Microspheres containing alumina and zirconia suitable for retroreflective articles

Abstract

Microspheres and retroreflective articles are described. In one embodiment, the microspheres include at least 40% by weight alumina, at least 35% by weight zirconia, and silica. In another embodiment, the microspheres include at least 80, 85, or 90% by weight of a combination of alumina, zirconia, and silica, and the microspheres have a density of at least 3.

Description

Summary of the Invention

[0001] Microspheres and retroreflective articles are described herein. In one embodiment, a retroreflective article is described that includes microspheres disposed on a surface of the article. In some embodiments, the article is a pavement marking or pavement marking tape. In other embodiments, the retroreflective article is a retroreflective element that includes a core particle that includes microspheres disposed on at least a portion of the surface of the core particle.

[0002] In one embodiment, the microspheres comprise at least 40% by weight alumina, at least 35% by weight zirconia, and silica.

[0003] In another embodiment, the microspheres comprise at least 80, 85, or 90 weight percent of a combination of alumina, zirconia, and silica, and the microspheres have a density of at least 3.4. [Brief description of the drawings]

[0004] [Figure 1] 1 is a cross-sectional view of an exemplary retroreflective element. [Diagram 2] FIG. 1 is a perspective view of an exemplary pavement marking. [Diagram 3] 1 is a cross-sectional view of an exemplary pavement marking tape. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0005] Microspheres generally comprise zirconia (i.e., ZrO2), aluminum oxide (i.e., Al2O3), and silica (i.e., SiO2). This base composition is referred to herein as "ZAS." Beads that comprise the ZAS base composition are referred to as "ZAS beads" or "ZAS microspheres." The terms "beads" and "microspheres" are used interchangeably and refer to particles that are substantially spherical.

[0006] The term "solid" refers to beads that are not hollow, i.e., that have no substantial cavities or voids. For use as lens elements, the beads are preferably spherical and preferably solid (i.e., non-porous). Solid beads are typically more durable than hollow beads. Solid beads can also focus light more effectively than hollow beads, resulting in higher retroreflectivity. The microspheres described herein are preferably transparent.

[0007] The term "transparent" means that the bead has the property of transmitting visible light when viewed under an optical microscope (e.g., 100x), so that an object beneath the bead, such as an object of the same nature as the bead, can be clearly seen through the bead when both are immersed in oil of approximately the same refractive index as the bead. The oil should have a refractive index close to that of the bead, but not so close that the bead appears to disappear (as in a perfect index match). The outline, perimeter, or edges of the object beneath the bead can be clearly discerned.

[0008] The transparent beads described herein typically have a refractive index of at least 1.72. In some embodiments, the transparent beads have a refractive index of at least 1.72, 1.73, 1.74, 1.75, 1.76, 1.77, 1.78, 1.79, 1.80, 1.81, 1.82, 1.83, 1.84, or 1.85. In some embodiments, the refractive index of the transparent beads is no greater than 1.92, 1.91, 1.90, 1.89, 1.88, 1.87, 1.86, 1.85, 1.84, or 1.83. In some embodiments, the refractive index of the transparent beads is no greater than 1.82, 1.81, 1.80, 1.79, 1.78, 1.76, or 1.75.

[0009] The described microspheres are particularly useful as lens elements in retroreflective articles. The articles of the invention share the common feature of including the described ZAS beads disposed on a surface of the article. Thus, at least a portion of the ZAS beads and / or reflective elements are exposed on the display surface of the article (e.g., a pavement marking). The microspheres and / or reflective elements are preferably embedded in the binder or core particles to a depth ranging from about 30% to about 60% of their diameter.

[0010] The pavement markings described herein include a binder. The binder adheres the microspheres or elements comprising the microspheres to the road surface. The road surface is typically substantially solid and predominantly inorganic. Typically, the road surface includes asphalt, concrete, and the like. The binder typically includes paint, thermoplastic material, thermosetting material, or other curable material. Common binder materials include polyacrylates, methacrylates, polyolefins, polyurethanes, polyepoxide resins, phenolic resins, and polyesters. In the case of reflective pavement marking paints, the binder can include a reflective pigment.

