HIGH ENTRANCE ANGLE RETROREFLECTIVE ARTICLE WITH SPHERICAL REFRACTING ELEMENTS

Abstract: Retroreflective article (80) that provides effective retroreflective brightness at very high entrance angles as well as low entrance angles and under wet conditions. The article comprises spherical refracting elements (60) having an index of refraction above 1.35 and below 1.75 adhered to the cover (66) film of an enclosed-lens retroreflective base sheet.

Description: HIGH ENTRANCE ANGLE RETROREFLECTIVE ARTICLE WITH SPHERICAL REFRACTING ELEMENTS Field Of Invention The present invention relates to a retroreflective article exhibiting high retroreflective brightness at high entrance angles and low entrance angles, and under wet conditions, and a method for making such an article. The article is well suited for use as a pavement marking or as a vertical barrier or delineator marking. 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. A common form of pavement markings 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 rely on an exposed-lens optical system comprising transparent microspheres partially embedded in a binder layer containing pigment particles, e.g., titanium dioxide (Ti0 2 ) or lead chromate (PbCr0 4 ), as reflectors. In use, light from the headlamp of a vehicle enters the microsphere and is refracted to fall on the reflective pigment particles. 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 at the air interface on the exposed portion of the microsphere. During rainy periods, the microspheres become wetted with water which reduces the their light refracting ability and results in much reduced retroreflective performance. One solution to this problem is raised pavement markings in which retroreflective elements are presented in somewhat vertical configurations. U.S. Patent Nos. 4,388,359 (Ethen et al.) , 4,988,555 (Hedblom) , and 4,988,541 (Hedblom) disclose pavement markings with protrusions carrying exposed-lens retroreflective elements on the sides thereof. It is also known to use enclosed-lens retroreflective structures on pavement markings. These structures are typically used as spot guides which are augmented with continuous paint or tape markings. See, e.g., U.S. Patent Nos. 5,277,513 (Flanagan et al.) and 5,340,231 (Steere et al.). Enclosed-lens retroreflective sheetings with flat cover films (also sometimes referred to as cover sheets, top sheets, top films, etc.) have been constructed as a means for improving wet retroreflectivity. See, e.g., U.S. Patent No. 4,025, 159 (McGrath) which discloses encapsulated-lens retroreflective articles and U.S. Patent Nos. 4,505,967 (Bailey) and 4,664,966 (Bailey et al.) which disclose embedded-lens retroreflective articles. U.S. Patent 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 which each have front and back (defined relative to the expected origin of light to be retroreflected) upwardly extending edge surfaces. The front edge surfaces are disposed across (i.e., relatively perpendicular to) the expected path of high-incidence angle light, whereby they transmit rather than reflect away a high percentage of incident light from approaching motor vehicles. The back edge surface is disposed to both reflect light transmitted through the front edge surface to a path within a predetermined angular range for retroreflection by the retroreflective elements, and to reflect light retroreflected by the retroreflective elements back through the front edge surface toward its source. A precise configurational relationship of the front and back upwardly extending edge surfaces of each projection must be established and maintained in order to retain adequate retroreflectivity. In addition, the longitudinally extending projections tend to make such sheeting less flexible. U.S. Patent 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. As with the article disclosed in U.S. Patent No. 4,145,112, maintenance of a precise configurational relationship between the two upward faces of the prisms is critical. U.S. Patent 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. The curved edges of the raised protrusions are said to reduce loss of incident light so that the marker is bright over wide angles of incident light upon the marker strip. In addition, incorporation of upwardly disposed retroreflective members in the protrusions results in a narrower entrance or incidence angle of light from approaching motor vehicles, permitting more effective retroreflection by the article. U.S. Patent 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 monolayer 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 improved retroreflection of light at high incidence angles. U.S. Patent No. 5,268,789 (Bradshaw) teaches an improved globule/microsphere retroreflective assembly and an improved method for making such an assembly. EP Patent No. 385746 Bl (Kobayashi et al.) discloses a pavement marking comprising a layer of large glass microspheres embedded in the top of retroreflective embedded-lens type base sheeting. The retroreflective pavement marking is said to be particularly useful in rainy conditions because the larger glass microspheres are partially exposed in air. However, the disclosed pavement marking is limited to using microspheres as a light gathering source. Furthermore, the pavement marker is only taught to increase retroreflectivity of its base sheeting at entrance angles of between 60° and 80°. It is known in the art that high-entrance angles, greater than approximately 85°, is more common for pavement marking applications. Furthermore, the reference discloses that the large glass microspheres having refractive index of less than 1.75 would not be suitable for the invention. Currently available pavement markings provide effective retroreflective response for only a narrower range of entrance angles than is sometimes desired. In addition, currently available pavement markings are not as effective retroreflectors as is desired for some applications. For example, current commercial flat pavement markings, relying on microspheres partially embedded in layers containing pigment particles, are most easily seen at distances of approximately 80 meters and less. At distances greater than this, retroreflective brightness declines due to the relatively larger entrance angles of the incident light and limited retroreflective efficiency. In addition to generally low retroreflectivity at high incidence angles, flat pavement markings are particularly difficult to see under rainy conditions. Raised pavement markings have better wet reflectivity because the rain will run off the raised portions. However, snow removal is frequently a problem on roads bearing raised pavement markings, as the snowplows have a tendency to catch on the raised protrusions and dislodge the markings from the road surface. The need exists for low profile retroreflective articles that exhibit high retroreflective brightness in a continuous line even at high incidence angles and which retain effective retroreflective brightness at high incidence angles even when wet. As used herein, "low profile" refers to an article low enough to sustain impacts from a snow plow after one winter season with minimal damage to the article. In addition, there exists a need for retroreflective articles exhibiting effective retroreflective response over a wide range of entrance angles for application to vertical surfaces such as guard rails, Jersey barriers, etc. Summary Of The Invention The present invention provides novel, preferably low profile, retroreflective articles that provide an unobvious combination of improved retroreflectivity at the very high entrance angles (88° or more) such as those at which pavement markings are observed, bright retroreflectivity at low entrance angles, and much greater retroreflectivity under wet conditions than typical pavement markings. The invention also provides a novel method for making such retroreflective