MXPA00002621A - Method of making retroreflective elements - Google Patents

Abstract

A method of making a ceramic retroreflective element having enhanced strength and increased retained reflectivity. The method of the present invention comprises forming retroreflective elements by the following steps:a) providing glass flakes;b) coating said glass flakes with a first barrier layer yielding coated glass flakes;c) providing optical elements;d) optionally coating said optical elements with a second barrier layer;e) blending said optical elements and said coated glass flakes;f) heating said optical elements and said coated glass flakes to spheroidize said flakes while agitating said optical elements and said coated glass flakes;g) further heating said optical elements and said spheroidized glass flakes to partially embed said optical elements in said spheroidized flakes while agitating said optical elements and said spheroidized flakes;and h) cooling said spheroidized flakes having partially embedded optical elements.

Description

METHOD OF ELABORATION OF RETRORREFLECTORS ELEMENTS FIELD OF THE INVENTION The present invention is concerned with a method to produce retroreflective elements that can be placed in pavement markings to guide and direct motorists traveling on a road.

BACKGROUND OF THE INVENTION The use of pavement markings (eg, paints, tapes and individually mounted articles) to guide and direct motorists traveling on a highway is well known. During the day, the markings can be sufficiently visible under ambient light to effectively signal and guide a motorist. However, at night, especially when the main source of illumination are the headlights of the motorist's vehicle, the markings are generally insufficient to properly guide the motorist because the light of the headlights hits the pavement and the marking to a angle of incidence very low and is reflected widely in the distance of the motorist. For this reason, improved pavement markings with retroreflective properties have been used. Retroreflection describes the mechanism where light incident on a surface is reflected from such REF.: 33091 so that much of the incident beam is directed back to its source. The most common retroreflective pavement markings such as highway lanes are made by dripping transparent glass or ceramic optical elements onto a newly painted line, so that the optical elements are partially embedded in it. The transparent optical elements act as a spherical lens and thus the incident light passes through the optical elements to the base paint or pigment particles that collide with the sheet therein. The pigment particles scatter the light by redirecting a portion of the light back to the optical element such that a portion is then directed back to the light source. In addition to providing the desired optical effects, pavement markings must withstand road traffic and exposure to the weather, adverse weather conditions and cost restrictions. Surfaces arranged somewhat vertical or upward provide a better orientation for retroreflection than horizontal surfaces; consequently, numerous attempts have been made to incorporate vertical surfaces in pavement markings, commonly by providing protrusions on the marking surface. In addition, vertical surfaces can prevent the accumulation of a layer of water on the retroreflective surface during rainy weather that otherwise interferes with the retroreflective mechanism. One means of providing vertical surfaces is to place raised pavement markers at intervals along a marking line (eg, U.S. Patent Nos. 3,292,507; 4,875,798). These markers are relatively large, several centimeters in general in width and 5 to 20 mm in height. Commonly, the markers require the assembly together of different components, some of which were previously molded individually. Accordingly, markers are relatively expensive to manufacture. The size of the markers subject them to substantial impact forces of passing vehicles. As a result, markers must be substantially secured to the pavement, increasing installation costs and removal costs when they wear out. In addition, because the markers are applied at intervals, the bright spots of light are discontinuous, instead of the desired continuous bright line. Pavement marking tapes embossed are a second means for providing vertical surfaces (e.g., U.S. Patent Nos. 4,388,359, 4,609,281 and 5,417,515). The selective placement of transparent optical elements on the vertical sides of the protuberances embossed in relief results in a highly effective marking material. NeverthelessSuch tapes are relatively expensive compared to conventional painted markings and thus their use is often limited to critical areas such as unlit intersections and railroad crossings. Also, these tapes are constructed with polymeric materials that are susceptible to wear. A third means for providing vertical surfaces for retroreflection is a composite retroreflective element or aggregate (e.g., U.S. Patent Nos. 3,254,563 4,983,458). Many variations are known, but the reflective elements essentially have a core with optical elements embedded in the surface of the core. Some known embodiments also contain optical elements scattered throughout the core that are exposed during wear. The core can be irregular in shape or can be formed into waits, tetrahedrons, disks, square mosaics, etc. The retroreflective elements are advantageous because they can be embedded in non-expensive painted markings. The retroreflective elements consist mainly of polymeric cores or binders. A pigmented or binder core often serves as a diffuse reflector. This "arrangement allows optical elements to be used on horizontal or vertical surfaces.

Other constructions have transparent optical elements that comprise a specular reflector such as metallic silver. The metal surface directs the light back to the source and a pigmented core is not necessary. Due to the geometry of the optical elements, a specular coated optical element would not be as effective if it is embedded in a pavement marking paint (a horizontal surface) and would be more highly effective if it is embedded in the vertical surfaces of a retroreflective element. Another construction of the retroreflective element, U.S. Patent No. 3,252,376, has only silver glass flakes that serve as a specular reflector on the surface of a spherical polymeric core without the use of spherical optical elements. Another known construction is a retroreflective element in which a plastic globule (lens) refracts the incident light on a layer of optical glass elements attached to the lower portion of the globule. Then the optical glass elements focus the light on a coating or specular film located below the optical elements, where the light is then reflected back along the original path towards the source (for example US Pat. Nos. 4,072,403; 4,652,172; 5,268,789).

