Road surfaces and other paved areas often must be marked to indicate various traffic control information, such as lane boundaries and striping, stop bars, and pedestrian lane markings at intersections and crosswalks. Many compounds have been devised to provide a long-lasting, highly visible road markings and include materials such as paint, plastic, and rubber formulations. However, each of these materials has its own deficiencies when used in traffic-intensive areas.
Painted road markings provide an suitable choice in many traffic situations. Unfortunately, painted markings wear out quickly in highly trafficked areas, such as travel lanes or at intersections. Painted markings are also drastically affected by cold weather road treatments, such as salt, sand, or gravel. In addition, snow plows and studded tires used in cold climates quickly wear the road marking paint from the road surface. As a result, painted road markings must generally be reapplied after each winter.
Preformed plastic road marking strips face the problem of satisfactory adherence to the road surface under constant heavy motor vehicle traffic. Unless; the pavement marker has a deformable layer of elastomeric materials which lacks memory positioned between the marker and the road surface, good adhesion will not always be achieved. Moreover, the constant flow of motor vehicle traffic on the stiff plastic marker can result in cracking and/or fractures. As a result, dirt may accumulate between the adhesive and the road surface and ultimately destroy the adhesive properties holding the plastic marking strip on the road surface.
Pavement marking sheet material made from unvulcanized elastomer precursors provide traffic control markings of superior durability over the plastic type because of greater deformability and reduced elasticity. Such sheet material is semi-rigid, exhibits very little rebound, and is able to flow over a broad temperature range. The materials deforms readily into intimate contact with the irregular pavement surface and absorbs the energy of wheel impacts without fracture. Further, the low elasticity of the precursor avoids the stretch-return action that has been found to loosen sheet material from a roadway.
Use of the elastomer materials has grown rapidly in recent years because they provide long life in heavy wear locations when compared to simple painted lines or plastic markings. Typically, preformed elastomer pavement marking materials comprise a continuous, wear-resistant top layer overlying a flexible base sheet, and are applied to substrates using pressure sensitive adhesive or contact cement. Typical formulations of elastomer-based pavement marking sheeting, such as disclosed in U.S. Pat. Nos. 4,117,192 to Jorgensen, include an acrylonitrile-butadiene elastomer polymer, a chlorinated paraffin extender resin, asbestos fiber filler, stearic acid, glass microspheres, silica or silica derivatives, and titanium dioxide.
Acrylonitrile-butadiene is the major polymer in the mixture and is preferred because it offers a high degree of oil resistance. An extender resin, such as a halogenated paraffin which is soluble in the polymer mixture is also included. Fillers, such as asbestos fibers, add reinforcement, surface hardness, and abrasion resistance properties to the final product. Glass microspheres are also included in the material to provide reflectivity at night and to give the sheet material skid-resistant qualities.
The above-described formulation of sheet material is deficient for some uses because asbestos fibers can constitute a large proportion of the inorganic filler in the sheet material. Asbestos fibers contribute importantly to the desired properties of the sheet material, but for toxicity reasons, use of such fibers has been virtually eliminated for many applications. Alternative fillers, such as polyethylene fibers as disclosed in U.S. Pat. No. 4,490,432 to Jordan, or reinforcing cellulose fibers, as disclosed in U.S. Pat. No. 5,139,590 to Wyckoff may substitute for asbestos filling material.
Although improvements have been made to the polymer component of the material (e.g., U.S. Pat. No. 5,077,117 to Harper and U.S. Pat. No. 4,282,281 to Ethen), most formulations of pavement marking materials continue to use titanium dioxide (TiO.sub.2) as an additional main filling component. The titanium dioxide also functions as a pigment to impart a white color, opacity, and brightness to the formulation. However, several deficiencies arise from the use of titanium dioxide in the above formulation. Titanium dioxide is an expensive material, generally costing between $1-1.50 per pound. Use of titanium dioxide can therefore account for a large portion of the cost of the road marking material. Titanium dioxide is also a heavy material and when utilized in a road marking formulation provides for difficult handling of the product and increased transportation cost. Titanium dioxide is also not completely compatible with many of the polymers and fillers in the formulations of the prior art. Long mixing times are frequently required to combine the titanium dioxide filler with the other ingredients in the formulation. The lack of compatibility also necessitates a long aging period between mixing and calendaring.