articles. In brief summary, an article of the invention comprises an enclosed-lens retroreflective base sheet and an array of spherical refracting elements on the front surface of the base sheet. The base sheet comprises an array of retroreflective elements beneath a continuous overlying transparent cover layer. The spherical refracting elements are disposed relative to the retroreflective base sheet such that light incident to the array of spherical refracting elements at a high entrance angle is refracted so as to be transmitted into the base sheet and retroreflected by the base sheet. Unlike the refracting elements in the articles disclosed in U.S. Patent Nos. 4,145,112 and 4,236,788, the front and back sides of the spherical refracting elements of articles of the invention need not possess a precise configuration relative to one another in order to achieve effective retroreflection. Unlike EP Patent No. 385746 Bl, the refracting elements are not limited to glass microspheres with an index of refraction of 1.75 to 2. As a result, retroreflective articles of the invention can be very easily and cheaply made. Retroreflective articles of the invention employ refraction at the front surface of the spherical refracting elements to direct light incident at high entrance angles into the base sheeting. As a result, articles of the invention provide surprisingly bright retroreflection and are surprisingly durable. Retroreflective articles of the invention are particularly well suited for applications where light is incident at high entrance angles greater than about 85°, e.g., at pavement marking geometries. Such applications include pavement markings and applications where the incident light may be from any direction, such as horizontal signs. Illustrative examples of such horizontal signs include the legends and symbols commonly placed on pavement in parking lots to denote handicapped parking, and the arrows and lane markings placed on the pavement at an intersection. In addition, retroreflective articles of the invention are also well suited for use on vertical surfaces, particularly those that are observed at high incidence angles such as guard rails, building walls along alleys, Jersey barriers, etc. An advantage of retroreflective articles of the invention is that in addition to exhibiting improved retroreflective brightness at high incidence angles, they also exhibit high retroreflective brightness at lower entrance angles, e.g., within 30° to 40° of normal, in which signs are often observed. This makes articles of the invention especially well suited for use on walls and barriers along highways and other applications where a vehicle may approach the structure from a wide range of angles at which effective retroreflective brightness is desired. For example, a first road may have a vertical barrier disposed substantially parallel to a portion thereof and a second road may intersect the first road. If the barrier has an article of the present invention on the surface thereof, it will provide effective retroreflection to vehicles approaching the barrier on either road, thereby increasing safety. Retroreflective articles of the invention can be used in curved formats, e.g., wrapped around traffic cones and barrels, on curved guardrails, etc., providing excellent retroreflective brightness along essentially the entire visible portion because of the exceptional entrance angularity of the articles. Unlike an exposed-lens reflective sheeting that will not retroreflect when wet, retroreflective articles of the invention are wet reflective. That is, the inventive article will retroreflect during rainy conditions, when the rain has subsided but the article is not yet dried, in the early morning hours when dew has collected on the article, or under similar conditions. In addition, in a pavement marking application the spherical refracting elements provide raised surfaces that also increase the article's retention of wet reflectivity by facilitating run-off of water. However, the relatively low profile of the raised surfaces enables the retroreflective article to maintain its utility even in areas where snowplows are used. In brief summary, the method of the invention comprises: (1) providing a retroreflective base sheet comprising an array of retroreflective elements and a cover layer; and (2) adhering an array of spherical refracting elements as described herein on the cover layer, the spherical refracting elements being disposed relative to the base sheet such that light incident to the array is refracted so as to be transmitted into the base sheet, retroreflected by the base sheet, and further refracted by the spherical refracting elements so as to be retroreflected by said article. The manufacturing process of the retroreflective article of the invention is much simpler relative to previous processes for fabricating retroreflective articles comprising a light-directing layer and a retroreflective base sheet. With previous retroreflective articles such as those disclosed in U.S. Patent Nos. 4,145,112 and 4,236,788, the light-directing layer must be carefully configured. In contrast, the spherical refracting elements of the invention can be randomly placed on the retroreflective base sheet if desired. Also, impacts and abrasion from road traffic that tend to distort the light-directing layer of previous retroreflective articles will have much less of an effect on the spherical refracting elements of the invention because maintaining a precise configuration is not critical to achieving retroreflection. Finally, because maintenance of precise geometries is not critical, softer more conformable materials can be chosen, thereby enhancing the ability of pavement markings of the invention to stay on the road. Brief Description Of The Drawings The invention will be further explained with reference to the drawings, wherein: Figure 1 is a plan view of a previously known pavement marking with a light-directing layer disposed above a base retroreflective layer; Figure 2 is a sectional view of the pavement marking shown in Figure 1; Figure 3 is a plan view of an illustrative retroreflective article of the invention; and Figure 4 is a sectional view of an illustrative retroreflective article. These figures, which are idealized and not to scale, are intended to be merely illustrative and non- limiting. Detailed Description of Illustrative Embodiments of the Invention The inventive retroreflective article has a novel optical system that increases the retroreflectivity of a base sheeting at high entrance angles without significantly compromising retroreflectivity at all other entrance angles. As used herein, "high entrance angles" mean angles greater than approximately 85° (a glossary of terms is provided at the end of this specification) . Because the inventive article is able to retroreflect light at the high entrance angles, it is useful for horizontal applications, such as pavement markings. Because the inventive sheeting has good angularity and also good head-on retroreflective brightness, it is useful for vertical applications, such as delineators and barrier markings. "Head-on" brightness denotes low entrance angles, typically from 0° up to approximately 30° to 40°. Figures 1 and 2 show a previously known retroreflective article such as is disclosed in U.S. Patent No. 4,145,112 wherein article 10 comprises light directing layer 12 with internally reflecting projections 16 and underlying retroreflective base sheeting 14 and underlying optional conformance layer 18. Typically such articles would further comprise an adhesive layer (not shown) on the under side of conformance layer 18 by which the article would be bonded to a desired surface, for example, a road surface (not shown) . As discussed above, projections 16 utilize internal reflection to redirect high entrance angle light into base sheeting 14 and then utilize internal reflection to redirect light