Polymeric retroreflective elements formed with a pigmented core and optical glass elements embedded in the vertical surfaces are described in U.S. Patent No. 3,418,896. These retroreflective elements are formed by extruding the pigmented polymer into rods of different cross-sectional shape. Optical glass elements are embedded in the surface of the polymer before it hardens, then the rods are cut to form the desired elements. Polymer retroreflective elements are undesirably susceptible to wear, especially in regions of high traffic and degradation due to exposure to the weather. In an attempt to overcome these limitations, retroreflective elements were constructed having a ceramic core and optical glass elements with a metallic specular coating. One type of construction is a rock or glass sphere core (U.S. Patent Nos. 3,043,196 3,175,935) covered by a polymeric binder with optical glass elements having a specular metallic coating embedded in the polymeric coating. Another construction described in U.S. Patent No. 3,556,637 has a glass sphere and a layer of glass optical elements attached to the bottom of the glass sphere with a polymeric binder. A metallic film under the glass optical elements acts as a specular reflector. Other known constructions include a composite lens element that serves as a retroreflective element and a slip resistant particle (EP 0,322,671). The slip-resistant particle that acts as a core (either a corundum particle or glass sphere) is coated with a pigmented polymeric binder that acts as a diffuse reflector. A ceramic element having glass optical elements embedded in a whole core of glass and on the surface of the core is described in U.S. Patent No. 3,171,827. A thin metallic film separates the optical elements from the glass core to provide an efficient specular retroreflective system. Alternatively, optical elements are used that have a high refractive index (greater than 2.0). It is said that these optical elements of high refractive index are able to reflect light without the need for a reflector support. A ceramic retroreflector element has a transparent glass sphere with smaller glass optical elements embedded in the surface is described in US Pat. Nos. 3, 274,288 and 3,486,952. Again, a thin metallic film separates the optical elements and the glass sphere to provide an efficient specular retroreflector system. The elements are formed by first coating the glass spheres with metallized optical elements using a temporary polymeric binder. Then the spheres are subjected to tumbling with the optical elements in excess in a rotary kiln. When the temperature exceeds the softening temperature of the glass spheres, the optical elements embed themselves on the surface of the spheres. Later, the film is etched or attacked by acid from the exposed portion of the optical elements. WO 97/28471 describes a retroreflective element comprising an opaque ceramic core and ceramic optical elements partially embedded in the core. The core, of diffuse reflective ceramic, in combination with the transparent optical elements embedded in the surface, provides a surprisingly bright retroreflective element without the gray coloration and susceptibility to corrosion associated with metallic specular reflectors. Although these fully ceramic retroreflective elements have a greatly improved resistance to wear and to the effects of weathering, improved crush resistance is desirable to increase the life of the retroreflective element.

BRIEF DESCRON OF THE INVENTION The present invention provides a method for making a ceramic retroreflective element having improved strength and increased retained reflectivity. The method of the present invention aims to form retroreflective elements by the following steps: (a) providing glass flakes; (b) coating such glass flakes with a first barrier layer producing coated glass flakes; (c) providing optical elements; (d) optionally coating such optical elements with a second barrier layer; (e) combining such optical elements and such coated glass flakes; (f) heating said optical elements and such coated glass flakes to convert such flakes into spheres while the optical elements and the coated glass flakes are agitated; (g) additionally heating the optical elements and the glass flakes converted into spheres to partially embed the optical elements in the flakes converted to spheroids as long as such optical elements and the flakes turned into spheroids are agitated and (h) to cool the flakes converted in spheroids that have partially embedded optical elements. Preferably, continuous agitation is provided throughout the process. The retroreflective elements are substantially spheroidal, which reduces sharp edges and tips which improves the crush resistance and fragmentation of the retroreflective element on a road.

BRIEF DESCRON OF THE DRAWINGS Figure 1 is a cross-sectional view of a spherical element 10 having a core 12 wherein the optical elements 14 are partially embedded in the core. Figure 2 is a profile of the retroreflective element showing a tracing or contour 16 of the area used to quantify the spherical character of the element. Figures 3a-d are contours of several different profiles of the retroreflective element. The proportions A / AO are (3a) 0.77; (3b) 0.88; (3c) 0.93; and (3d) 0.97. The figures, which are ideal and are not to scale, are proposed only as illustrative and not limiting.

DETAILED DESCRON OF ILLUSTRATIVE MODALITIES The present invention provides a method for making particularly useful retroreflective ceramic elements for imparting retroreflection to liquid pavement markings. The ceramic retroreflector element is bonded together in final form without the aid of polymeric materials. These retroreflective elements may be free of metals or alternatively, the optical elements may be partially coated with a metallic layer. The resulting ceramic retroreflective elements are substantially spheroidal. This shape reduces the sharp edges and points which improves the resistance to crushing and the fragmentation of the retroreflective elements on the road. Additionally, the low porosity of the spheroidal shapes formed by the reconfiguration of the dense glass flakes can improve the internal resistance of the retroreflective element. This increased resistance is evident by improved crush resistance and improved fragmentation resistance. WO 97/28471 describes various methods for preparing ceramic retroreflective elements. One of the most convenient methods involves stirring a mixture of glass flakes (typically 0.5 to 1.5 mm thick by 1 to 3 mm wide) with spherical optical elements at a temperature higher than the softening point of the glass flakes. . The resulting retroreflective element retains the general shape of the original glass flake. The present invention provides a method for converting the glass flakes into spheroids prior to the embedding of the optical elements. The leaflets must be converted into spheroids before the incrustation of the optical elements because once the optical elements are embedded in the surface of the glass flake, additional changes in the form can no longer be presented. Without being limited by the theory, the presence of the optical elements can inhibit the changes of form due to the transformation in spheroid of a glass flake requires a reduction in surface area. Thus, the optical elements would have to be removed from the surface of the glass flake to allow a reduction in the surface area. A spheroidal retroreflective element is defined by comparing the area covered by the profile of the retroreflective element to the area of a circle having an equivalent perimeter. When this ratio is greater than about 0.90, the retroreflective element is considered to be spheroidal.

