WO2010014929A2

WO2010014929A2 - Process for forming a reflective surface

Info

Publication number: WO2010014929A2
Application number: PCT/US2009/052448
Authority: WO
Inventors: Aaron P. Bates; Randall Craft
Original assignee: Bates Aaron P; Randall Craft
Priority date: 2008-08-01
Filing date: 2009-07-31
Publication date: 2010-02-04
Also published as: WO2010014929A3

Abstract

Processes form a retroreflective surface on a substrate. In one example, the substrate is in condition for accepting a polymer coating and the process includes providing a powder coating material and applying the powder coating material to an outer surface of the substrate. The process can further include heating the powder coating material to a partially cured state, providing a reflective material, and applying a retroreflective layer on the surface of the powder coating material by at least partially embedding the reflective material in the partially cured powder coating material. The process further includes secondary heating to the powder coating material to a final cure state, wherein the substrate is not pretreated prior to application of the powder coating step. Further processes include applying a polymeric powder coating material in a molten state to the substrate by flame spraying. Still further processes include applying a polymeric coating material in a molten state to the substrate by extrusion. After application by flame spraying or extrusion, reflective material can be embedded to form a retroreflective surface on the substrate.

Description

PROCESS FOR FORMING A REFLECTIVE SURFACE

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of U.S. Provisional Application No. 61/085,564, filed August 1, 2008, the entire disclosure of which is hereby incorporated herein by reference.

FIELD

[0002] The present invention relates to a novel process for forming a retroreflective surface on a substrate. More particularly, the present invention relates to a process for forming a retroreflective surface on a substrate for applications such as, but not limited to, highway products, guardrails, road markers, airport runways, or signs, bicycles, automobiles, mailboxes, clothing, safety apparel and the like.

BACKGROUND

[0003] Most highway products such as guardrails, guidance lines, such as centerlines, edge lines and lane markers, as well depend upon some sort of light-reflecting device for making them more visible at night when the only source of illumination is the light from the motor vehicle head lamps. Such reflecting devices can be cube corners, glass microspheres, or simply light colored objects protruding above the pavement surface.

[0004] A plain white line painted on the surface or even a plain white plastic line adhered to the road surface is not easily visible even at a distance as near as 100 feet because of the extremely shallow angle of the light emanating from vehicle head lamps which impinge upon the road surface. Most of the incidental light is scattered and thus reflected away from the vehicle and very little returns by reflection for the operator to detect. Use of light-reflecting devices such as those mentioned above, incorporated within the painted or other light-colored line, can increase a motorists detection of the line out to many hundreds of feet. For example, the incorporation of transparent glass microspheres, ranging in size from a few thousandths of an inch in diameter to as much as a tenth of an inch, produce a better light reflection through an effect in which the microspheres serve as miniature optical lenses which focus the incident light from the headlamps into a tiny spot located a slight distance behind the rear surface of the microspheres. The focused spot of light falling upon a pigmented material after undergoing scattering is then partially reflected back upon itself and reaches the motorist's eyes by a phenomenon called retro-reflection. Because of light scattering by the pigmented binder in which the microspheres are partially embedded, only a small percentage of the incident light is returned by retro-reflection; but even this is considerably more light than is the case of an ordinary painted line. During daylight, the ordinary painted line is easily seen by a motorist for thousands of feet because of the abundance of ambient overhead skylight incident upon the line.

[0005] The principle of using glass microspheres as light-reflecting lenses for highway markers was disclosed as early as 1936 in U.S. Pat. No. 2,043,414. Soda-lime- silicate glass, such as window glass with a refractive index of 1.5, is commonly used as the medium for the microspheres because it is relatively inactive chemically and is a very hard material. This glass, forming the microspheres, generally causes the incident light to come to a focus some distance behind the rear surface of the microsphere. An increase in the brightness can result, however, when the light comes to a focus upon the rear surface of the microsphere itself. This occurs when a glass with a higher index of refraction is used. The distance behind the rear surface of a glass microsphere where the incident light comes to a focus is a function of the refractive index of the glass. As the refractive index increases from a value of approximately 1.5, the focus point moves in closer to the rear surface of the microsphere, reaching this surface when a refractive index value of approximately 1.9 is attained. At this point, the majority of the incident light is returned back upon itself in a retro- reflected beam.