retroreflected by base sheeting 14 back toward the source. Layer 12 is shown adhered to base sheeting 14 with intermediate adhesive layer 13. Base sheeting 14 comprises an array of cube corner retroreflective elements 20 on the back side of its main layer 22 and seal film 24 sealed to main layer 22 with a network of interconnecting bonds 26 to provide the interface on cube corner elements 20 necessary for retroreflection. I. General Structure of the Inventive Article An illustrative retroreflective article of the invention, in this case a pavement marking, is shown in Figure 3. Pavement marking 30 comprises retroreflective base sheeting 32 with an array of spherical light refracting elements 34 on the top surface thereof, and optional conformance layer 36 underlying base sheeting 32 and optional adhesive layer 38 underlying conformance layer 36. Different types of retroreflective sheeting may be used as base sheeting 32. The retroreflective base sheetings typically do not, on their own, provide sufficient retroreflectivity at extremely high entrance angles, for examples at angles of 85° to 89°. However, when these base sheetings are used in a composite article of the invention, very good retroreflective performance at both high entrance and low entrance angles is achieved. Spherical refracting elements 34 adhere to the relatively flat front face of the retroreflective base sheeting, e.g., are partially embedded therein. Because of their placement, these spherical refracting elements capture light that would ordinarily be specularly reflected at high entrance angles. The captured light is refracted by the spherical refracting elements so as to enter base sheeting 32, is retroreflected by base sheeting 32 and is refracted again so as to be directed towards the originating light source. A retroreflective article of the invention may contain colorants in at least some portion of it, for example in the spherical refracting elements and/or in one or more components of the base sheeting. Illustrative examples of common colorants include white, yellow, and red, although other colorants may be used as desired. Also, a thin, high abrasion resistance and/or dirt resistance coating may be applied to the top surface of the retroreflective article to protect it from traffic wear and dirt accumulation. Preferably, the coating will be light transmissive and will not reduce the skid resistance of the article. Another illustrative retroreflective article of the invention is shown in Figure 4. Pavement marking 80 comprises spherical refracting elements 60, anti-skid particles 62, high angularity retroreflective base sheeting 82, optional conformance layer 74, optional adhesive layer 76, and an optional liner 78. Base sheeting 82 further comprises retroreflective elements 68 embedded in transparent polymer matrix 65, specular coat 70, adhesive layer 72, and cover layer 66 from which spherical refracting elements 60 protrude. II. Retroreflective Base Sheetings Retroreflective base sheetings used in the invention preferably possess good angularity; that is, the base sheetings' retroreflectivity is still substantial at relatively high entrance angles of about 80° or more. All component layers of the retroreflective base sheeting preferably adhere together in all types of weather conditions, even under repeated impact and shear stress resulting from road traffic passing over the sheeting in the case of pavement marking applications. Also, base sheetings used in the invention are inherently wet reflective because the important components of the optical system are enclosed within the sheeting and are not exposed to water. Retroreflective base sheets used herein comprise a relatively flat cover layer on the front surface thereof. The cover layer protects underlying components of the base sheet and may be single layer or multilayer. The cover layers are typically polymers but may be other light transmissive material if desired. They can be selected to separately optimize distinct characteristics of the sheeting. The spherical refracting elements may be adhered to the base sheeting by being embedded either in the cover layer or in an additional layer that is adhered to the front of the cover layer. Different types of retroreflective base sheeting may be used for the present invention. Illustrative examples of retroreflective base sheetings that can be used in the invention include, but are not limited to, embedded-lens retroreflective sheetings and encapsulated- lens retroreflective sheeting (that is, both microsphere type and cube-corner type) . Illustrative encapsulated-lens sheetings include microsphere-based retroreflective sheetings that comprise a monolayer of transparent microspheres partially embedded in a binder layer with a reflective layer on the rear (i.e., embedded) portions thereof. An air interface is provided by a cover layer disposed in front of the microspheres. Alternatively, a cube-corner type sheeting that comprises a monolayer of cube-corner retroreflective elements having an air interface protected by a sealing layer can also be used. Cube corner type sheeting wherein the cube corner elements have been covered with a specularly reflective metal layer may also be used. In a cube-corner type sheeting, the cover layer may be an integral part of the cube corner formations or may be an independent film. U.S. Patent No. 4,025,159 (McGrath) discloses some microsphere type and cube-corner type encapsulated-lens retroreflective sheetings that can be used herein. Illustrative embedded-lens sheetings include microsphere-based retroreflective sheetings that comprise (1) a monolayer of transparent microspheres whose front and rear surfaces are embedded in a transparent matrix and (2) a reflective layer disposed from the rear surfaces of the microspheres by a selected distance. U.S. Patent No. 4,505,967 (Bailey) discloses an embedded- lens retroreflective sheeting that is particularly well suited and preferred for use herein. An illustrative example of a cube-corner, embedded-lens sheeting comprises a monolayer of cube-corners whose front and read rear surfaces are embedded in polymeric matrices and a reflective layer specularly coated or metallized on the surface of the cube-corners. It is known in the art that metallization of a cube-corner sheeting increases the entrance angularity of the sheeting. Embedded-lens retroreflective sheetings are typically more preferred than encapsulated-lens retroreflective sheetings when used as pavement markers. It is believed that the solid construction of the embedded-lens sheeting would be more durable when subjected to traffic conditions because it has no internal voids like encapsulated-lens sheeting. Embedded-lens retroreflective sheetings are available in commercial forms that are quite durable and flexible. They are available in embodiments that provide effective bright retroreflective performance at higher entrance angles than many encapsulated-lens systems are capable of. Moreover, the reflective layer in many embedded-lens sheetings is aluminum and aluminum conformance layers are commonly used in pavement marking materials. This similarity may minimize any possible corrosion problems that may arise if different metals were used. Microsphere-based enclosed-lens optical systems utilize the light bending and focusing effect of the microspheres to refract light onto a reflector member which is reflected and then refracted back toward its origin. The degree of refraction and thus the optimal placement of the specular reflector depends on the relative refractive indices of the cover layer above the microspheres, the microspheres, and the spacing layer between the microspheres and the reflector member, if any. For example, when used with cover layer and spacing layer materials having refractive index of approximately 1.5, a 2.25 refractive index microsphere will focus the light behind it at a distance of about 0.44 