The retroreflective elements consist of a layer of ceramic optical elements such as transparent ceramic microspheres, partially embedded in the surface of a diffusely reflecting opaque ceramic core, in such a way that some of the incident light to the exposed surface of the optical elements it is refracted by this to the core, wherein some of it is reflected to re-enter the embedded portion of the optical element and be refracted in such a way that it leaves the exposed portion of the optical element in a general direction towards the source of the optical element. light. Commonly, the retroreflective elements range in size from about 0.5 mm to about 3 mm in diameter. "Ceramics" is used in the present to refer to inorganic materials that can be either crystalline (a material that has an atomic structure in a geometric form sufficient to produce a characteristic X-ray diffraction pattern) or amorphous (a material that does not have order of longitudinal rank in its atomic structure as evidenced by the lack of a characteristic X-ray diffraction pattern). Amorphous ceramics are commonly known as glasses. The opacified ceramic cores of this invention will often contain a mixture of amorphous (glass) and crystalline phases.

Optical elements A wide variety of ceramic optical elements (e.g. microspheres) can be employed in the present invention. Commonly for an optimum retroreflective effect, the optical elements have a Refractive Index of 1.5 to approximately 2.6. the optical elements preferably have a diameter compatible with the size, shape and geometry of the core or glass flakes. The currently preferred core dimensions range from about 0.5 to about 5 mm in diameter. In general, optical elements of about 50 to about 1000 microns in diameter can be employed appropriately. Preferably, the ratio of the diameter of the optical elements to the diameter of the core is not greater than about 1: 2. Preferably, the optical elements used have a relatively narrow size distribution for effective coating and optical efficiency. The optical elements comprise an amorphous phase, a crystalline phase or a combination, as desired. The optical elements consist preferably of inorganic materials that are not easily susceptible to abrasion. Suitable optical elements include glass-formed microspheres, which preferably have refractive indices of about 1.5 to about 1.9. The most widely used optical elements are made of sodium carbonate - lime - silicate glass. Although the durability is acceptable, the Refractive Index is only about 1.5, which greatly limits its retroreflective brilliance. Optical glass elements of higher refractive index, of improved durability, may be used herein as taught in U.S. Patent No. 4,367,919. Preferably, when optical glass elements are used, the manufacture of the retroreflective element occurs at temperatures lower than the softening temperature of the glass optical elements, in such a way that the optical elements do not lose their shape or degrade in any other way. . The softening temperature of the optical elements or the temperature at which the glass flows should generally be at least about 100 ° C, preferably about 200 ° C, greater than the process temperature used to form the retroreflective element. . Additional improvements in durability and refractive index have been obtained using microcrystalline ceramic optical elements as described in U.S. Patent Nos. 3,709,706; 4,166,147; 4,564,556; 4,758,469 and 4,772,511. Preferred ceramic optical elements are described in U.S. Pat. Nos. 4,564,556 and 4,758,469 which are incorporated herein by reference in their entirety. These optical elements comprise at least one crystalline phase which contains at least one metal oxide. These ceramic optical elements can also have an amorphous phase such as silica. The optical elements are resistant to scratching and fragmentation, are relatively hard (Knoop hardness greater than 700) and are made in such a way that they have a relatively high refractive index. Optionally, the optical elements can be coated in steam with a metal (for example aluminum). See U.S. Patent No. 2,963,378 (Palmquist et al.) Incorporated by reference herein, for a description of the vapor coated optical elements. The optical elements may comprise silicone, alumina, silica, titania and mixtures thereof. When using optical elements having a crystalline phase, the manufacturing temperature of the retroreflective element preferably does not exceed the temperature at which the crystal growth occurs in the crystalline component of the optical elements, otherwise, the optical elements are They can distort or lose their transparency. The transparency of the optical elements depends in part on maintaining the size of the crystal at a size smaller than the size at which they begin to scatter visible light. In general, the process temperature used to form the retroreflective element is limited to about 1100 ° C and preferably less than 1050 ° C. Higher process temperatures can cause the optical elements to cloud with a corresponding loss in retroreflective effectiveness . The optical elements can be colored to match the binder (eg, marking paints) in which they are embedded. Techniques for preparing colored ceramic optical elements that can be used herein are described in U.S. Patent No. 4,564,556. Dyes such as ferric nitrate (for red or orange) can be added in the amount of about 1 to about 5% by weight of the total metal oxide present. The color can also be imparted by the interaction of two colorless compounds under certain processing conditions (for example, Ti02 and Zr02 can interact to produce a yellow color).

