of pressure and heat, and the carrier
12
is shown being removed to expose the lower portions of the beads to air, thus permitting the applique to retroreflect light that is incident from below (from the perspective of
FIG. 1
d
).
The applique just described can be sold in transfer sheet form (
FIG. 1
c
) or instead in the more finished form of
FIG. 1
d.
Another known exposed lens retroreflective applique is similar to that just described, except the bead bond layer
18
is a printable ink such as plastisol ink that functions not only as bead bond but also as a transfer adhesive, so that layers
20
and
22
are omitted. Further, such plastisol ink bead bond layer
18
can be screen printed in an image-wise pattern on the exposed portions of some beads
10
but not of others, after the dielectric mirror has been formed on the beads.
FIG. 2
shows an example of a graphic
26
useable as the image-wise pattern for the applique. This type of applique is sometimes referred to in the art as a “transfer graphic”. Plastisol inks that are used for this purpose are available from Plast-O-Meric Inc. (a subsidiary of The Geon Company, Avon Lake, Ohio) under item series SX864.
In known transfer graphic appliques, the bead bond
18
can be printed in an organic solvent-free form and then gelled by applying heat for a short period of time. This gelled state results from partial dissolution of the particles in the plasticizer and partial coalescence of the particles to form a weak image-wise film that is generally dry to the touch and can withstand mild rubbing without smearing. Once in the gelled state, the further application of heat during lamination causes the gelled plastisol to temporarily soften and flow and/or penetrate a desired substrate, such as fabric
24
. During this process the particles are further dissolved by the plasticizer, and upon cooling, they are fused into a rugged image-wise film.
Transfer graphic appliques can be sold in transfer sheet form without bead bond (
FIG. 1
a
), in a kit comprising such a transfer sheet and a separate supply of printable ink bead bond material, or instead in the more finished form of
FIG. 1
d.
If the image-wise bead bond layer
18
is applied in the form of graphic
26
(FIG.
2
), then once the carrier
12
is removed, the beads
10
, dielectric mirror
14
,
16
, and bead bond material
18
remain adhered to the substrate in the shape of the image-wise pattern or graphic
26
. In those areas, the applique provides retroreflection of incident
Ligtenberg Norman D.
Robbins William B.
Shusta Jeanine M.
3M Innovative Properties Company
Jensen Steven C.
Phan James
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