times its radius. The thickness of the spacing layer preferably approximates this so that light is focused onto the specular reflector. Any deviation away from these precise optical relationships will tend to result in losses of retroreflectivity of the base sheeting. Thus, the cover layer preferably remains firmly attached to the microsphere layer, the microspheres preferably are stably positioned in the matrix, and all layers through which light must pass to be retroreflected are preferably clear and distortion-free. In addition, the specular reflector, typically vapor deposited aluminum, preferably remains as a substantially continuous, distortion-free layer without cracking or corroding. The spacing layer- specular layer interface preferably remains smooth and wrinkle-free. Very small changes in these optical relationships will tend to result in degradation of the retroreflective performance of the base sheeting and thus of any article made using such base sheeting. Although extremely small changes may not cause objectionable brightness loss, slight changes can severely affect these precise relationships. It is surprising that any retroreflective sheeting made using these precise optical relationships can withstand repeated traffic impact and shear stresses in combination with other effects of sunlight, rain, road oil, road sand, road salt, and vehicle emissions. When light enters embedded-lens retroreflective sheeting at high entrance angles and passes through a microsphere, it tends to be focused at the side of the microsphere rather than at the back as happens when light is incident in a more perpendicular fashion at low incidence angles. Therefore, it is important to maintain the right spacing between the microspheres and reflective layer. As will be understood by those skilled in the art, the thickness of the space coat layer can be controlled partly by fabrication methods. When the spacing layer tends to conform hemispherically, that is, concentricly to the back side of the microspheres, optimum spacing for a variety of entrance angles can be achieved. U.S. Patent No. 4,505,967 (Bailey) discloses embedded-lens retroreflective sheeting suitable for use herein and discusses in detail the relationship between configuration of the spacing layer and retroreflective response of the sheeting. 3M SCOTCHLITE Brand Reflective License Plate Sheeting No. 3750 is an illustrative example of a commercial retroreflective sheeting that can be used in the invention. It is important that the light-transmissive cover layer disposed in front of the retroreflective elements be durable because the pavement marking will, in some applications, be exposed to high volumes of traffic. The cover layer is preferably substantially continuous. Compositions of the cover layer should be selected to as to provide a strong bond to the spherical refracting elements. Preferably, the cover layer is a thermoplastic polymer. Illustrative examples of thermoplastic polymers suitable for use in the invention include polyurethanes, polyethylene acid copolymer consisting of ethylene methacrylic acid (EMAA) , ethylene acrylic acid (EAA) , ionically crosslinked EMAA or EAA. A preferred material is an aliphatic polyurethane because of its high impact resistance, flexibility at low temperatures, color, clarity, abrasion resistance, and bond strength to preferred base sheetings. The cover layer is preferably also resistant to dirt build-up, clear, flexible enough to conform to the road surface, bonds to inorganic anti¬ skid particles, and does not appreciably change color during use. If desired, the spherical refracting elements may be treated with materials to further increase their bond to the cover layer. III. Spherical Refracting Elements In accordance with the present invention, there is an array of spherical refracting elements adhered to the cover layer of the base retroreflective sheeting. As used herein, array means a plurality of spherical refracting elements, whether or not the elements are arranged in an ordered pattern or randomly. The desired properties of the spherical refractive elements include a high degree of clarity, and a glossy scratch-resistant surface. The clarity of the elements is important so that incident light is transmitted through the element with minimal loss such that a majority of that light will be retroreflected back to the source. The surface of the element is preferably scratch-resistant so that it will remain glossy, and light will not be scattered by scratches. The elements are preferably of sufficient hardness to resist the flattening effects of traffic, and should not soften appreciably at temperatures below 75°C (170°F) . In addition, the elements must not crack upon traffic impact at temperatures from -40°C to 75°C (-40°F to 170°F) . The elements preferably also adhere well to the retroreflective base sheeting, and are preferably oil- resistant, dirt-resistant, and moisture-resistant. Other desirable or preferred properties of the elements include low color and low cost. Spherical refracting elements applied to the base retroreflective sheeting in accordance with the invention may be any of glass, ceramic, thermoset polymers, or thermoplastic polymers. Selection will be based in part upon the properties desired of the resultant article, e.g., abrasion resistance, resistance to weathering, etc. Illustrative examples of thermoset or thermoplastic polymers suitable for use as spherical refractive elements include polycarbonates, acrylics, polyurethanes, polyvinyl chloride, and polyolefin copolymers, such as, polyethylene acid copolymer consisting of ethylene methacrylic acid (EMAA) , ethylene acrylic acid (EAA) , ionically crosslinked EMAA or EAA. A preferred material is an aliphatic polyurethane because of its high impact resistance, flexibility at low temperatures, color, clarity, abrasion resistance, and bond strength to preferred base sheeting cover layer. The spherical refracting elements preferably have a refractive index above 1.35 and below 1.75, more preferably between about 1.4 and 1.7. Unlike EP Patent No. 385746 Bl (Kobayashi et al.), higher refractive indices are not necessary to carry out this invention because the retroreflective base sheeting possesses good angularity; that is the base sheetings' retroreflectivity is still substantial at relatively high entrance angles of about 80° or more. In general, large glass microspheres (e.g., those approaching 2 millimeters in diameter) with higher refractive indices (greater than about 2) are difficult and expensive to manufacture. Typically a portion of each refracting element will be exposed above the cover layer with the remaining portion embedded in the cover layer. For example, typically about 10 to 70 percent of the element's diameter will be embedded. Preferably, about 30 to 70 percent of the each spherical refracting element diameter will be exposed. More preferably, about 40 to 60 percent of each spherical refracting element diameter will be embedded. Preferably spherical refracting elements are embedded such that a major portion of light incident the article at between about 70° and 90° is refracted so as to enter the retroreflective base sheet. In accordance with the invention, the retroreflective brightness of an article of the invention at entrance angles above 89°, and typically above 85°, is higher than the retroreflective brightness of the base sheet alone. Typically it is preferred that the spherical refracting elements are in the range of about 0.5 to about 4.0 millimeters, and more preferably about 1 to about 2 millimeters in diameter. The elements disclosed in U.S. Patent Nos. 4,145,112 (Crone) and 4,236,788 (Wyckoff) require the second face of the element to be oriented in conjunction with the first face and of such quality to reflect light into the base sheeting. In contrast, the spherical refracting elements of the invention rely upon refraction, typically at