Glass flakes The diffuse reflection exhibited by the glass flakes is an important factor in determining the retroreflective performance of a retroreflective element of the invention.

Glass is an attractive core material because it can be processed at low temperatures and thus at a lower cost. However, conventional glasses tend to be single-phase, fully dense materials, which do not provide the desired light scattering for use as core materials. A special class of ceramics containing both glass phases and crystalline phases that provide excellent dispersion is known. These materials are known as clear opaque ice when they are applied as a coating on a ceramic and as opaque porcelain enamels when applied as a coating on a metal. Because clear opaque ices and opaque porcelain glazes contain a large portion of glass, they are frequently referred to and referred to herein as opaque glasses. Silicates having a refractive index commonly in rarigo from about 1.5 to about 1.6 are used in opaque glasses and in opaque porcelain enamels. To obtain an appropriate difference in refractive index, a dispersion phase with a high refractive index is desirable for use in opacified glass. Materials (opacifiers or opacifying agents) that are commonly used for this purpose include tin oxide (Sn02) with a refractive index of approximately 2.04.; zircon (ZrSi04) with refractive index from about 1.9 to about 2.05; calcium titanate (CaTi03) with a refractive index of approximately 2.35 and titania (Ti02), anatase and rutile, with a refractive index that ranges from approximately 2.5 to approximately 2.7. Other illustrative opacifying agents suitable for use herein include, but are not limited to CaTiOSi0 (Refractive Index from about 1.95 to about 2.09); Ca3Ti207 (refractive index from about 2.16 to about 2.22); Na2Ti2Si209 (Refractive index from about 1.91 to about 2.02); BaTi03 (Refractive index of approximately 2.4); MgTi205 (refractive index from about 2.11 to about 2.23) and MgTi03 (refractive index from about 1.95 to about 2.3). Preferably, the crystalline phase required by the opacity and thus sufficient light scattering, is obtained by dissolving the opacifier in the molten glass, cooling the glass to prevent the crystalline phase from precipitating and then allowing the crystalline phase to precipitate when reheated. at a sufficient temperature to allow precipitation to occur, but low enough to avoid the rapid growth of crystals. However, in some cases, the opacifier may not dissolve in the glass and may be added to the glass as a separate component. Most opaque titania glasses contain 15 to 20% by weight of titania which is mostly in solution until the porcelain enamel is heated, commonly at a temperature of about 700 ° C. Titania precipitates in crystals, usually around 0.2 microns in size. The zircon has a solubility in many glasses of about 5% by weight at about 1200 ° C. The customary amount of zircon in the clear ice is from about 8 to about 10% by weight, so much of the zircon is precipitated from the glass, some of the zircon remains undissolved in the molten glass. Accordingly, the zircon raw material used in the clear ice is preferably ground to a fine crystal size (that is, it ranges from about 0.05 microns to about 1.0 miera) (before the addition to the glass formulation) Many variations of titania and zircon opacified glass are sold commercially Glass and opacifier are available as a single homogeneous material (ie, the manufacturer has combined and heated the ingredients together to form a melt and then cooled and ground the resulting material which is then sold as flakes or powders.) The glass flakes and opacifying powder can also be obtained separately and can then be combined in the manufacturing process.Zirconia (Zr02) can also be used as an opacifying additive. , zirconia frequently reacts with silica in the base glass to form zircon, if desired, additional opacifier ional can be added to an opacified alkaline flux. For example, additional zircon powder can be added to an alkaline zinc zinked glass alkaline flux. When opacifiers are used in this way, powders in the size range of 0.05 to 1 mire are particularly useful. This size helps in the complete dissolution of the powder in the glass or in cases where the glass is already saturated with the opacifier, ensures that the undissolved material is in the range of size desired for the dispersion. Preferably, during the manufacturing process, the pulverized opacifier and the glass powder are thoroughly and uniformly mixed. The complete mixture is preferred to avoid agglomeration of any of the components. Commonly, as is known in the art, by proper mixing by the use of dispersants, agglomeration can be prevented. The glass flakes are substantially free of porosity when visually observed using a power optical microscope 10. Commonly, the core (glass flakes) ranges in size from about 0.5 mm to about 4 mm in diameter, preferably about 1.2 mm to approximately 2 mm in diameter. Preferably, the core material does not react with or solubilize the optical elements, since this tends to reduce the transparency and may distort the shape of the optical element, thereby deteriorating the retroreflective performance of the final product.

Barrier layer materials The first barrier layer covers the glass flakes to prevent the optical elements from partially embedding in the glass flakes (ie the core) prior to the formation of spheroids. Without being limited by theory, it is believed that the barrier layer increases the softening temperature on the surface of the glass flake, which allows the glass flake to become spheroid before the embedding of the optical element. The material of the first barrier layer is incorporated into the glass flakes. Suitable materials of the first layer - barrier include but are not limited to sun silica, titania powder, mica powder and mixtures thereof. When a pulverized material is used as the first barrier layer, preferably the powder adheres naturally to the surfaces of the glass flakes and is evenly distributed over the surface during the combination. For ease of processing, the currently preferred material is sun silica. When sun silica is used as the material of the first barrier layer, the glass flakes are coated with a thin continuous film, which commonly has a coating thickness less than 1 miera. When a powder material is used as a first barrier layer, the powder is usually finely milled by an average of less than 1 miera. Commonly from about 0.01 to about 0.5% by weight of the material of the first barrier layer is used based on the glass flake, preferably about 0.025 to about 0.3% by weight based on the glass flakes. Material levels of less than about 0.01% by weight adversely impact the formation of spheroids and material levels greater than about 0.5% by weight require an increased temperature for an embedding of the optical element. The increased temperature increases the production costs and can affect the coloration of the glass flakes.