[0011] For reflective sheeting suitable for reflective signage, apparel, or other applications, the binder that secures the beads is typically transparent. The transparent binder may be applied to a reflective base or applied to a release-coated support from which the bead-attaching film is peeled off after the binder solidifies and subsequently applied to a reflective base or reflective coating or plating. The reflective elements and / or microspheres that contain microspheres are typically coated with one or more surface treatments that alter the wetting properties of the pavement marking binder and / or improve adhesion of the reflective elements or microspheres that contain microspheres in the binder. The reflective elements are preferably embedded in the pavement marking binder by about 20-40%, more preferably about 30%, of their diameter so that the reflective elements are adequately exposed. Surface treatments that control wetting include various fluorine compound derivatives, such as those available under the trade name "Krytox 157 FS" from Du Pont, Wilmington, DE. Various silanes are suitable as adhesion promoters, such as those available under the trade designation "Silquest A-1100" from OSI Specialties, Danbury, Conn.

[0012] In some embodiments, the retroreflective article is a retroreflective element that includes a core particle that includes microspheres at least partially embedded in the core. With reference to FIG. 1, the retroreflective element 200 includes ZAS microspheres 117 alone or in combination with higher refractive index beads (e.g., having a refractive index of at least 2.20 or greater) 116 partially embedded in the surface of the core 202. The core is typically substantially larger than the beads. For example, the average core diameter may range from about 0.2 to about 10 millimeters. The core may include an inorganic material. Glass ceramics are also useful as core materials. The crystalline phase acts to scatter light, resulting in a translucent or opaque appearance. Alternatively, the core may include an organic material, such as a thermoplastic or bonded resin core, i.e., a cross-linked cured resin, such as an epoxy, polyurethane, alkyd, acrylic, polyester, phenolic, etc. Various epoxies, polyurethanes, and polyesters are generally described in U.S. Patents 3,254,563, 3,418,896, and 3,272,827. The core may be a composite material comprising inorganic particles coated with an organic material. In the latter case, the organic material functions as a binder to adhere the beads to the outer surface of the core.

[0013] Retroreflective elements may be prepared from non-diffusely reflective bonded resin cores in combination with specularly reflective microspheres (e.g., vapor coating the microspheres with aluminum), but this approach results in less durable retroreflective elements due to the use of metals that may be susceptible to chemical degradation. Less durable retroreflective elements are also obtained by incorporating metals (e.g., aluminum) into the core. In some embodiments, the retroreflective elements include at least one non-metallic light-scattering material dispersed within the core. Reflective elements may be made by known processes such as those described in U.S. Pat. No. 5,917,652, U.S. Pat. No. 5,774,265, and U.S. Patent Application Publication No. 2005 / 0158461.

[0014] The retroreflectivity of the microspheres and / or retroreflective elements for an incidence angle of -4° and an observation angle of 0.2° (determined according to the test method in the Examples) is at least 8, 9, 10, 11, 12, 13, 14, or 15 (Cd / m 2 In some embodiments, the retroreflectivity of the microspheres and / or retroreflective elements is 15, 14, 13, 12, 11, 10, 9, or 8 (Cd / m 2 ) / lux or less. Lower refractive index and brightness (in air) microspheres can be used in conjunction with higher refractive index beads.

[0015] In some embodiments, the beads and / or retroreflective elements are used in liquid applied marking (e.g., pavement) applications. For example, referring to FIG. 2, beads 117 and / or reflective elements 200 are sequentially or simultaneously dropped onto liquefied binder 10 or are synthesized in liquefied binder provided on road surface 20. In other embodiments, the beads and / or reflective elements are used in retroreflective sheeting, including exposed lenses, encapsulated lenses, embedded lenses, or encapsulated lens sheeting. Representative pavement marking sheeting materials (tapes) are described in U.S. Pat. No. 4,248,932 (Tung et al.), U.S. Pat. No. 4,988,555 (Hedblom), U.S. Pat. No. 5,227,221 (Hedblom), U.S. Pat. No. 5,777,791 (Hedblom), and U.S. Pat. No. 6,365,262 (Hedblom).

[0016] Pavement marking tapes and sheet materials generally include a backing, a layer of binder material, and a layer of beads partially embedded in the layer of binder material. The backing, which is typically less than about 3 millimeters thick, can be made from a variety of materials, such as polymeric films, metal foils, and fiber-based sheets. Suitable polymeric materials include acrylonitrile-butadiene polymers, grindable polyurethanes, and neoprene rubbers. The backing can also include particulate fillers or skid-resistant particles. The binder material can include a variety of materials, such as vinyl polymers, polyurethanes, epoxides, and polyesters, and can optionally include colorants, such as inorganic pigments, including specular pigments. The pavement marking sheet can also include an adhesive, such as a pressure-sensitive adhesive, a contact adhesive, or a hot melt adhesive, on the bottom of the backing sheet.