a single (i.e., the front) face. This eliminates the need to configure the first and second faces to achieve retroreflection off the second face, and also eliminates the need for the second face to be of a quality capable of reflection, for example, polished. The quality necessary for reflection is typically more stringent than that required for refraction. Also, the articles disclosed in these references are not known to have utilized base sheets that were capable of retroreflection of high incidence angle light such as are preferred for use in the present invention. The spacing between spherical refracting elements may be uniform, or the elements may be arranged in a random manner. A less than optimum spacing of the elements may be used in those applications where optimum brightness is not required. This feature of random placement permits simplified, less expensive manufacturing. However, in the present invention, uniform placement in a specific pattern can be advantageous because the elements may be placed in specific locations in such relation to one another that each element does not shadow other elements. In this manner, the majority of the incident light will be captured by the elements so as to optimize the retroreflective brightness at pavement marking geometries. For instance, spherical refracting elements may be spaced apart so as to minimize shadowing of adjacent elements at anticipated entrance angles and to permit greater surface contact of vehicle tires with the spaces between light spherical refracting elements which may contain protruding skid control particles. Preferably, the surface area occupied by the spherical refracting elements is less than fifty percent of the base sheet in order to allow for maximum conformance of the inventive article, e.g., to the road, a guard rail, or other structure. More preferably, the surface area covered by the spherical refracting elements is less than twenty-five percent. The entire inventive article, including the spherical refracting elements, can be protected such as by a protective coating. Such a coating has the advantages of providing abrasion and/or dirt resistance. Illustrative examples of protective coating compositions include, but are not limited to, ceramer coatings or crosslinked water-based polyurethane coatings. As used herein, "ceramer" refers to a fluid comprising surface-modified colloidal silica particles dispersed in a free-radically polymerizable organic liquid. Advantages of the coating include the ability to withstand outdoor conditions with excellent resistance to moisture, light and heat; resistance to abrasion; resistance to chemical attack and coloration by automobile engine oil and carbon black (for example, carbon black of tires) ; desirable optical properties such as transparency; good adhesion to the spherical refracting elements; and good flexibility. In a first step, a ceramer precursor coating composition is applied to the surface of retroreflective article, preferably including the top surface of the spherical refracting elements and portions of the base sheet not covered by spherical refracting elements. The coating composition comprises about 20 weight percent (wt%) to about 80 wt% of ethylenically unsaturated monomers; about 10 wt% to about 50 wt% of acrylate functionalized colloidal silica; and about 5 wt% to about 40 wt% of N, -disubstituted acrylamide monomer or N-substituted-N-vinyl-amide monomer; wherein said percentages are weight percents of the total weight of said coating. The composition is then cured to form a retroreflective article having an abrasion-resistant, light transmissive ceramer coating. The ceramer composition can be applied by any of a number of methods known in the art, including spraying, rolling, dip coating, or knife coating. Assignee's pending U.S. Patent Application Serial No. 08/444076 (filed May 19, 1995, and incorporated herein by reference in its entirety) discloses the use of a ceramer on pavement markers and retroreflective sheetings. An illustrative example of a crosslinked water- based polyurethane protective coating suitable for use in the invention comprises NEOREZ Brand R-960 polyurethane resin crosslinked with CX100 Brand crosslinking agent (both available from ICI Resins, Wilmington, Massachusetts) . As will be understood by those skilled in the art, other water-based systems and crosslinking agents may be used in formulating a protective coating. IV. Methods of Manufacture One method of the invention comprises: (1) providing a retroreflective base sheet comprising an array of reflective elements and a cover layer and (2) adhering an array of spherical refracting elements on the cover layer, the spherical refracting elements being disposed relative to the base sheet such that light incident to the array at a high entrance angle is refracted so as to be transmitted into the base sheet, retroreflected by the base sheet, and further refracted by the spherical refracting elements so as to be retroreflected by said article. In a typical fabrication process to make a pavement marking, a base sheet (for example, 3M SCOTCHLITE Brand Reflective License Plate Sheeting No. 3750), would be applied to an aluminum conformance layer. Subsequently, spherical refracting elements would be adhered to the cover layer of the base sheet by embedding into the cover layer or an additional layer thereover. Typically, the more deeply the spherical refracting elements are embedded the more resistant they are to being dislodged. In addition, if embedded to at least 50 or more of their diameter, they tend to be less subject to collection of dirt and debris (that would reduce retroreflective performance of the article) than if more shallowly embedded. In the fabrication process, it is typical to add anti-skid particles, if used, at the same time the spherical refracting elements are attached to the base sheet. Also, colorants, e.g., dyes and/or pigments, may be introduced at an appropriate time during the fabrication process, depending on where in the article a colorant is desired. Components of the inventive article that lie underneath the retroreflective base sheet are preferably selected to fit the application desired. For example, a scrim adhesive (i.e., a polymeric scrim that has been saturated with an adhesive) imparts additional strength as well as selected adhesive characteristics to the retroreflective article. Suitable conformance layers. adhesive layers, reinforcing layers, etc. can be readily selected by those skilled in the art. V. Colorants Numerous methods may be used to add colorants to selected portions or the entirety of the retroreflective article. In pavement marking applications, illustrative examples of desirable colorants include, among others, white, yellow, red, and blue colors. The colorants may be light transmissive or opaque, as desired. Typically, if the colorant is disposed within the optical path it is preferably light transmissive such that retroreflective performance is not undesirably reduced. However, it will be appreciated that in some instances it may be desirable to use an opaque colorant disposed in a location that will reduce retroreflective brightness while providing some other desired effect, e.g., more brilliant overall color or appearance. Light transmissive colorants can enhance both day time and night time color of the inventive article. In pavement marking applications, as well as others, it is important that a motorist distinguish between colored markers, for example between yellow and white markings. One way to obtain night time color involves placing a light transmissive colored material in the optical path. In one approach, color is achieved using a colored base sheeting. For example, in Figure 4, light transmissive matrix 65 can be made with the desired color, for example yellow. In a encapsulated-lens cube- corner base sheeting, the cube-corners themselves may.be colored. Another