A second barrier layer can optionally be coated on the optical elements. This layer can help prevent the first barrier layer from coming out of the glass flakes. Otherwise, the powder originally adhered to the glass flakes can be transferred to the surface of the optical elements that are normally present in large volume during processing. When a pulverized material is used as the second barrier layer, the powder preferably adheres naturally to the surface of the glass flakes and is evenly distributed over the surface during mixing. Commonly, when added, from about 0.01 to about 0.3% by weight, preferably about 0.05 to about 0.2% by weight of the material of the second barrier layer is used to coat the optical elements. If more material is used from the second barrier layer, the brightness of the retroreflective element is likely to be smaller. ' Suitable materials of the second barrier layer include, but are not limited to, titania, zirconia and silica (sol). It is also possible to avoid the loss of the first barrier material of the glass flakes by sticking it to the surface of the glass flakes. A convenient method to accomplish this is to heat the coated glass flakes to a temperature just above the softening point of the glass, causing the powder to partially embed itself on the glass surface. The barrier layer materials can not interact openly with other components of the retroreflective element. Preferably, the barrier layer materials have a high refractive index which contributes to the scattering of the light because the materials can remain on the glass flakes or optical elements, particularly on the portion of the optical elements in contact with the glass flakes. The material of the first and second barrier layers may be the same or may be different. If the same material is used for each barrier layer, the particle size used for each layer may differ. When the barrier layer on the glass flakes is not glued to the glass it may be desirable to wash the retroreflective elements after forming to improve the brilliance. Washing removes the barrier material adhered to the outer surfaces of the optical elements to improve the brilliance.

Optional additives Other materials can be included within the retroreflective elements. These can be materials added to the core material during the preparation, added to the core material by the supplier and / or added to the retroreflective elements during coating with the optical elements. Illustrative examples of such materials include pigments, slip resistant particles, particles that improve the mechanical bond between the retroreflective element and the binder. Pigments can be added to the core material to produce a colored retro-reflective element, in particular yellow may be desirable for yellow pavement markings. For example, zircon doped with praseodymium ((Zr, Pr) Si04) and Fe203 or NiO in combination with Ti02 can be added to provide a yellow color to better match aesthetically with a yellow liquid pavement marking frequently used in centerlines. You can add zinc cobalt silicate ((Co, Zn) 2Si04) to match a blue color marking. Colored clear ice or porcelain enamels can also be purchased commercially to impart color, for example, yellow or blue. Pigments that improve optical behavior can be added. For example, when neodymium oxide (Nd203) or neodymium titanate (Nd2Ti05) is added, the perceived color depends on the illumination light.

The slip resistant particles can be replaced by some of the optical elements on the surface of the retroreflective elements. They are useful on retroreflective and non-reflective pavement markings to reduce the lowering by pedestrians, bicycles and motorized vehicles. Slip resistant particles can be for example ceramics such as quartz, aluminum oxide, silicon carbide or other abrasive media. Preferred slip-resistant particles include calcined ceramic spheroids having a high alumina content as taught in U.S. Patent Nos. 4,937,127; 5,056,253; 5,094,902 and 5,124,178, the disclosures of which are incorporated herein by reference. Slip resistant particles typically have sizes ranging from about 200 to about 800 microns.

Method The present invention provides a method for forming ceramic retroreflective elements having a substantially spheroidal shape. The resulting retroreflective elements have improved strength and retained reflectivity.