[0006] If the rear surface of a microsphere is covered with a highly specular light- reflecting metal such as aluminum, chromium, silver or some other specularly reflective material, then the entire incident light beam is returned except for small losses due to absorption and other minor effects, such as spherical aberration. Even without such a reflective coating, however, the returned light beam is considerably brighter than it would be with a lower refractive index glass. This effect is achieved because the scattered light in the focused spot is very near the rear surface of the sphere itself and thus most of it re-enters the sphere and produces a brilliant retroreflected beam. [0007] Certain known techniques for producing a retroreflective surface on a substrate using reflective elements embedded in a binder utilize conventional coating techniques such as painting, laminating or dipping of the substrate in the binder. Such techniques are relatively expensive, inefficient and generate a large amount of waste and pollution.

[0008] Electrostatic powder coating is a technique whereby an electrostatically charged particulate is adhered to an exposed surface of a neutrally charged object. This particulate can comprise any of a number of compounds, including a variety of thermoset and thermoplastic materials. The charged particles adhere to the surface of the object and are subsequently permanently bonded thereto by curing the powder coating using heat or some other method. The resulting coating provides exceptional toughness and impact resistance as well as resistance to environmental and chemical exposure. Fluidized bed powder coating is a technique in which powder particles are dispersed throughout a chamber by low pressure air or other gas. When a preheated substrate is introduced into the chamber, the particles strike the substrate where they melt and cling to its surface. Subsequent curing of the melted particles permanently bonds them to the substrate.

[0009] The use of powder coating techniques for coating and coloring the exposed surfaces of finished articles has increased in recent years, taking the place of traditional painting and dipping techniques. Powder coating techniques offer numerous advantages over conventional coating processes utilizing paint, lacquer or other solvent-based carriers.

[0010] A first, and perhaps the most important advantage, is the fact that powder coatings are applied without the use of solvents, thereby greatly reducing the amount of polluting volatile organic compounds released into the atmosphere. This allows the coating industry to meet ever increasingly strict environmental regulations and worker safety concerns easily and inexpensively. This aspect of powder coating, along with the fact that excess powder spray can be collected for reuse, also reduces the cost of disposal of potentially hazardous and flammable waste.

[0011] Thus, because powder coating provides many advantages over traditional coating techniques, a need exists for a method of producing retroreflective surfaces on a substrate utilizing reflective elements in a powder coating process. [0012] A prior patent which discloses application of a retroreflective surface in conjunction with a powder coating process is disclosed in U.S. Patent No. 6,623,793, the disclosure of which is incorporated herein by reference in its entirety. However, while the process of the 6,623,793 is very effective, a modified process which involves fewer steps is desirable.

SUMMARY

[0013] In one example aspect, a process is provided for forming a retroreflective surface on a substrate which is in condition for accepting a polymer coating. The process includes providing a powder coating material and applying the powder coating material to an outer surface of the substrate. The process further includes heating the powder coating material to a partially cured state. The process still further includes providing a reflective material and applying a retroreflective layer on the surface of the powder coating material by at least partially embedding the reflective material in the partially cured powder coating material. The process further includes secondary heating the powder coating material to a final cure state, wherein the substrate is not pretreated prior to applying the powder coating material to the outer surface of the substrate.

[0014] In another example aspect, a process is provided for forming a retroreflective surface on a substrate. The process includes providing a polymeric powder coating material and applying the polymeric powder coating material in a molten state to the substrate by flame spraying. The process further includes providing a reflective material and applying the reflective material to the molten state polymeric powder coating material. The process further includes finish curing the powder coating material to form a retroreflective surface on the substrate.

