RETROREFLECTIVE MATERIAL

Abstract: Retroreflective pavement marking material (10) comprising on the top surface thereof an exposed-lens retroreflective sheet (12) that comprises a monolayer of retroflective elements wherein first portions of the monolayer are arranged in an upwardly contoured profile (18) and second portions of the monolayer are arranged in a lower, substantially planar position (20).

Description: RETROREFLECTIVE MATERIAL Field of Invention The present invention relates to a retroreflective marking material. 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 materials 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 (Ti0 2 ) or lead chromate (PbCr0 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. 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. U.S. Patent Nos. 4,388,359 (Ethen et al.), 4,988,555 (Hedblom) , 4,988,541 (Hedblom) , and 5,227,221 (Hedblom) disclose pavement marking sheets with protrusions bearing microspheres partially embedded in and protruding from a binder layer containing pigment materials. Such constructions will provide improved retroreflective brightness at high entrance angles but are more expensive to manufacture. Also, the failure mode of many pavement marking materials is weakening and loss of the adhesive bond to the road surface such that significant portions of a marking tape are dislodged. The need exists for inexpensive low profile retroreflective articles that exhibit high retroreflective brightness even at high incidence angles, that retain effective retroreflective brightness at high incidence angles even when wet, and that are durable. 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, bride abutments, walls, etc. Summary of Invention The present invention provides novel materials that exhibit surprisingly effective retroreflective properties and that can be made relatively inexpensively. Materials of the invention exhibit effective retroreflective performance at high entrance angles (e.g., 85° or more), and are well suited for use on vertical and horizontal surfaces. In addition, marking materials of the invention exhibit surprisingly good adhesion to the road, thereby providing longer useful life. A glossary of some of the terms used in this application appears at the end of this specification. Much of the following description is made with specific reference to embodiments of the invention to be used as retroreflective pavement marking materials. It will be understood that many aspects of such embodiments will also be relevant to embodiments used as retroreflective markings on vertical surfaces. In brief summary, retroreflective materials of the invention comprise a top layer comprising a plurality of exposed-lens retroreflective elements, e.g., transparent microspheres partially embedded in and protruding from a binder layer containing pigment particles. First portions of the layer are arranged in an upwardly (i.e., relatively normal to the body of the layer) contoured profile and second portions of the layer are arranged a lower, sometimes substantially planar position. Typically a material of the invention will comprise a plurality of such first portions and second portions in a single continuous sheet. Retroreflective marking materials of the invention can be made in flexible form and can be made in desired color. In addition, the materials provided herein can be made low profile and highly conformable so as to conform to road surfaces very well. If made with suitable materials, they are resistant to damage from traffic and withstand exposure to water, salt, oils, ultraviolet radiation, sand abrasion, high temperatures, etc. making them well suited for use on roads. They can be manufactured in continuous processes and can be manufactured quite inexpensively. When marking materials of the invention are applied to roads to provide traffic control markings, the delineations are more visible to motorists and thus improved safety is possible. 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., pavement markings. They are also well suited for applications where the incident light may come from a number of orientations. Examples include intersections and so-called horizontal signs. Illustrative examples of 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, bridge abutments, posts, traffic barrels, etc. Brief Description of Drawing The invention will be further explained with reference to the drawing, wherein: Figure 1 is a plan view of a portion of an illustrative pavement marking material of the invention; Figure 2 is a cross section of the material shown in Figure 1; and Figure 3 is a cross section of another illustrative pavement marking materials of the invention. These figures, which are idealized, are not to scale and are intended to be merely illustrative and non-limiting. Detailed Description of Illustrative Embodiments Articles of the invention typically comprise a retroreflective top layer sheet, configuration member, optional backing material, and adhesive layer. The configuration member is used to impart upwardly contoured portions to the article. Upwardly is used in this context with reference to the general elevation of the first portions apart from the second portions and the fact that pavement markings are applied to generally horizontal surfaces. It will be understood to also refer to the manner in which first portions protrude beyond second portions in vertical markings. An illustrative marking material of the invention is shown in Figures 1 and 2. Material 10 comprises top layer 12 with microspheres 16 and skid particles 17 protruding from binder layer 14. First portions 18 of top layer 12 are arranged in an upwardly contoured profile and second portions 20 of top layer 12 are arranged a lower, sometimes substantially planar position. Marking material 10 further comprises a backing made up of optional conformance layer 22 and adhesive layer 24 by which it can be secured to a desired substrate such as a road (not shown) . In the embodiment shown, first portions 18 are elevated above second portions 20 by configuration member 19. The various component elements of the retroreflective sheet 