The retroreflective elements are formed using continuous agitation. A continuous process or a batch process can be used where the retroreflective elements are continuously agitated. The first stage involves coating the glass flakes with a first barrier layer. Normally, from about 0.01 to about 0.5 weight percent, based on the weight of the glass flakes of the material of the first barrier layer are mixed with the glass flakes until a layer of substantially continuous material is coated on the surface of the glass flakes. Optionally, the glass flakes can be heated after coating to bond the barrier layer to the glass flakes. The gluing step is especially useful when a powder material is used as the material of the first barrier layer. For example, the glass flakes can be heated to a temperature ranging from about 500 ° C to about 700 ° C for about 1 to about 2 minutes. The time and temperature depends on the material and the method of agitation. The heating conditions are preferably such that the "barrier powder" layer is firmly attached to the glass flakes and is not removed in subsequent handling or contact with the optical elements., a second barrier layer can be coated on the surface of the optical elements. The material of the second barrier layer is combined with the optical elements until a substantially continuous layer of material is coated on the surface of the optical elements. In the third stage, optionally coated optical elements are mixed with the coated glass flakes. Preferably, the ratio of the optical element to glass flakes is about 10: 1 on a weight basis. This ratio may vary with the equipment, processing conditions, etc. However, the ratio of optical elements to glass flakes is such that there are sufficient optical elements to minimize agglomeration, that is, adhesion of the glass flakes together during the subsequent processing steps. In the fourth stage, the mixture of the optionally coated optical elements and the coated glass chips are heated or heated (that is, heat treatment applied to a ceramic to consolidate or densify a ceramic or alter its condition in some way) to form in spheroids the glass flakes. Typically, the heating temperature is in the range of about 750 ° C to about 875 ° C for about 2 to about 3 minutes. During the heating process, the mixture of glass flakes and optical elements is continuously stirred, for example in a rotary kiln. At this high temperature, the opacifier precipitates on the glass, the flakes become spheroids and the optical elements are partially embedded in the glass. The heating temperature allows the glass flakes to soften, but it is low enough to avoid damage to the optical elements. While the temperature is high, the optical elements are partially embedded in the spheroidal or core glass flakes. Preferably, the optical elements are embedded to a depth sufficient to retain the optical elements in the core during processing and use. For spheroidal optical elements, an incrustation greater than 30% of the diameter will usually effectively retain the optical element in the core. Preferably, the optical elements are embedded at a depth of about 30% to about 80% of their average diameter, more preferably, about 40% to about 60% of their average diameter. If the optical elements are embedded to a depth less than about 30% of their diameter, they tend to dislodge easily from the surface of the retroreflective element. When the depth of incrustation is greater than 80%, the amount of light able to access the optical element is undesirably restricted. The retroreflective elements of the present invention are normally substantially covered by the optical elements. The surfaces of the retroreflective elements designed to retroreflect the light preferably do not contain larger portions that are hollow than optical elements. The optical elements are in essence packed tightly on the surfaces intended to retroreflect the light. After the embedding of the optical element, the spheroidal retroreflective element is allowed to cool to room temperature. The cooling speed affects the resistance of the retroreflective element. If the retroreflective element is cooled too quickly, the retroreflective element can be fractured by thermal shock. For example, cracks or small defects can result, which decreases the resistance to fragmentation and decreases the resistance to crushing.

Evaluation procedures 1. Spherical character of the retroreflective elements The spherical character of a batch of retroreflective elements was determined from the profiles of a sample of retroreflective elements. The deviation of these profiles from that of a sphere was measured and the percentage of the retroreflective elements that meet the criteria established for the spherical retroreflective elements was determined. Individual retroreflective elements were observed in a microscope equipped with a television camera. The image was captured on a computer video card and analyzed using the public domain NIH Image Programming Elements Program (developed at the National Institutes of Health in the United States of America and available on the Internet at http: // rsb .info.nih.gov / nih-image /) downloaded in August 1997. The profile of each retroreflective element was tracked using the NIH Image polygon tool. The tracking followed a path along the surface of the glass cores, ignoring the profile of the individual optical elements. The thick black line of Figure 2 illustrates the tracking of a profile of the typical retroreflective element. The length of the tracking perimeter (P) and the enclosed area (A) were determined by the measurement feature in NIH Image. The spherical deflection of the profile was quantified by comparing the profile area with the area of a circle that has the same perimeter. The equivalent area (Ao) was determined from the formula: Ao = P2 / 4p The proportion A / Ao will approach a value of 1.0 as the profile becomes more circular and will be less than one for non-circular profiles. Figure 3 shows some exemplary traces and the corresponding values of A / Ao. This method can be misleading if a retroreflective element is observed in only one direction. For example, a disk observed in one direction may have a circular profile, but a rectangular profile when the observation direction is rotated 90 degrees. To avoid this ambiguity. Each retroreflective element was observed in two directions separated by a 90 degree rotation. The lowest value of A / A was used to characterize the retroreflective element. A sample of 20 retroreflective elements was measured for each process batch. The percent of retroreflective elements that have an area ratio greater than or equal to about 0.90 was used to quantify the spherical character of the batch. 2. Resistance to fragmentation and crushing resistance of retroreflective elements The experience of testing of ceramic retroreflective elements as highway markings revealed three types of degradation: (1) crushing, where a retroreflective element was broken into very small pieces, (a) fragmentation, where a small portion of the retroreflective element was broken, especially sharp corners and (3) cutting of the optical element, where the ceramic optical elements were broken in such a way that the exposed portion of the optical elements was lost and those portions of the optical elements embedded in the glass core persisted. All three types of damage are observed in the crush resistance test that was developed to predict highway performance. A 50 g sample of the retroreflective elements was placed in a porcelain grinding jar of size 00, 11.4 cm in internal diameter, volume of 1300 ml (Norton Chemical Process Products, Akron, OH) together with eight spheres of high density alumina, 3.8 cm in diameter, density 3.4 g / cc (US Stoneware Corp., Mahwah, NJ). The milling jug was rotated at 60 revolutions per minute for six 10-minute intervals. After each interval, the fraction of retroreflective elements ground to less than 18 mesh (1 mm) was filtered and discarded. The percentage of the original retroreflective elements remaining after the six intervals was reported as the crush resistance. Preferably, the retroreflective element has a crush resistance greater than about 70 percent. 3. Retroreflective Brilliance of Retroreflective Elements The coefficient of retroreflection (R), following procedure B of the E809-94a standard of the ASTM was measured at an input angle of -4.0 degrees and an observation angle of 0.2 degrees. The photometer used for these measurements is described in U.S. Defensive Publication No. T987,003. The retroreflective elements were placed in a small disk, in an amount sufficient to cover the background with several layers of retroreflective elements. The surface of the retroreflective elements was leveled and the disc was positioned in the photometer in such a way that the light beam fell completely within the area covered by the elements. Preferably, the retroreflective element has a coefficient of retroreflection greater than about 3 candelas / lux / square meter.