approach is to use a colored cover layer. For example, inventive article can be made with a light transmissive yellow, red, or blue colored cover layer. Also, colored spherical refracting elements may be used. When a light transmissive colored cover layer is used in conjunction with a light transmissive colored spherical refracting elements, a very effective colored retroreflective article results. Alternatively, a light transmissive colored layer can be applied onto the top of base sheeting. A colorless cover layer could be applied on top of the colored base sheet. This approach has the advantage of burying the colored layer to enhance its durability. Also, of multiple color layers in a pattern may be applied to form desired symbols or legends. Opaque colorants are typically used mainly to enhance the day time color of the inventive article and preferably lie outside the optical path so as not to reduce retroreflective performance. Thus, a base sheeting that is gray colored initially, due to the aluminum reflective layer, can be changed to a desired color by the addition of an opaque colorant. For example, one approach to make a colored article would entail applying white, opaque segments on top of the inventive article or using opaque spherical refracting elements. Although these particular segments and opaque elements will not retroreflect incident light, they will increase the true color of the sheeting when used in small quantities. Alternatively, between the spherical refracting elements, pellets of white pigmented resin might be applied to the cover layer and heated so as to cause them to melt and adhere thereto. Also, segments of desired colors could be applied to some portions of the spherical refracting elements as well as to the base sheet, albeit at some reduction in retroreflective response. For example, a method for making a colored retroreflective article comprise of the following steps: (1) providing a retroreflective base sheet comprising an array of retroreflective elements and a cover layer, (2) laminating a conformance layer to the bottom of the base sheet, (3) adhering an array of spherical refracting elements on the cover layer of the base sheet, (4) debossing the spherical refracting elements to yield a relatively flat top surface, (5) applying a colored layer onto the top surface; and (6) embossing the base sheet so that the spherical refracting elements protrude from the base sheet. As used herein, "debossing" refers to reverse embossing; i.e., to make a textured surface be relatively flat. Spherical refracting elements that originally protrude from the top surface of the base sheet are pushed down so that they become relatively level with the base sheet. One way to deboss involves feeding the base sheet with its attached conformance layer and spherical refracting elements through a set of rollers. For instance, the spherical refracting elements would contact a steel roll while the conformance layer would contact a rubber roll that can deform under lamination pressure. Pressure is applied to push the spherical refracting elements down into the conformance layer. After debossing, it is not necessary for the top surface of the sheeting to be perfectly smooth. Some surface topography is allowed. Preferably, the resultant surface of the base sheeting is as nearly flush with the spherical refracting elements as possible. After debossing, a colored layer is applied to portions of the base sheet, portions of the spherical refracting elements, and to the anti-skid particles, if any, by any convenient technique. An opaque colored layer may also be transferred onto selected portions of the base sheet before adding spherical refracting elements thereto. For example, a method for making a colored retroreflective article comprise the following steps: (1) providing a retroreflective base comprising an array of reflective elements and a thermoplastic cover layer, (2) providing a discontinuous thermoset polymer on the cover layer in a regular pattern to yield a partially printed base sheet, (3) heating the partially printed base sheet to soften the cover layer, (4) depositing spherical refracting elements onto the partially printed base sheet while the cover layer is softened and such that the spherical refracting elements can selectively adhere thereto, and (5) cooling. Portions of the thermoset polymer may contain a colorant as desired. For example, in one embodiment, all area of the base sheeting could be printed with a thermoset colored layer except for the regions beneath the spherical refracting elements, i.e., where they are bonded to the cover sheet. In another embodiment, a light transmissive thermoset polymer could be printed to encircle the spherical refracting elements at a defined radius. The balance of the sheeting could be printed with another colored thermoset polymer, for example white. The regions immediately beneath the spherical refracting elements would be unprinted. Because it is possible for a light ray to enter the base sheeting outside the spherical refracting elements, the region of light transmissive polymer encircling the spherical refracting elements would still allow the light ray to enter the base sheeting and be retroreflected by the base sheeting. This same method has the advantage of placing the spherical refracting elements in an ordered fashion thereby increasing the optical efficiency of the article while minimizing the amount of spherical refracting elements used for cost effectiveness. The composition of the colored layer, if any, should be resistant to solvents, traffic wear, and ultraviolet light. An example of a colorant solution comprises 78 weight percent (wt%) NEOREZ Brand R960 water-based urethane resin (from Zeneca Resins, Wilmington, Massachusetts) , 19 wt% WW3000 Brand titanium dioxide dispersion (from Heucotech Ltd., Fairless Hills, Pennsylvania) and 3 wt% CX100 crosslinker (from Zeneca Resins, Wilmington, Massachusetts) . It will be obvious to those skilled in the art that other colored layer compositions may be used. It will be obvious for one skilled in the art to use a combination of opaque and light transmissive colorants. For example, light transmissive colored spherical refracting elements lying in the optical path could be used with an opaque colored layer lying outside the optical path. In this way, an article would be have effective day and night time colors. Thus, any of the above combinations of opaque and light transmissive colored systems could be used. VI. Anti-Skid Particles Anti-skid particles are a common component of many pavement marking articles to increase the skid resistance of the pavement marker and have been used extensively in the art. They can be placed anywhere on the surface of the article where there is contact with the tires of the vehicles. Typically, anti-skid particles can be randomly sprinkled on to the cover film of the base sheet while it is in a softened state. It has been found that anti-skid particles can be preferentially deposited close to the zenith of the spherical refracting elements. For example, a web of base sheeting with the spherical refracting elements thereon could be kiss-coated with a binder composition. Kiss-coating refers to a coating method where a composition is desirably coated to only the top portions of the spherical refracting elements; i.e., the solution is allowed to "kiss" the tops of the spherical refracting elements only. This process is done by controlling the gaps between the coating rolls and maintaining the web such that only the tops of the spherical refracting elements are allowed to touch the coating composition. As the composition remains wet, copious amounts of anti-skid particles are sprinkled on to the web. Because the rest of the base sheeting is dry, the particles adhere only to the wet areas. Excess anti-skids particles are vibrated off the web. Subsequently the web is sent through a series of