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. The top layer comprises microspheres protruding from a binder layer containing pigment particles. An advantage of such lens systems is that they can be made in a variety of desired, brilliant or true day time colors. This can be particularly useful in embodiments intended to be applied to concrete or other relatively light road surfaces where brighter colored markings are desired to achieve better contrast and visibility during the day. Desired color under daytime and retroreflective viewing conditions can be achieved by selection of suitable pigment and dye materials for the binder layer and selection of colorless or colored microspheres. Typically in the case of pavement marking applications, the backing materials of the article will comprise an optional conformance layer and/or an optional scrim layer for the reasons known in the art. Those with ordinary skill in the art will be able to readily select suitable conformance layer and scrim materials. The adhesive provides a mechanism for attaching the retroreflective article to a surface, e.g., a roadway. If desired, a layer of adhesive or primer may be applied to the surface prior to laminating the retroreflective article. Selection of a suitable adhesive, e.g., pressure sensitive adhesive, hot melt adhesive, activated adhesive, etc., will depend in part upon the application conditions, the ambient environment of the substrate, and the nature of the layer of the article to which the adhesive layer must adhere. Suitable adhesive layers may be readily selected by those with ordinary skill in the art. Typically, pavement marking materials of the invention can be wound into roll form. In such instances it may be desired to use a release liner or apply a release treatment, e.g., silicone, to the top surface of the marking. As described above, portions of the top layer have a profile with first portions upwardly contoured relative to second portions. These upwardly contoured portions, with their vertical profiles, have several advantages such as: (1) high entrance angle light will be more effectively retroreflected, (2) rain water, if present, will drain off, and (3) increased adhesion to the roadway as tires repeatedly contact the upwardly contoured portions. The desired selectively upwardly contoured profile is defined by the configuration member which may be a single member, e.g., a imesh of connected fibers, or an array of separate members;, e.g., an array of independent fibers or nodule.s. It is preferably a mesh, e.g., of point bonded nylon cr polyester fibers, which is discontinuous with openings between fibers. The openings permit surrounding components, e.g., the top layer and adhesive layer, to bond directly together through the configuration member. Configuration members may be of any suitable shape so long as they elevate some portions of the retroreflective sheet as described herein. The configuration members should be flexible so as to permit easy conformance to the road. The configuration members used herein are typically polymeric. Illustrative examples of polymeric materials include polyurethanes and polyolefin copolymers such as polyethylene acid copolymer consisting of ethylene methacrylic acid (EMAA) , ethylene acrylic acid (EAA) , ionically crosslinked EMAA or EAA. In some instances the first portions will be in the form of discrete protrusions. These can be achieved using an array of nodules as the configuration member. The nodules may be substantially independent or may be connected by relatively thinner filaments. The latter embodiment is typically easier for handling purposes during fabrication of the retroreflective marking material. In some embodiments, the first portions will be in the form of strips that may be substantially non- intersecting or that intersect in an interconnected network. Each strip may be substantially straight, they may be curved, they may be arranged in a nested array of sinusoids, or other configuration as desired. The shape and arrangement of the first portions defines the shape of the second portions. Some illustrative examples of shapes of the second portions are rectangles, diamonds, hexagons. If the first portions are intersecting, the second portions may be separated from one another. If the first portions are discrete and do not connect, the second portions are adjoining. Combinations of such arrangements may be used as desired. Other components of the article should be sufficiently conformable so that the desired upwardly contoured profile of the top sheet can be achieved. Preferably, the configuration member will self adhere to the conformance layer, if present, as this is believed to be a more durable final product. In addition to providing the functions disclosed herein, the mesh can impart improved mechanical properties to the pavement marking material in similar manner as the scrims disclosed in U.S. Patent Nos. 4,146,635 (Eigenmann) and 4,299,874 (Jones et al.). In some embodiments, some parts of the upwardly contoured portions will be higher than other parts of the first portions. For instance, in rectangular embodiments as described above, the upwardly contoured portions that are perpendicular to the direction of travel may be raised higher than those that are parallel to the direction of travel. In this way, the higher upwardly contoured portions provide effective retroreflective performance. The lower upwardly contoured portions permit water, if any, to drain away. Whatever shape of contour is selected, the height of the upwardly contoured portions is preferably selected with distance between these portions in mind. For example, the upwardly contoured portions along a line of sight from an approaching vehicle should be placed far enough apart so as to minimize shadowing. The distance between the upwardly contoured portions (i.e., the width of the second portions) is preferably at least 3 millimeters wide. Typically, the tops of the first portions are elevated to at least 0.1 millimeters, preferably between about 1 to 4 millimeters, higher than the second portions. Closer spacing between upwardly contoured portions may be used without