Applications The retroreflective elements made using the method of the present invention can be dropped or cascaded onto binders such as wet paint, heat-hardenable materials or hot thermoplastic materials (for example, U.S. Patent Nos. 3,849,351, 3,891,451, 3,935,158, 2,043,414, 2,440,584 and 4,203,878). In these applications, the binder (that is, the paint, thermosetting material or thermoplastic material) forms a matrix that serves to retain the retroreflective elements in a partially incised and partially protruding orientation. The matrix can be formed from durable systems of two components, such as epoxies or polyurethanes or from thermoplastic polyurethanes, alkyd resins, acrylics, polyesters and the like. Alternative coating compositions that serve as a matrix and include the retroreflective elements described herein are also contemplated within the scope of the present invention. Normally, the retroreflective elements made using the method of the present invention are applied to a road or other surface by means of the use of conventional delineation equipment. The retroreflective elements are dropped in a random position or a prescribed configuration if desired on the surface and each retroreflective element comes to rest, in such a way that it is embedded and adhered to the paint, thermoplastic material, etc. If different sizes of retroreflective elements are used, they are usually evenly distributed over the surface. When the paint or other film-forming material is fully cured, the retroreflective elements are held firmly in place to provide an extremely effective reflector marker. The retroreflective elements of the present invention can also be used in preformed tapes used as pavement markings. The following examples illustrate several specific features, advantages and other details of the invention. The particular materials and amounts cited in these examples, as well as other conditions and details, should not be construed in a manner that unduly limits the scope of this invention. The percentages are given by weight.

Examples Example 1: Without layers without barrier (comparative) Glass flakes of mesh size -11, +18 (XT-1370, Ferro Corp., Cleveland, OH) were mixed with ceramic optical elements (zirconia-silica, with a refractive index of 1.76, prepared as described in U.S. Patent 4,564,556) in a weight ratio of about 10: 1 of optical elements to flakes. The mixture was heated in a rotary tube oven at a temperature of about 775 ° C, the residence time in the hot zone was about 2 minutes. During this process the optical elements were embedded at about half their diameter to the surfaces of the glass flakes. Excessive optical elements were separated from the combined retroreflective elements by filtering the exit of the rotary kiln through an 18 mesh screen. The finished reflective elements retained the original glass flake shape, except for some rounding of the sharp edges. The classification of retroreflective element shapes using the area ratio technique indicates that only approximately 10.0% exceeds the value of approximately 0.90 used to represent a spherical retroreflective element. The crush test used to determine crush resistance and fragmentation produces a survival value of approximately 57.8%. A value of retroreflective brightness (RA) of approximately 5.8 (candelas / lux / square meter) was measured in the retroreflective elements.

Example 2: Powder barrier layers on glass flakes and ceramic optical elements Glass mesh size -11 flakes, +18 (XT-1370) were mixed with 0.3% by weight of Ti02 powder (R700, Dupont Chemicals, Wilmington, DE). During mixing, Ti02 naturally coats the surfaces of the glass flakes. The ceramic optical elements (zirconia-silica with a refractive index of 1.76) were mixed with approximately 0.3% by weight of Ti02 powder (R700). Vigorous agitation caused the Ti02 to coat the surfaces of the optical elements. The coated glass flakes were combined with the optical elements coated in a rotary kiln at a weight ratio of approximately 10: 1 optical elements to flakes. The oven temperature was approximately 825 ° C and the residence time in the hot zone was approximately two minutes. The excess optical elements were separated from the finished retroreflective elements by filtering the output of the rotary kiln through a 18 mesh screen. The Ti02 powder was washed from the exposed surfaces of the optical elements by tumbling the retroreflective elements in a container containing water for about 1 hour.

The reflective elements were predominantly spherical in shape with the ceramic optical elements embedded to approximately half their diameter in the opaque glass core. When the core of the retroreflective element is fractured, it is found to be substantially free of pores, that is, only an occasional isolated pore is observed. The classification of retroreflective element shapes using the area ratio technique indicates that approximately 90.0% exceeds the value of approximately 0.90 used to represent a spherical retroreflective element. The crush test used to determine crush resistance and fragmentation yield a survival value of approximately 71.2%. A retroreflective brightness value (R?) Of approximately 4.0 (candelas / lux / square meter) was measured in the retroreflective elements.