[0015] In still another example aspect, a process is provided for forming a retroreflective surface on a substrate. The process includes providing a polymeric coating material and applying the polymeric coating material in a molten state to the substrate by extrusion. The process further includes providing a reflective material and applying the reflective material to the molten state polymeric coating material. The process further includes finish curing the polymeric coating material to form a retroreflective surface on the substrate. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] These and other features, aspects and advantages of the present invention are better understood when the following detailed description of the invention is read with reference to the accompanying drawings, in which:

[0017] FIG. 1 is one example process for forming a retroreflective surface on a substrate;

[0018] FIG. IA is an example process of applying powder coating material to an outer surface of a substrate by positioning the substrate in a fluidized bed of powder material;

[0019] FIG. IB is an enlarged view of a portion of the retroreflective surface on the substrate;

[0020] FIG. 2 is another example process for forming a retroreflective surface on a substrate; and

[0021] FIG. 3 is yet another example process for forming a retroreflective surface on a substrate.

DETAILED DESCRIPTION

[0022] The present invention will now be described more fully hereinafter with reference to the accompanying drawings in which example embodiments of the invention are shown. Whenever possible, the same reference numerals are used throughout the drawings to refer to the same or like parts. However, this invention may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. These example embodiments are provided so that this disclosure will be both thorough and complete, and will fully convey the scope of the invention to those skilled in the art.

[0023] As shown in FIG. 1, in a first embodiment of the present invention, a polymer process 100 is disclosed in which a reflective material 132 is embedded in a polymeric binder 114a coated on a substrate 104. The process can involve a conveyor 102 but other techniques can be used in further examples. The reflective surface on the substrate finds particular usefulness in the manufacture of high visibility road signs, but is also applicable in any application in which an object must be visible from an extreme distance and/or at night.

[0024] Due to the higher temperatures necessary to cure many traditional polymeric coatings, the substrate is typically metal or some other material capable of withstanding elevated temperatures. Thus, for ease of discussion, the invention will be described with the use of a metal substrate (e.g., galvanized steel) although high temperature plastics exist which are made primarily for powder coating, and can withstand the high temperature requirements needed for powder coating process. One example plastic can comprise conductive Noryl GTX PPE-PA alloys available from GE Plastics. It is also important to note that certain polymeric coatings, including certain powder coatings that cure at somewhat lower temperatures, are now available, making the use of wood and other temperature sensitive materials as the substrate practical.

[0025] According to a further aspect of the invention, the substrate to be coated does not need to be treated in any special manner prior to application of the polymeric coating. In fact, certain pretreatment processes are detrimental to the adherence of the polymeric coating and should be avoided. For example, pretreatment of galvanized steel will result in improper adherence of the polymer to the surface. Accordingly, as shown in the drawings a pretreatment zone 106 is illustrated as optional and may actually be undesirable in certain applications.

[0026] In accordance with the inventions a substrate is coated first with a polymeric material. Different polymeric materials may be used to coat the substrate, depending on the application, the substrate, and the final properties desired in the finished coated product. Any conventional polymeric coating may be utilized in the present invention. These polymeric materials are available from various suppliers in assorted grades. Generally, for metal substrates, suitable polymeric coating materials include thermoplastics, thermosets, and mixtures thereof. Particular examples of suitable materials can be readily selected by those skilled in the art in view of the instant disclosure. Thermoplastics, such as nylon, polyethylene, polypropylene and polyvinyl chloride, are compounds that melt and flow under the application of heat but do not undergo any chemical change. Thus, they can be cooled and re-melted numerous times. Further, thermoplastics are ideal when employing coating processes such as flame spraying.

[0027] Thermosets are compounds that will chemically crosslink (cure) under the application of heat. After cooling, thermosets will not re -melt when reheated once they are cured. Various types of thermosets may be used as the coating in the present invention including, but not limited to, acrylics, epoxies, polyesters, polyurethanes and various hybrids and combinations thereof. Suitable acrylic compounds include hydroxyl functional acrylics and glycidyl methacrylate acrylics (GMA). Epoxies are among the most prevalent coatings, and in particular powder coatings, in the industry and exhibit good toughness and weather resistance. In addition, one or more different compounds can be combined to form a hybrid coating, often as mixtures of powders, which may exhibit some or all of the properties of the individual components. In this way, a formulator can tailor the coating to provide the exact properties desired. Due to their popularity and for ease of description, the invention will be described using a thermoset material as the coating. Therefore, mention will be made in subsequent steps of the curing of the powder. As described above, however, the invention contemplates the use of a thermoplastic material as the coating as well. In addition, the invention contemplates the application of the polymer as an extrusion or by flame spraying, thereby eliminating the need for partial, and possibly final, curing.