[0017] Patterned retroreflective (e.g., pavement) markings advantageously implement vertical surfaces (e.g., defined by protrusions) with microspheres partially embedded therein. Because light sources typically strike pavement markings at high approach angles, vertical surfaces with embedded microspheres provide more efficient retroreflection. Vertical surfaces also reduce the exposure of the microspheres to water during periods of rain, thereby improving retroreflective performance.

[0018] For example, FIG. 3 shows a patterned pavement marking 100 including a (e.g., elastomeric) polymer base sheet 102 and a number of protrusions 104. For illustrative purposes, only one protrusion 104 is covered with microspheres and anti-skid particles. The base sheet 102 has a front surface 103 from which the protrusions extend, and a back surface 105. The base sheet 102 is typically about 1 millimeter (0.04 inches) thick, but may be other dimensions if desired. Optionally, the maker 100 may further include a scrim 113 and / or an adhesive layer 114 on the back surface 105. The protrusions 104 have a top surface 106, a side surface 108, and in the illustrative embodiment are about 2 millimeters (0.08 inches) high. Protrusions having other dimensions may be used if desired. As shown, the side surface 108 meets the top surface 106 at a rounded top portion 110. The side surface 108 preferably forms an angle of about 70° at the intersection of the front surface 103 and the lower portion 112 of the side surface 108. The protrusion 104 is coated with a pigmented binder layer 115. Embedded in the binder layer 115 are a plurality of ZAS microspheres 117 and a plurality of second microspheres 116 (e.g., having a higher refractive index than the ZAS microspheres). Optionally, anti-slip particles 118 may be embedded in the binder layer 115.

[0019] Pavement marking sheets can be made by various known processes. A representative example of such a process includes coating a mixture of resin, pigment, and solvent onto a backing sheet, dripping beads as described herein onto the wet surface of the backing, and curing the construction. A layer of adhesive can then be coated onto the bottom of the backing sheet. US Patent No. 4,988,541 (Hedblom) discloses a preferred method of making patterned pavement markings, which is as follows: Optionally, if necessary, a scrim (e.g., woven or nonwoven) and / or an adhesive layer can be attached to the backside of the polymer base sheet.

[0020] In some embodied retroreflective articles, two types of microspheres are used, one type being the ZAS beads described herein, and the second type being "high refractive index microspheres" having a refractive index of, for example, at least 2.1, 2.2, or 2.3, typically 2.45 or less. In some embodiments, one of the two types of microspheres is larger. For example, ZAS microspheres in the size range of 50 to 150 micrometers in diameter can be arranged in combination with larger or smaller beads.

[0021] ZAS microspheres, alone or in combination with optional non-ZAS beads, are typically present in an amount of at least 15, 20, 25, 30, or 35 weight percent of the total amount of microspheres in the reflective article, in some embodiments, ZAS microspheres, alone or in combination with optional non-ZAS beads, are present in an amount of up to 85, 80, or 75 weight percent of the total amount of microspheres.

[0022] The microspheres are preferably selectively disposed on the side and top surfaces of the protrusions, while leaving the valleys between the protrusions substantially empty to minimize the amount of microspheres and thereby minimize manufacturing costs. The microspheres may be disposed on either the side surfaces as well as the top surfaces of the protrusions to achieve efficient retroreflection.

[0023] The binder layers of Figures 2 and 3 and the core of the retroreflective element shown in Figure 1 contain optically transparent materials so that light entering the retroreflective article is not absorbed, but instead retroreflected by scattering or reflection from pigment particles in the optically transparent material. Vinyl, acrylic, epoxy, and urethane are examples of suitable media. Urethanes, such as those disclosed in U.S. Patent No. 4,988,555 (Hedblom), are preferred binder media, at least for pavement markings. The binder layer typically covers selected portions of the protrusions so that the base sheet remains substantially free of binder. To facilitate coating, the media is preferably a liquid having a viscosity of less than 10,000 centipoise at the coating temperature.

[0024] The binder layers of Figures 2 and 3 and the core of Figure 1 typically include at least one pigment, such as a diffuse or specular reflective pigment.