significant effects on retroreflective brightness but would require a larger number of configuration members than would be optimally necessary. Likewise, larger spacing between upwardly contoured portions may be used but reduced brightness at closer viewings will result. However, brightness at larger viewings will be unaffected. As used herein, "closer viewings" refer to shorter distances between the vehicle and the inventive material, e.g., a distance of about 30 meters. "Longer viewings" refer to longer distances between the vehicle and the inventive material, e.g. a distance of about 120 meters. Those skilled in the art will be able to select and design suitable contour patterns in accordance with the invention. Marking material of the invention can be readily made by laminating a preformed top layer, a configuration member, conformance layer, if any, and adhesive layer, simultaneously or separately, in the desired order. The other components may be assembled and the top layer formed thereon if desired. An advantage of the present invention is that a mesh or net used as the configuration member is easily worked with and can be readily incorporated into a marking material to achieve the desired upwardly contoured profile more easily and less expensively than by selectively embossing protrusions in a member and then applying or forming a binder layer material with protruding microspheres thereon. In some instances, it will be desired to partially embed skid control particles, sometimes referred to as anti-skid particles, in the top layer to improve the traction of vehicles passing thereover. Anti-skid particles are well known in the art. Suitable embodiments can be readily selected by those skilled in the art. Typically, anti-skid particles can be randomly sprinkled on the top layer while the binder is in a softened state. It has been found that anti-skid particles can be preferentially deposited close to the zenith of the upwardly contoured portions. For example, a web of retroreflective sheet with upwardly contoured portions could be kiss-coated on the top of the first portions with a binder composition, and as the composition remains wet, copious amounts of anti-skid particles are sprinkled on to the web. Because the rest of the retroreflective sheet 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 secured to the top regions of the upwardly contoured portions thereby providing anti-skid resistance. This approach may result in some loss of retroreflective performance Material of the invention may be wound upon itself into a roll form. The projections made by the upwardly contoured profiles are not substantial enough to interfere with wind-up. In a typical embodiment as a pavement marking for marking lanes, material of the invention will be between about 7.5 and 30 centimeters (3 and 12 inches) wide and at least 30 centimeters (12 inches) long. Usually, the pavement marking is much longer. Marking material of the invention will typically be formed in sizes conforming to standard materials. In the United States, pavement marking tapes are typically about 4, about 6, or about 8 inches wide. In Europe, pavement marking tapes are typically about 15 or 30 centimeters wide. Glossary The following definitions are used herein: "Reference Axis" is the line normal to a retroreflective article at the point where light is incident thereto. "Entrance Axis" (sometimes also referred to as the "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" 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 marking material comprising (1) a top layer comprising a plurality of microspheres partially embedded in and protruding from the front surface of a binder layer containing pigment particles wherein first portions of said top layer are arranged in an upwardly contoured profile and second portions of said top layer are arranged in a lower, substantially planar position; (2) a configuration member which defines said first and second portions; and (3) an adhesive layer. 2. The material of claim 1 wherein said material further comprises a conformance layer. 3. The material of claim 2 wherein said conformance layer is disposed between said top layer and said configuration member. 4. The material of claim 2 wherein said conformance layer is disposed between said configuration member and said adhesive layer. 5. The material of claim 1 wherein the tops of said first portions are elevated at least 0.1 millimeter higher than said second portions. 6. The material of claim 1 wherein the tops of said first portions are elevated about 1 to 4 millimeters higher than said second portions. 7. The material of claim 1 wherein said second portions are at least 3 millimeters wide. 8. The material of claim 1 wherein said first portions are in the form of discrete protrusions. 9. The material of claim 1 wherein said first portions are in the form of strips. 10. The material of claim 9 wherein said strips are substantially non-intersecting. 11. The material of claim 9 wherein said strips intersect in an interconnected network. 12. The material of claim 9 wherein said strips are substantially straight. 13. The material of claim 9 wherein said strips are arranged in a nested array of sinusoids. 14. The material of claim 1 wherein said first portions are connected such that said second portions are in the form of one of the following: rectangles, diamonds, hexagons. 15. The material of claim 1 wherein the retroreflective brightness of said material at an entrance angle above 85° is higher than the retroreflective brightness of said article without said first portions. 16. The material of claim 1 further comprising anti-skid particles protruding from the front surface thereof. 17. The material of claim 1 wherein said marking has a width of between about 7.5 and 15 centimeters and is longer than 30 centimeters. 18. A roadway surface marked with a material of claim 1. 19. The material 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.

Inventors: BRADSHAW THOMAS I, BRADSHAW, THOMAS, I.
Applicants: MINNESOTA MINING & MFG, MINNESOTA MINING AND MANUFACTURING COMPANY
Application year: 1996
Main classification: E01F9
IPC codes: E01F9, G02B5