ovens to dry, cure, or solidify the wet binder composition. As a result, the anti-skid particles are selectively secured to the top regions of the spherical refracting elements thereby providing anti-skid resistance. VII. Applications Retroreflective articles of the present invention may be used advantageously in a number of different applications, particularly in wet conditions and where light is incident at high entrance angles. In particular, the articles are well suited for use as pavement markings or horizontal signs. Because of their high retroreflectivity at both high and low entrance angles, the articles are also well-suited for vertical applications, such as use on Jersey barricades or guard rails; for curved surfaces applications such as traffic barrels, tubes, and cones; for vehicle surfaces; and for other applications where the exceptional effective entrance angularity of the article will be advantageous. For example, because many embodiments of sheeting of the invention can provide effective retroreflection over all entrance angles from 0° to nearly 90°. As a result, when the sheeting is wrapped around an object such as a telephone pole or barrel, the entire surface of the sheeting that is within the line of sight can provide effective retroreflection including portions on the surface of the article that are curving away from the observer. This increases the effective retroreflective area, providing a more visible marking and thereby enhancing safety. In addition, a single marking such as a stripe on a guard rail, Jersey barrier, or wall that is parallel a first road and perpendicular to a second road that intersects the first road on the opposite side of the first road from the second road can provide very bright and effective retroreflective response visible to drivers of vehicles on both the first and second roads. Another advantage of the present invention is that because the retroreflective article is visible from many orientations. This omni-directional feature makes the invention particularly well suited for horizontal signing applications, intersection markings, etc. where vehicles may approach from a number of angles. The ease of coloring this sheeting also makes it particularly useful for horizontal signs. Transparent color layers may be applied onto the sheeting in a graphic pattern so that the retroreflected light has nearly the same coloration and pattern as is seen in a daytime viewing. It is especially useful if the ink is applied beneath the cover layer so as to be protected from road abrasion by both the elements and the continuous overlying transparent solid covering. This feature is particularly important as commonly used inks are thin, and can thus be quickly worn away by road traffic if left exposed. Material of the invention may be wound upon itself into a roll. The projections made by the spherical refracting elements are not substantial enough to interfere with wind-up. VIII. Examples The invention will be further explained by the following illustrative examples which are intended to be non-limiting. Wet Retroreflectivity The wet retroreflectivity of the reflective sheetings was measured using a LTL 2000 (available from Delta Light & Optics, Lyngly, Denmark) which measures retroreflective brightness at a 88.76° entrance angle and a 1.05° observation angle. Such a configuration is similar to that which would be experienced by a driver of an average automobile 30 meters away from the reflective pavement marking. The sheeting was first laid horizontally in the test area and then flooded with a solution of tap water and 0.1 weight percent AJAX Brand dishwashing soap. The solution was allowed to run off, and brightness measurements taken within about 10 seconds. Soap is added to the water to increase surface wettability of the sheeting. The soap also better simulates the effect of rain after the reflective pavement marking has been on the road for some time, when it has been subjected to increased wettability due to the actions of sun, abrasive grit and sand, and dirt accumulations. Example 1 A thermoplastic polyethylene methacrylic acid copolymer resin (NUCREL Brand 699, available from DuPont) was extruded onto a polyethylene terephthalate (PET) carrier to yield an EMAA cover film 0.25 millimeters (0.0098 inch) thick. A high angularity base sheeting (3M SCOTCHLITE Brand Reflective License Plate Sheeting No. 3750 "the 3750 sheeting") was prepared in the following manner. It was primed with a water-based polyethylene acrylic acid solution (ADCOTE Brand 50T4983, available from Morton Chemical Co., Seabrook, New Hampshire) under common gravure coating techniques using a 150 line quadrangular gravure pattern. The solution was applied onto the front surface, i.e. the reflective side, of the 3750 sheeting, and dried in a forced air convention oven for one minute at 93 °C (200 °F) to yield a primed 3750 sheeting. The EMAA cover film was hot laminated to the primed 3750 sheeting in a nip created by a hot can and a rubber roll under the following conditions. The PET carrier of the EMAA cover film was allowed to contact a hot can heated to 150 °C (300 °F) ; the EMAA film was exposed to the atmosphere. The primed sheeting, with the reflective side exposed, was allowed to contact a rubber roll. The two films contacted each other in the nip whereby the EMAA film was laminated to the primed 3750 sheeting to yield a composite sheeting. Both the hot can and the rubber roll turned at a surface speed of 6.1 meters/minute (20 ft/min.). On the composite sheeting, the PET carrier was stripped away from the EMAA film. The liner on the 3750 sheeting was also removed thereby exposing the pressure sensitive adhesive (PSA) that came with the 3750. A 0.075 millimeter (0.003 inch) aluminum foil (No. 1145-0 single rolled foil available from A. J. Oster Foils, Inc.) was laminated to the PSA. Glass microspheres, with an average diameter of about 1.5 millimeters and a 1.5 refractive index (A135 Glass Beads, available from Potters Bros.), were sprinkled onto the foil laminated composite sheeting. The entire article was heated in a 205 °C (400 °F) oven for two minutes. No melting of the glass microspheres occurred. The microspheres embedded themselves into the EMAA cover layer through gravitational effects. Also, the EMAA resin wicked up around the microspheres due to capillary action. Approximately 83,000 beads per square meter of sheeting were used. The wet retroreflective brightness in millicandela/lux/meter 2 was measured as described above under "Wet Retroreflectivity". For this example, the wet retroreflectivity was determined to be 700 to 800 millicandela/lux/meter 2 . When dry, the sample had retroreflective brightness of 840 to 960 millicandela/lux/meter 2 . Glossary The following definitions are used herein when discussing the geometry of retroreflection: "Reference Axis" is the line normal to the retroreflective article at the point where light is incident thereto. "Incidence Axis" is the axis defined by the path of incident light from the light source, e.g., the headlight of a motor vehicle, to the point of incidence on the article. "Entrance Angle" (sometimes referred to as "Incidence Angle" or "Angle of Incidence" and also as β) is the angle between the Reference Axis and the Incidence Axis. "Observation Axis" is the axis defined by the path of retroreflected light from the point of incidence on the article to the observation point, e.g., the eyes of the driver of the motor vehicle. "Observation Angle" (sometimes referred to as α) is the angle between the Entrance Axis and Observation Axis. "Entrance Plane" is the plane defined by the Reference Axis and the Incidence Axis. "Observation Plane" is the plane defined by the Observation Axis and the Incidence Axis. Various modifications and alterations of this ■ invention will become apparent to those skilled in the art without departing from the scope and spirit of this invention.