[0028] Added to the polymeric material can be a wide variety of other materials including, but not limited to, reinforcing fillers, extenders, pigments, processing aids, accelerators, cure agents, lubricants, coupling agents, plasticizers, preservatives, flow agents and other modifiers. These additional materials may be added in any concentration that does not adversely affect the properties of the polymeric coating.

[0029] There are several basic methods that may be used for applying the polymeric coating material to the substrate. The first application method is the use of an electrostatic spray. This option is shown in Zone 108 of FIG. 1 wherein powder coating material 112, such as polymer powder, in a holding bin 110 can be supplied through a delivery hose to a spray gun by air conveyance. The powder is electrostatically charged, either in the spray gun or at an electrode, and is deposited as a layer 114 on a grounded substrate 104 by means of a static charge. There are two common charging processes that may be used in spray coating: corona charging and triboelectric charging.

[0030] Corona charging applies an electric charge (usually negative) to the powder as it exits the spray gun. A high voltage power supply creates a concentrated charge at an electrode positioned at the tip of a spray gun, creating an electric field which causes the adjacent air to ionize and generate a corona, creating negative ions. As the powder particles pass through the corona field, they are bombarded by the negative ions of the corona, which transfer their charge to the powder particles.

[0031] Triboelectric charging is a method in which static electricity is generated by rubbing the powder particles against materials that readily accept electrons. As the powder particles move down the barrel of the gun, they rub against the interior of the gun barrel and transfer electrons to it. The positively charged powder particles then exit the gun and adhere to the surface of the substrate.

[0032] As an alternative to spray coating, a polymeric powder coating may be applied using a fluidized bed method as shown in zone 108b of FIG. IA. In this method, low pressure air or gas suspends the powder particles in a closed coating chamber, creating a cloud-like suspension of powder. When a preheated substrate is introduced into the chamber, the powder strikes the substrate, melting and clinging to the substrate's surface.

[0033] As shown in the process of FIG. 2, an additional method of applying the polymeric coating includes heating the polymeric material to a molten state (e.g., see molten polymeric material 210 in holding bin) and then extruding the polymeric material into a layer 214 and applying the extruded polymeric to the surface to be treated.

[0034] As shown in the process of FIG. 3, a further method of applying the polymeric material involves the use of flame spraying the polymeric material onto the surface to be treated. An example flame spraying zone 308 is shown in FIG. 3. Flame spraying is a technique where a flame spraying device or "gun" is used to apply or "spray" on coating of molten polymeric material to a substrate. In one example, polymeric powder 310 is heated with heating element 312 to spray the molten spray to form layer 316 to the surface to be treated.

[0035] The flame-spray technique was recently developed for application of thermoplastic powder coatings. The thermoplastic powder is fluidized by compressed air and fed into a flame gun where it is injected through a flame of propane, and the powder melts. The molten coating particles are deposited on the workpiece, forming a film on solidification. Since no direct heating of the workpiece is required, this technique is suitable for applying coatings to most substrates. Metal, wood, rubber, and masonry can be successfully coated by this technique. This technology is also suitable for coating large or permanently- fixed objects.

[0036] The choice of powders is dependent on the end-use application and desired properties. Powders are typically individually formulated to meet specific finishing needs. Nevertheless, powder coatings fall into two basic categories: thermoplastic and thermosetting. The choice is application dependent. However, in general, thermoplastic powders are more suitable for thicker coatings, providing increased durability, while thermosetting powders are often used when comparatively thin coatings are desired, such as decorative coatings. The principal resins used in thermoplastic powders use primarily epoxy, polyester, and acrylic resins.