Example 3: Powder barrier layer attached to the glass flakes, layer without barrier on the ceramic optical elements Glass flakes of mesh size -11, +18 (XT-1370) were mixed with 0.3% by weight of powder Ti02 (R700). The coated flakes were heated to a temperature of about 650 ° C in a rotary tube oven with a residence time in the hot zone of about 2 minutes. Then the heated flakes were combined with uncoated ceramic optical elements (zirconia-silica with a refractive index of 1.76) in a rotary kiln at a weight ratio of approximately 10: 1 of optical elements to flakes. The oven temperature was approximately 825 ° C and the residence time in the hot zone was approximately two minutes. Excessive optical elements were separated from the retroreflective elements terminated by filtering the output of the rotary kiln through an 18 mesh screen. The reflective elements were predominantly spherical with the ceramic optical elements embedded in approximately half of their diameter in the opaque glass core. When the core of the retroreflective element is fractured, it is found to be substantially free of pores, that is, only an occasional isolated pore is observed. The classification of retroreflective element shapes using the area ratio technique indicates that approximately 80.0% exceeds the value of approximately 0.90 used to represent a spherical retroreflective element. The crush test used to determine crush resistance and fragmentation yield a survival value of approximately 80.2%. A retroreflective brightness (RA) value of approximately 4.5 (candelas / lux / square meter) was measured in the retroreflective elements.

Example 4: Colloidal sun barrier layer on the glass flakes, layer without barrier on the ceramic optical elements. A 0.05% by weight Si02 coating was applied to the glass flakes of mesh size -10, +18 (XT-1370) using colloidal silica sol (1042; Nalco Chemical Company of Chicago, IL 60638). The silica sol was filtered through a Whatman # 54 filter paper, then diluted to a silica content of approximately 0.4% by weight with additional water. The diluted sol was mixed with the glass flakes and drummed in a heated rotating drum until it dries. The flakes were then combined with uncoated ceramic optical elements (zirconia-silica with a refractive index of 1.76) in a rotary kiln at a weight ratio of approximately 10: 1 optical elements to flakes. The oven temperature was about 800 ° C and the residence time in the hot zone was about two minutes. The excess optical elements were separated from the retroreflective elements terminated by filtering the output of the rotary kiln through an 18 mesh screen.

The reflective elements were predominantly spherical in shape with the ceramic optical elements embedded to approximately half their diameter in the opaque glass core. When the core of the retroreflective element is fractured, it is found to be substantially free of pores, that is, only an occasional isolated pore is observed. The classification of retroreflective element shapes using the area ratio technique indicates that approximately 90.0% exceeds the value of approximately 0.90 to represent a spherical retroreflective element. The crush test used to determine the resistance to crushing and fragmentation produces a survival value of approximately 77.0%. A retroreflective brightness (RA) value of approximately 5.6 (candelas / lux / square meter) was measured in the retroreflective elements. It is noted that, with regard to this date, the best method known to the applicant to carry out the aforementioned invention is that which is clear from the present description of the invention.

Claims (10)

1. CLAIMS Having described the invention as above, the content of the following claims is claimed as property: 1. A method for making a retroreflective element, characterized in that it comprises the steps of: (a) providing glass flakes; (b) coating the glass flakes with a first barrier layer producing coated glass flakes; (c) provide optical elements; (d) optionally coating the optical elements with a second barrier layer; (e) combining the optical elements and the coated glass flakes; (f) heating the optical elements and the coated glass flakes to convert the flakes into spheroids, while the optical elements and the coated glass flakes are agitated; (g) additionally heating the optical elements and the spheroidal glass flakes to partially embed the optical elements in the spheroidal flakes while stirring the optical elements and the spheroidal flakes; and (h) cooling the spheroidal glass flakes having partially embedded optical elements.
2. 2. The method according to claim 1, characterized in that the glass flakes comprise opacified glass and the opacified glass comprises one or more agent (s) -opacifying agent (s) selected from the group consisting of Ti02 (anatase), Ti02 (rutile) and ZrSi04.
3. 3. The method according to claim 1, characterized in that the glass flakes range in size from approximately 0.5 mm to approximately 4 mm.
4. 4. The method according to claim 1, characterized in that the glass flakes are substantially free of porosity. The method according to claim 1, characterized in that the first barrier layer comprises a material that increases the softening temperature of the glass flakes on the surface of the glass flake while allowing the glass flakes to turn into spheroids. The method according to claim 1, characterized in that the glass flakes are heated after coating the glass flakes with the first barrier layer to glue the first flake barrier layer. The method according to claim 1, characterized in that the second barrier layer comprises a material that prevents the first barrier layer from leaving the glass flakes to coat the optical elements. The method according to claim 1, characterized in that the first barrier layer and the second barrier layer comprise different materials. The method according to claim 1, characterized in that the retroreflective element has a coefficient of retroreflection (RA) greater than about 3 candelas / lux / square meter. A pavement marking characterized in that it comprises: (a) a binder material and (b) one or more retroreflective elements made according to the method of claim 1. M ALL OF ELABOR SUMMARY OF THE INVENTION A method for making a ceramic retroreflector element having improved strength and increased reflectivity-retained is described. The method of the present invention comprises forming retroreflective elements by the following steps: (a) providing glass flakes, (b) coating the glass flakes with a first barrier layer producing coated glass flakes, (c) providing optical elements. (d) optionally coating the optical elements with a second barrier layer; (e) mixing the optical elements and the coated glass chips; (f) heating the optical elements and the coated glass flakes to convert the flakes into spheres so much that the optical elements and the coated glass flakes are agitated, (g) additionally heating the optical elements and the glass flake-shaped sheets to partially embed the optical elements in the spheroidal flakes while the optical elements are agitated and the optical elements are agitated. the spheroidal flakes and (h) cool the spherical glass flakes that have partially incrust optical elements ados.