Claims: What is claimed is: 1. A retroreflective article comprising: (a) a retroreflective base sheet comprising an array of retroreflective elements and a cover layer; and (b) an array of spherical refracting elements adhered to the front surface of said base sheet such that some of the light incident to said array of spherical refracting elements is refracted so as to be transmitted into said base sheet, retroreflected by said base sheet, and further refracted by said spherical refracting elements so as to be retroreflected by said article, said spherical refracting elements having an index of refraction above 1.35 and below 1.75. 2. The article of claim 1 wherein said base sheet comprises at least one of the group consisting of embedded-lens retroreflective sheeting or encapsulated- lens retroreflective sheeting. 3. The article of claim 1 wherein said cover layer comprises thermoplastic polymers. 4. The article of claim 1 wherein the retroreflective brightness of said article at an entrance angle above 85° is higher than the retroreflective brightness of said article without said array of spherical refracting elements. 5. The article of claim 1 further comprising anti-skid particles. 6. The article of claim 1 wherein at least some of said spherical refracting elements comprise material selected from the group consisting of glass, ceramic, and polymeric material. 7. The article of claim 1 wherein at least some of said spherical refracting elements comprise polymeric material selected from the group consisting of fluoropolymer, polycarbonate, acrylic, polyester, polyurethane, polyvinyl chloride, polyolefin copolymers, and blends thereof. 8. The article of claim 1 wherein at least some of said spherical refracting elements comprise thermoplastic material. 9. The article of claim 1 wherein at least some of said spherical refracting elements comprise thermoset material. 10. The article of claim 1 wherein said spherical refracting elements have an average diameter of between about 0.5 and 4 millimeters. 11. The article of claim 1 wherein said spherical refracting elements have an average diameter between about 1 and 2 millimeters. 12. The article of claim 1 wherein said spherical refracting elements have an index of refraction between about 1.4 and 1.7. 13. The article of claim 1 wherein a portion of said spherical refracting elements is embedded in said cover layer. 14. The article of claim 13 wherein said portion is between about 30 to 70 percent of said spherical refracting elements' diameter. 15. The article of claim 13 wherein said portion is between about 40 to 60 percent of said spherical refracting elements' diameter. 16. The article of claim 1 wherein said spherical refracting elements are in contact with less than fifty percent of the surface area of said retroreflective base sheet. 17. The article of claim 1 wherein said spherical refracting elements are in contact with less than twenty-five percent of the surface area of said retroreflective base sheet. 18. The article of claim 1 wherein said spherical refracting elements are randomly arranged on said front surface of said base sheet. 19. The article of claim 1 wherein said spherical refracting elements are uniformly arranged on said front surface of said base sheet. 20. The article of claim 1 wherein said spherical refracting elements are placed in a regular pattern on said front surface of said base sheet. 21. The article of claim 1 wherein said base sheet comprises an embedded-lens retroreflective sheeting comprising a monolayer of transparent microspheres, a transparent matrix in which the front surfaces of said microspheres are embedded, an associated reflective means behind said microspheres, and a cover layer disposed in front of said transparent matrix, said spherical refracting elements being adhered to said cover layer. 22. The article of claim 21 further comprising a colorant in at least one of the group consisting of said spherical refracting elements, said cover layer, and said transparent matrix. 23. The article of claim 1 wherein said base sheet comprises a monolayer of cube-corner retroreflective elements. 24. The article of claim 1 further comprising a discontinuous colorant containing layer covering the top portions of at least some of said spherical refracting elements and portions of said cover layer between said spherical refracting elements. 25. The article of claim 1 applied to a surface on which motor vehicles travel. 26. The article of claim 1 applied to a vertically disposed surface selected from the group consisting of guard rails, Jersey barriers, building wall, fence, utility pole, traffic cone, and vehicle side. 27. The article of claim 25 wherein at least a portion of said surface is curved. 28. A method for making a retroreflective article comprising (a) providing a retroreflective base comprising an array of retroreflective elements and a cover layer; (b) adhering an array of spherical refracting elements having an index of refraction above 1.35 and below 1.75 on the front surface of said cover layer such that some of the light incident to said array of spherical refracting elements is refracted so as to be transmitted into said base sheet, retroreflected by said base sheet, and further refracted by said spherical refracting elements so as to be retroreflected by said article. 29. The method of claim 28 further comprising at least one of the group consisting of incorporating at least one colorant in said spherical refracting elements, incorporating at least one colorant in said cover layer, or incorporating at least one colorant in a layer formed on said cover layer. 30. A method for making a colored retroreflective article comprising: (a) providing a retroreflective base sheet having a major surface comprising an array of retroreflective elements and a cover layer; (b) applying a conformance layer to said major surface of said base sheet; (c) adhering an array of spherical refracting elements having an index of refraction above 1.35 and below 1.75 on said cover layer; (d) debossing said spherical refracting elements to yield a relatively flat top surface; then (e) applying a colored layer onto said top surface; and (f) embossing said base sheet such that said spherical refracting elements protrude from said base sheet. 31. The method of claim 30 wherein said colorant comprises polymers resistant to solvents, traffic wear, and ultraviolet light. 32. The method of claim 30 wherein said colored layer comprises an opaque colorant. 33. A method for making a colored retroreflective article comprising: (a) providing a retroreflective base sheet comprising an array of reflective elements and a thermoplastic cover layer; (b) applying a layer of thermoset polymer to some portion of said cover layer; (c) heating said cover layer so as to soften it; (d) depositing spherical refracting elements having an index of refraction above 1.35 and below 1.75 to adhere selectively to said softened cover layer; and (e) cooling said cover layer so that it hardens. 34. The method of claim 33 wherein said layer of thermoset polymer is light transmissive. 35. The method of claim 33 wherein said thermoset polymer comprises colorants selected from the group consisting of light transmissive colorants and opaque colorants.

Inventors: BAILEY TERRY R, BELISLE LOUIS C, HAUNSCHILD DALE H, JACOBS GREGORY F, SCHUELER DAVID G, STUMP LARRY K, BAILEY, TERRY, R., BELISLE, LOUIS, C., HAUNSCHILD, DALE, H., JACOBS, GREGORY, F., SCHUELER, DAVID, G., STUMP, LARRY, K.
Applicants: MINNESOTA MINING & MFG, MINNESOTA MINING AND MANUFACTURING COMPANY
Application year: 1996
Main classification: E01F9
IPC codes: E01F9, G02B5