[0037] The concentration of powder in air must be controlled to maintain a safe working environment. Despite the absence of flammable solvents, any finely divided organic material, such as dust or powder, can form an explosive mixture in air. This is normally controlled by maintaining proper air velocity across face openings in the spray booth. In the dust collector, where the powder concentration cannot be maintained below the lower explosive limit, either a suppression system or a pressure relief device must be considered.

[0038] The thickness of the applied polymeric coating can be controlled to provide the results desired based on the particular application intended for the finished product. A typical polymeric coating thickness is from about 0.5 thousandths of an inch (0.5 mils) to about 50 mils. In spray coating techniques, including both powder coating and flame spraying, controlling the thickness of the powder coating can be accomplished by varying the rate of flow out of the spray gun as well as the distance between the gun and the substrate to be coated. In fluidized bed coating techniques, the thickness of the powder is primarily controlled by varying the amount of time that the substrate is left in the coating chamber.

[0039] In the case of powder coating using the spray technique or the fluidized bed application, once the substrate has been coated to the desired thickness, it is heated, in zone 120, to above the melting temperature of the polymeric coating 114 an order to melt and, in the case of thermoset polymers, at least partially cure the coating. This heating can be accomplished by any method that provides acceptable results. As shown in the figures, 122 represents conduction heating while 124 represents a device for emitting infrared or ultraviolet radiation 126. It will also be appreciated that convection heating may be used in further examples. One or more or all heating techniques may be used in the heating zone(s) of the processes herein.

[0040] Convection heating uses hot air to transfer heat from the energy source to the article being heated. The most common convection systems use a gas flame and blower to provide circulation of heated air in an oven chamber. Other convection systems utilize electric infrared elements which, while cleaner, are generally more expensive to operate. In convection heating, the entire object including the substrate must be brought to the cure temperature of the polymer. If the substrate is large, it may take a substantial amount of time to fully heat, lengthening the time required to cure the polymer coating. Since the entire oven chamber is heated evenly however, it is relatively easy to achieve consistent cure over the entire surface of even complex shaped objects.

[0041] Short wave, high-intensity infrared radiation 126 provides a direct, radiant method of heating. Unlike convection heating, radiation heating 126 does not require the medium to be heated for heat transfer to take place. Thus, since the air and substrate do not need to be heated, substantial savings in cure time may be realized. However, a direct line between the surface to be heated and the radiator is necessary for optimum and consistent results. Substrates with complex shapes may heat unevenly, resulting in uneven cure in various locations on the substrate surface. Radiation heating is best used with products of consistent and simple shape.

[0042] As stated previously, the polymeric coating 114a is heated such that it melts and flows together, forming a continuous film on the substrate surface. The polymer is heated to such a degree that it partially cures, forming a viscous fluid film in which a reflective material 132 may be subsequently partially embedded. This partial curing is typically at the gel point of the polymer. The temperature and length of time necessary to achieve this partial cure will vary depending on the identity of the polymeric coating. Thus, for an acrylic urethane powder coating, the partial cure step might include heating the coating for about 15 minutes at about 375 ⁰F.

[0043] As the partially cured polymer coated substrate exits from the heating chamber 120, and while it is still hot, a retroreflective layer is applied to the surface of the substrate on top of the partially cured polymer 114a at an application zone 130. The retroreflective layer comprises a reflective material 132. The polymer coating 114a should be sufficiently tacky or gelled such that the reflective material easily adheres thereto. The reflective material 132 can be applied in any manner such that it partially embeds in the partially cured polymer and bonds thereto. In one embodiment, the reflective material 132 is embedded in the partially cured polymer 114a such that at least a part of the upper surface of the reflective material is exposed to the atmosphere, thereby forming a retroreflective layer and better permitting retroreflection of incident light by the final product.