[0025] Specular pigment particles are generally thin and plate-like and are part of the binder layer, the organic core of the element (a core that essentially contains only organic binder material), or an organic binder coating on inorganic particles that together make up the composite core of the element. Light that strikes the pigment particle is reflected at an angle equal to but opposite to the angle at which it was incident. Suitable examples of specular pigments include pearlescent pigments, micas, and nacreous pigments. Typically, the amount of specular pigment present in the binder layer ranges from at least 15 weight percent up to 40 or 50 weight percent. Pearlescent pigment particles are often preferred for color accuracy.

[0026] Instead of or in addition to combining transparent beads with a reflective (e.g., pigmented) binder and / or element core, the beads may include a reflective (e.g., metallic) coating. Typically, the metallic coating is absent on the portion of the outer surface of the bead oriented to receive the retroreflected light, and is present on the portion of the outer surface of the bead oriented opposite the direction in which the retroreflected light is incident. For example, in FIG. 1, a metallic coating may be disposed at the interface between the bead 117 and the core 202. In FIG. 3, a reflective layer may be disposed at the interface between the bead 117 and the binder 115, as shown in U.S. Pat. No. 6,365,262. The metallic coating may be disposed on the beads by physical vapor deposition means, such as evaporation or sputtering. Full coverage metallic coatings disposed on the beads may be partially removed by chemical etching.

[0027] The components of the beads are described as oxides, i.e., the form in which they exist in fully processed glass and glass ceramic beads and retroreflective articles, and which accurately describes the chemical elements and their proportions in the beads. The starting materials used to make the beads may contain some chemical compounds other than oxides, such as dispersants that volatilize during the melting and spheronization process.

[0028] The microspheres described herein typically contain at least 40% alumina (Al2O3) by weight. In typical embodiments, the microspheres contain no more than 60, 59, 58, 57, 56, or 55% alumina by weight. In some embodiments, the microspheres contain at least 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, or 60% alumina by weight. Microspheres containing at least 40, 41, 42, 43, 44, or 45% alumina by weight have good crush resistance. In other embodiments, the microspheres contain no more than 54, 53, 52, 51, 50, 49, 48, 47, 46, or 45% alumina by weight.

[0029] The microspheres described herein typically contain at least 30, 31, 32, 33, 34, or 35% by weight of zirconia (ZrO2). In typical embodiments, the microspheres contain up to 55, 54, 53, 52, 51, or 50% by weight of zirconia. When the amount of zirconia is about 50% by weight, the amount of alumina is typically greater than 30% by weight and / or the amount of silica is less than 20% by weight. In some embodiments, the microspheres contain at least 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, or 50% by weight of zirconia. Microspheres containing at least 45, 46, 47, 48, 49, or 50% by weight of zirconia have good crush resistance. In other embodiments, the microspheres comprise up to 49, 48, 47, 46, 45, 44, 43, 42, 41, or 40 weight percent zirconia.

[0030] In some embodiments, the total amount of alumina and zirconia is at least 80, 85, 90, or 95% by weight of the microsphere.

[0031] In some embodiments, the microspheres contain about the same amount of alumina as zirconia. In other words, the weight ratio of alumina to zirconia is about 1:1. For example, the microspheres may contain about 40% alumina and about 40% zirconia by weight. As yet another example, the microspheres may contain about 45% alumina and about 45% zirconia by weight.

[0032] In other embodiments, the microspheres contain more alumina than zirconia. For example, the microspheres may contain about 55% alumina and about 35% zirconia by weight. In some embodiments, the weight ratio of alumina to zirconia is at least 1.1:1, 1.2:1, 1.3:1, 1.4:1, 1.5:1, or 1.6:1.

[0033] In other embodiments, the microspheres contain more zirconia than alumina. For example, the microspheres may contain about 40% alumina and about 50% zirconia by weight. In some embodiments, the weight ratio of zirconia to alumina is at least 1.1:1, 1.2:1, 1.3:1, 1.4:1, 1.5:1, or 1.6:1.

[0034] The microspheres described herein typically contain silica. In some embodiments, the microspheres contain at least 1, 2, 3, 4, or 5% silica by weight. In some embodiments, the microspheres contain no more than 30, 29, 28, 27, 26, 25, 24, 23, 22, 21, or 20% silica by weight. In some embodiments, the microspheres contain at least 6, 7, 8, 9, or 10% silica by weight. In some embodiments, the microspheres contain at least 11, 12, 13, 14, or 15% silica by weight. Microspheres containing less than 15, 14, 13, 12, 11, or 10% silica by weight have good crush resistance. In some embodiments, the microspheres contain at least 16, 17, 18, 19, or 20% silica by weight. In some embodiments, the microspheres contain no more than 20, 19, 18, 17, 16, or 15% silica by weight. In some embodiments, the microspheres contain no more than 14, 13, 12, 11, or 10% silica by weight. In some embodiments, the microspheres contain no more than 9, 8, 7, 6, or 5% silica by weight.

