Stock material or miscellaneous articles – Structurally defined web or sheet – Continuous and nonuniform or irregular surface on layer or...
Reexamination Certificate
1999-04-02
2002-09-17
Watkins, III, William P. (Department: 1772)
Stock material or miscellaneous articles
Structurally defined web or sheet
Continuous and nonuniform or irregular surface on layer or...
C428S143000, C428S325000, C428S149000, C428S166000, C428S156000, C428S161000, C428S162000, C428S163000, C428S220000, C428S343000, C359S529000, C359S531000, C359S532000, C359S533000, C359S534000, C359S537000, C359S536000, C359S542000, C427S163400, C404S014000
Reexamination Certificate
active
06451408
ABSTRACT:
TECHNICAL FIELD
The present invention relates to retroreflective articles that exhibit good retroreflective brightness under wet and dry conditions and good retroreflective brightness at high entrance angles.
BACKGROUND
Pavement markings, such as those on the centerline and edgeline of a roadway, are important in order to provide visual guidance for motor vehicle drivers. Pavement marking materials are used as traffic control markings for a variety of uses, such as short distance lane striping, stop bars, and pedestrian pavement markings at intersections and long line lane markings, etc. on roadways. A common form of pavement marking material is adhesive-backed tape that is applied to the roadway surface in desired location and length; the top surface of the tape having selected color and typically retroreflective characteristics.
Currently, many flat pavement markings typically rely on an exposed-lens optical system comprising transparent microspheres partially embedded in a binder layer containing reflective pigment particles such as titanium dioxide (TiO
2
) or lead chromate (PbCrO
4
) as the retroreflector mechanism. In use, light from the headlamp of a vehicle enters the microsphere and is refracted to fall on the reflective pigment. Some portion of the light is returned generally along the original entrance path in the direction of the vehicle so as to be visible to the driver. The amount of refraction and the amount of light gathering of these microspheres is dependent in part upon maintaining a low index of refraction air interface on the exposed portion of the microsphere. During rainy periods, the microspheres become wetted with water which reduces their light refracting ability resulting in much reduced retroreflective performance.
It is also known to use enclosed-lens retroreflective structures on pavement markings. See, e.g., U.S. Pat. No. 4,875,798 (May) and U.S. Pat. No. 5,340,231 (Steere et al.). Enclosed-lens retroreflective sheetings with flat cover films (also sometimes referred to as top films, top sheets, cover sheets, etc.) have been constructed as a means for providing effective retroreflection under wet conditions. See, e.g., U.S. Pat. No. 4,025,159 (McGrath) which discloses encapsulated-lens retroreflective articles and U.S. Pat. No. 4,505,967 (Bailey) and U.S. Pat. No. 4,664,966 (Bailey et al.) which disclose embedded-lens retroreflective articles.
Currently available low profile pavement markings provide effective retroreflective response for only a narrower range of entrance angles than is sometimes desired. For example, flat pavement markings, relying on microspheres partially embedded in layers containing diffuse pigments as described above, are most easily seen at distances of approximately 80 meters and less. At distances greater than this, retroreflective brightness declines due in part to the relatively larger entrance angles of the incident light and in part to inherently limited retroreflective brightness. In addition to generally low retroreflectivity at high incidence angles, flat pavement markings are particularly difficult to see under rainy conditions for the reasons discussed above. Raised pavement markers typically have better wet reflectivity because the rain will run off the raised portions and they sometimes use reflective systems that are inherently retroreflective when wet. However, snow removal is frequently a problem on roads bearing raised pavement markers, as the snowplows have a tendency to catch on the raised protrusions and dislodge the markers from the road surface. Also, raised pavement markers mounted as spot delineators provide relatively poor daytime road delineation and thus commonly need to be augmented with continuous painted or tape line markings.
To expand the effective entrance angle range of a pavement marking material, combination of a light directing means with an enclosed-lens retroreflector has been proposed. For example, U.S. Pat. No. 4,145,112 (Crone) discloses an article comprising an underlying base retroreflective layer and a light directing layer comprised of a longitudinally-extending series of short transparent projections. U.S. Pat. No. 4,236,788 (Wyckoff) discloses a related type of pavement marker strip wherein the two sides of the transverse prisms are adjusted to provide for downward internal reflection into the base sheet from one side and refraction to the space between successive prisms into the base sheet from the other side. U.S. Pat. No. 3,920,346 (Wyckoff) discloses a saw-tooth-like marker strip comprising protrusions with curved edges and having upwardly disposed retroreflective members embedded therein.
U.S. Pat. No. 4,072,403 (Eigenmann) discloses a retroreflective assembly that is particularly useful for situations in which retroreflection is required in rainy conditions. The assembly disclosed therein comprises a transparent globule with a layer of transparent microspheres on certain portions of the globule and a reflective layer disposed behind the microspheres. The retroreflective assemblies, sometimes referred to as “globule/microsphere retroreflective assemblies”, are positioned on the top surface of a pavement marking where they provide effective retroreflection of light at high incidence angles. U.S. Pat. No. 5,268,789 (Bradshaw) discloses an improved retroreflective assembly of this type and an improved method for making such an assembly.
EP Patent publication 385746 B1 (Kobayashi et al.) discloses a high-brightness all-weather pavement marking. The marking includes glass microspheres of relatively large diameter partially embedded in a first transparent resin layer that serves to hold the microspheres. The pavement marking further includes glass microspheres of relatively small diameter that are buried in a second transparent resin layer. In the lower portion of the second transparent resin, behind the rear of the small glass microspheres, lies a reflecting layer. In use, incident light strikes the large glass microspheres, travels through the second transparent resin layer, strikes the small glass microspheres, is reflected by the reflecting layer, and returns in the direction from which it came, i.e., through the small glass microspheres, through the second transparent resin layer, and finally through the large glass microspheres. The small glass microspheres and large glass microspheres work together to retroreflect incident light. It is also described that the large glass microspheres, being partially exposed in air, can receive a large amount of incident light, even if such light is at a large incidence angle. It is further described that the pavement marking is useful in rainy conditions.
There still exists a need for a low profile retroreflective article that exhibits good retroreflective characteristics under dry and wet conditions, preferably at high incidence angles in both daytime and nighttime, possesses suitable skid resistance, and is suitably durable.
SUMMARY OF INVENTION
Applicants have found that an article having a combination of exposed-lens retroreflective elements and enclosed-lens retroreflective sheeting provides retroreflectivity under dry and dynamic wet conditions at very high entrance angles (e.g., 80° or more, preferably 85° or more). The inventive articles are well suited for use on vertical and horizontal surfaces, but are particularly beneficial for use on horizontal surfaces, such as roadway pavement. The invention also provides a method for making such materials. Much of the following description is made with specific reference to embodiments of the invention to be used as pavement marking articles.
One aspect of the present invention provides a retroreflective article including an enclosed-lens retroreflective sheeting having a first surface and a second surface, wherein, under dry conditions, the sheeting exhibits a coefficient of retroreflective luminance at a simulated 30 meter viewing geometry as measured by ASTM D 4061-94 of about 40 (millicandela/m
2
)/lux, and under dynamic wet conditions, the sheeting exhibits an increase in coeffic
Ethen John L.
Haenggi Robert A.
Haunschild Dale H.
Stump Larry K.
Zender Mark D.
3M Innovative Properties Company
Watkins III William P.
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