[0044] When using flame coating (see zone 308) or extrusion processes (see zone 208) for application of the polymeric coating 214, 316 to the substrate 104, the retroreflective layer may be formed by application of the reflective material 132 to the coated substrate immediately after application of the molten polymer so that the reflective material partially embeds within the molten polymeric layer. The reflective material may be, for example, glass microspheres (as shown) or prism shaped. Glass microspheres (see 132 in FIG. IB) that are retro-reflective by the nature of their composition can be used in the flame coating method, and these spheres can further be hemispherically coated with metal (metalized) so that they are more reflective, but can also be uncoated. Prisms can be polymeric translucent prisms wherein the shape and composition of the prisms also makes them retro-reflective.

[0045] Various known reflective materials can be utilized in forming the retroreflective layer according to the process described herein. Reflective materials include, but are not limited to, glasses, ceramics, metals, plastics and other similar types of reflective materials known in the art. In one embodiment, the reflective material 132 comprises numerous distinct reflective optical elements. These optical elements are generally small grains or particles that act as lenses to diffract and reflect incident light.

[0046] The reflective optical elements can be any desired shape, such as triangular, square, pentagonal, hexagonal, prismatic, etc. In one embodiment, the reflective elements are substantially spherical. Such spherical reflective elements are known in the art as microspheres.

[0047] A wide variety of ceramic optical elements (e.g. microsphere) may be employed in the present invention. Typically, for optimal retroreflective effect, the optical elements have a refractive index of about 1.5 to about 2.6. Generally, optical elements of about 5 to about 1000 micrometers in diameter may be suitably employed. In one embodiment, the optical elements used have a relatively narrow size distribution for effective coating and optical efficiency.

[0048] The optical elements may comprise an amorphous phase, a crystalline phase, or a combination, as desired. Also, the optical elements may be comprised of inorganic materials that are not readily susceptible to abrasion. Suitable optical elements include microspheres formed of glass having indices of refraction of greater than about 1.5 and typically from about 1.5 to about 1.9. The optical elements most widely used are made of soda-lime-silicate glasses. Although the durability is acceptable, the refractive index is only about 1.5, which greatly limits their retroreflective brightness. Higher-index glass optical elements of improved durability that can be used herein are taught in U.S. Pat. No. 4,367,919.

[0049] When glass elements are used, the fabrication of the retroreflective layer occurs at temperatures below the softening temperature of the glass optical elements, so that the optical elements do not lose their shape or otherwise degrade. The optical elements' softening temperature, or the temperature at which the glass flows, generally should be greater than the process temperature used to form the retro-reflective layer. This is typically about 100 ⁰C to about 200 ⁰C above the process temperature used to form the retro-reflective layer.

[0050] The optical elements can be colored to match the binder (e.g. marking paints) in which they are embedded. Techniques to prepare colored ceramic optical elements that can be used herein are described in U.S. Pat. No. 4,564,556. Colorants such as ferric nitrate (for red or orange) may be added in the amount of about 1 to about 5 weight percent of the total metal oxide present. Color may also be imparted by the interaction of two colorless compounds under certain processing conditions (e.g. TiO₂ and ZrO₂ may interact to produce a yellow color). Further, a pigmented translucent layer may also be used to impart a color to the finished product.

[0051] Other materials may be included within the retroreflective layer. These may be materials added to the optical elements during preparation, added to the optical elements by the supplier, and/or added to the retroreflective layer during coating with the optical elements. Illustrative examples of such materials include pigments and skid-resistant particles.

[0052] Pigments may be added to the optical elements to produce a colored retroreflective element. In particular, yellow may be desirable for yellow pavement markings. For example, praseodymium doped zircon ((Zr, Pr)SiO₄) and Fe₂O₃ or NiO in combination with TiO₂ may be added to provide a yellow color to better match aesthetically a yellow liquid pavement marking often used in centerlines. Cobalt zinc silicate ((Co, Zn)₂ SiO₄) may be added to match a blue colored marking. Colored glazes or porcelain enamels may also be purchased commercially to impart color, for example yellow or blue.

[0053] Pigments which enhance the optical behavior may be added. For example, when neodymium oxide (Nd₂O₃) or neodymium titanate (Nd₂TiOs) is added, the perceived color depends on the spectrum of the illuminating light.