[0035] In some embodiments, the total amount of zirconia, alumina, and silica totals at least 85, 90, 95, or 100% by weight of the microsphere. When the total amount of zirconia, alumina, and silica totals at least 85, 86, 87, 88, 89, or 90% by weight of the microsphere, the microsphere exhibits good crush strength.

[0036] The microspheres described herein may also contain other metal oxides, the total amount of metal oxides being up to 15 or 10% by weight.

[0037] Such other metal oxides are selected so as not to impair the properties (e.g., brightness and / or durability) of the ZAS microspheres. Other metal oxides may be selected to be added to lower the melting point of the material, which results in easier processing. Suitable other metal oxides include, for example, LiO2, Na2O, K2O, and alkaline earth oxides such as BaO, SrO, MgO, and CaO, AhO3, ZnO, SiO2, and B2O3. Other metal oxides may be selected and added to increase the refractive index. Suitable other metal oxides include, for example, rare earth oxides such as titania and lanthana.

[0038] In some embodiments, the microspheres include alkaline earth oxides, such as MgO and / or CaO. The amount of alkaline earth oxide(s) (e.g., MgO, CaO, or the sum thereof) is typically less than 10, 9, 8, 7, 6, or 5% by weight of the microsphere. In some embodiments, the amount of alkaline earth oxide(s) is at least 1, 2, 3, 4, or 5% by weight of the microsphere.

[0039] In some embodiments, the microspheres comprise titania. The amount of titania is typically less than 10, 9, 8, 7, 6, or 5% by weight of the microsphere. In some embodiments, the amount of titania is at least 1, 2, 3, 4, or 5% by weight of the microsphere.

[0040] In some embodiments, the microspheres include lanthana (La2O3). The amount of lanthana is typically less than 15, 14, 13, 12, 11, or 10% by weight of the microsphere. In some embodiments, the amount of lanthana is at least 1, 2, 3, 4, or 5% by weight of the microsphere. In some embodiments, the amount of lanthana is at least 6, 7, 8, 9, or 10% by weight of the microsphere.

[0041] In some embodiments, the microspheres include a combination of titania and lanthana. In this embodiment, the amounts of titania and lanthana are within the ranges described above. In particular, a sufficiently high brightness can be obtained without the presence of titania and lanthana.

[0042] In yet other embodiments, the microspheres include one or more (e.g., transition) metal oxides to impart color and / or fluorescence, as known in the art. Such colorants include, for example, Fe2O3, CoO, Cr2O3, NiO, CuO, MnO2, VOs, and the like. Typically, the beads include about 5% by weight or less (e.g., 1%, 2%, 3%, 4%) of colorant, based on the total weight of the bead. Rare earth elements, such as praseodymium, neodymium, europium, erbium, thulium, ytterbium, and the like, may also be optionally included for color or fluorescence. Preferably, the microspheres are substantially free of lead oxide (PbO) and cadmium oxide (CdO).

[0043] In some embodiments, the microspheres contain little or no other metal oxides, hi some embodiments, the amount of other metal oxides is no more than 10, 9, 8, 7, 6, 5, 4, 3, 2, 1, or 0.5% by weight of the microsphere.

[0044] In some embodiments, the microspheres typically have a density of at least 3.4 or 3.5. The density of the microspheres is typically no greater than 3.95. In some embodiments, the density is at least 3.6, 3.7, 3.8, or 3.9. In some embodiments, the density is less than 3.9, 3.8, 3.7, or 3.6.

[0045] Beads can be made and used in a variety of sizes. It is not common to intentionally form beads with a diameter of less than 10 micrometers, but some beads with a diameter of up to 2 or 3 micrometers are sometimes formed as a by-product in the manufacture of larger beads. Thus, beads are typically at least 20, 30, 40, 50, 60, 70, 80, 90, or 100 micrometers. For example, in some embodiments, the d10 of the particle size distribution, measured as described in the Examples, is typically at least 80, 90, or 100 micrometers. Beads are typically 1-2 mm or less. More commonly, beads are 750, 500, or 300 microns or less. In some embodiments, the d90 of the particle size distribution is 300, 350, 200, or 150 microns or less.