[0054] Skid-resistant particles may be substituted for some of the optical elements. They are useful on retroreflective and non-retroreflective pavement markings to reduce slipping by pedestrians, bicycles, and motor vehicles. The skid-resistant particles can be, for example, ceramics such as quartz, aluminum oxide, silicon carbide or other abrasive media. Preferred skid-resistant particles include fired ceramic spheroids having a high alumina content as taught in U.S. Pat. Nos. 4,937,127; 5,053,253; 5,094,902; and 5,124,178, the disclosures of which are incorporated herein by reference. Skid-resistant particles typically have sizes ranging from about 200 to about 800 micrometers.

[0055] As shown in zone 130, the optical elements can be applied to the partially cured polymeric coating or to the molten flame coated or extruded polymer by any effective means. A pneumatic or hydraulic powered dispensing machine can be used to deposit the optical elements on the polymeric coating. The optical elements should be deposited with such a velocity that the optical elements are partially embedded in the polymer coating with at least a portion of the surface of a sufficient number of optical elements still exposed to provide the desired retroreflectivity to the finished article. Pressure may be applied to the optical elements after they have been deposited to assure that they are securely embedded in the polymeric coating. [0056] Once the retroreflective layer is deposited on the partially cured powder coating, the resulting assembly can then be heated, in zone 140, to completely cure the polymeric coating. In the case of thermoset polymers, this allows the thermoset to fully crosslink and reach is maximum toughness and durability. This finishing cure step also bonds the retroreflective layer to the cured polymeric coating, securing the two together more tightly and making the optical elements less likely to become dislodged. Again, the temperature and length of time necessary for final curing will depend on the identity and thickness of the polymeric coating.

[0057] Optionally, as shown in zone 150, an additional clear coating 154 may be applied after the coating is fully cured. In one example, a quantity of clear coating 152 can be extruded into a layer 154 of appropriate thickness although other techniques are possible in further examples. This clear coating may be added to provide protection for the retroreflective layer and inhibit the dislocation of optical elements. This coating may comprise any clear material that does not unduly affect the retroreflective properties of the product.

[0058] The end products for which the coating process can be utilized include any known product for which a reflective surface is desired. Such products include, but are not limited to, highway signs, roadside safety products, auto parts (cars, motorcycles, trucks, buses, etc.), bicycles, railroad cars, railroad signs and crossing gates, loading and freight dock markings, airport runways, parking lots and garages, mailboxes and virtually anywhere light delineation is needed.

[0059] It will therefore be appreciated that the process of the present invention can involve coating a substrate or material with both a polymeric coating material and a retroreflective material. The polymeric coating material can provide a tough, corrosion resistant protective layer on the substrate and also acts as a binder in which the retroreflective material is subsequently embedded. The process of the present invention can include the following steps:

1. Providing a polymeric coating material;

2. Applying the polymeric material to a substrate; 3. Heating the polymeric material to a partially cured state (optional if

polymeric material is applied in a molten state);

4. Apply retroreflective material;

6. Finish curing the powder coating (optional if the polymeric material is

applied in a molten state;

7. Apply clear coat or translucent composition (optional step)

[0060] The powder coating can be applied to its surface by one of several methods, such as, but not limited to, electrostatic spray, fluidized bed treatment, extrusion or flame spraying. Any of the various known polymeric coatings can be used in the coating process according to the present invention. Typical polymeric materials useable in the coating process include, but are not limited to, epoxy compounds, polyesters, acrylics, polyester urethanes, acrylic epoxies, thermoplastics, thermosets, olefin polymers, ethylene acrylics, along with various hybrids and combinations thereof. Partial curing of the polymeric coating, when required, is accomplished by conventional methods such as oven curing or curing with infrared radiation. This partially cured polymer acts as a binder in which reflective elements can be subsequently embedded. The partial curing step comprises approximately 35% of the total cure time or to the gel point of the polymer.

[0061] After the polymer coating is partially cured, or, in the case of when a molten layer of polymeric material is applied, after application of the molten layer of polymeric material, a retroreflective layer is applied to the substrate on top of the partially cured polymer or the molten polymer. The retroreflective layer is comprised of a reflective material. Suitable reflective materials for the retroreflective layer include glasses, ceramics, metal flakes, plastics and other reflective materials known in the art. The reflective material can be in the form of small beads, chips, flakes or prisms, collectively known as reflective elements, and can be applied in any conventional manner, such as fluidized bed, sprayed on methods and or a roll process/film transfer, where the beads are applied from a roll of them like tape, over the molten polymer, and then run through a roll process transferring the beads from the film to the molten polymer. Suitable reflective elements in the present invention include ceramic or glass beads or microspheres as well as retroreflective prisms. Typical of these include beads made from soda-lime-silicate glasses (also known as barium titanate glass beads).

[0062] After the reflective material is applied to the powder-coated substrate, the powder is fully cured, if not applied in a molten state, to intimately bond the reflective material to the cured powder layer. An optional clear coat or translucent composition may subsequently be added to provide additional protection and adhesion.

[0063] It will be apparent to those skilled in the art that various modifications and variations can be made to the present invention without departing from the spirit and scope of the invention. Thus, it is intended that the present invention cover the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.

Claims

What is claimed is:

1. A process for forming a retroreflective surface on a substrate which is in condition for accepting a polymer coating, the process comprising: providing a powder coating material; applying the powder coating material to an outer surface of the substrate; heating the powder coating material to a partially cured state; providing a reflective material; applying a retroreflective layer on the surface of the powder coating material by at least partially embedding the reflective material in the partially cured powder coating material; and secondary heating the powder coating material to a final cure state, wherein the substrate is not pretreated prior to applying the powder coating material to the outer surface of the substrate.

2. The process according to claim 1, wherein the step of providing a powder coating material is performed by providing a thermoset or thermoplastic polymer.

3. The process according to claim 1, wherein the step of providing a reflective material is performed by providing a plurality of ceramic optical elements.

4. The process according to claim 3, wherein the ceramic optical elements are microspheres.

5. The process according to claim 4, wherein the microspheres are barium titanate glass beads.

6. The process according to claim 4, wherein the microspheres have a refractive index of at least about 1.5.

7. The process according to claim 6, wherein the refractive index is about 1.9.

8. The process according to claim 3, wherein the ceramic optical elements have a particle size of from about 50 to about 1000 micrometers in diameter.

9. The process according to claim 1, wherein the step of applying the powder coating material to an outer surface of the substrate is performed by electrostatically spray coating the powder coating material on the substrate.

10. The process according to claim 1, wherein the step of applying the powder coating material to an outer surface of the substrate is performed by positioning the substrate in a fluidized bed of the powder coating material.

11. The process according to claim 1 , wherein the heating steps are accomplished by using a convection heating system.

12. The process according to claim 1, wherein the steps of partially curing the powder coating and final curing the powder coating are performed by heating the powder coating using infrared or ultraviolet radiation.

13. A process for forming a retroreflective surface on a substrate, the process comprising: providing a polymeric powder coating material; applying the polymeric powder coating material in a molten state to the substrate by flame spraying; providing a reflective material; applying the reflective material to the molten state polymeric powder coating material; and finish curing the powder coating material to form a retroreflective surface on the substrate.

14. The process according to claim 13, wherein the substrate is not pretreated prior to the step of applying the polymeric powder coating material.

15. The process according to claim 13, wherein the reflective material comprises ceramic optical elements including barium titanate glass microspheres.

16. The process according to claim 13, wherein the reflective material comprises ceramic optical elements including a refractive index of from about 1.5 to about 2.6.

17. The process according to claim 13, wherein the retroreflective surface is further coated with a protective coating.

18. A process for forming a retroreflective surface on a substrate, the process comprising: providing a polymeric coating material; applying the polymeric coating material in a molten state to the substrate by extrusion; providing a reflective material; applying the reflective material to the molten state polymeric coating material; and finish curing the polymeric coating material to form a retroreflective surface on the substrate.

19. The process according to claim 18, wherein the substrate is not pretreated prior to the step of applying the polymeric coating material.

20. The process according to claim 18, wherein the retroreflective surface is further coated with a protective coating.