[0046] The microspheres described herein can be prepared by any suitable method. In some embodiments, shaped precursor green particles were prepared from a slurry according to the general teachings of U.S. Patent No. 8,701,441 (Kramlich et al.), which is incorporated herein by reference. To form glass microspheres, the shaped precursor green particles were fed into a methane / oxygen torch flame (i.e., flame former), thereby producing glass microspheres.

[0047] The glass precursor composition used to form the spheres comprises glass precursor particles and, optionally, at least one of the following liquids: water, a volatile organic liquid, and a temporary binder, i.e., a binder that dissipates during the high temperature treatment used to form the spheres. The glass precursor particles are preferably dispersed such that the composition forms a dispersion (e.g., a slurry). One example of a useful glass precursor composition is one that comprises glass precursor particles and water and is in the form of a slurry.

[0048] The liquid (e.g., water) is typically present in the glass precursor composition in an amount of at least 5, 10, 15, or 20% by weight of the slurry composition. In some embodiments, the liquid is present in an amount of no more than 50, 40, or 30% by weight of the slurry composition.

[0049] The aqueous glass precursor composition can include other additives including, for example, hydrocolloids (e.g., xanthan, maltodextrin, galactomannan, and tragacanth), polysaccharides, natural gums (e.g., gum arabic), starch derivatives, surfactants (e.g., cationic, anionic, nonionic, and zwitterionic), including, for example, sodium lauryl sulfate, polysorbates, and sodium 2-ethylhexyl sulfate, and combinations thereof.

[0050] Examples of useful volatile organic liquids include methanol, ethanol, isopropyl alcohol, butyl alcohol, heptane, and toluene.

[0051] Useful temporary binders include, for example, water-soluble and water-dispersible binders including dextrin, starch, cellulose, hydroxyethyl cellulose, hydroxypropyl cellulose, carboxyethyl cellulose, carboxymethyl cellulose, carrageenan, scleroglycan, xanthan gum, guar gum, hydroxypropyl guar gum, and combinations thereof. Other suitable binders are described in U.S. Patent No. 8,701,441.

[0052] The shaped microparticles are then passed through a flame or other source of sufficient thermal energy (e.g., a gas-fired or electric furnace) to form molten glass droplets. Any suitable sphere-forming process and apparatus can be used, including, for example, glass, glass-ceramic, glass-bonded ceramic, and crystalline ceramic sphere manufacturing processes and apparatus.

[0053] In one useful method, the shaped microparticles are in the form of a free-flowing powder, and the passing includes dispersing the free-flowing powder in a flame. The flame has a temperature sufficient to convert, e.g., melt, the glass precursors present in the shaped microparticles to a homogenous state. The flame temperature is selected to be suitable for melting and fusing the shaped microparticles into glass droplets. Useful flame temperatures are at least about 2000 K, at least about 3000 K, or even from about 3000 K to about 5000 K. The flame can be generated by any suitable fuel and oxidant source, including, for example, natural gas, hydrogen, oxygen, acetylene, air, and mixtures thereof.

[0054] The duration of the shaped microparticles in the flame is referred to as the "residence time". The residence time is selected to achieve spheres with the desired property(ies). Variables that affect the residence time include, for example, flame speed, flame size, flame shape, flame temperature, volume of the shaped microparticles, composition of the shaped microparticles, density of the shaped microparticles, and density of the spheres. The molten droplets can be maintained in the flame for a time sufficient to convert the molten droplets into spheres through any suitable mechanism, including, for example, directing a gas flow under the molten droplets, allowing the molten droplets to fall freely through a heated zone, and combinations thereof.

[0055] The fused glass droplets form spheres, which are then quenched to form spheres. Various quenching methods are suitable, including, for example, air cooling (e.g., by free-falling through a sufficient distance of space), rapid cooling, and combinations thereof. Useful rapid cooling methods include allowing the spheres to continue to free-fall through a cooling zone or into a cooling medium, such as water, oil, or combinations thereof. Alternatively, or in addition, gas (e.g., air or argon) can be sprayed into the free-falling stream of the fused spheres to accelerate and cool them to form solid, transparent glass microbeads.

[0056] The spheres are then collected and optionally further processed, including, for example, screening (also called classification, sieving, and sizing), heat treating (e.g., to induce crystallinity in the spheres to form glass-ceramic, glass-bonded ceramic, and crystalline ceramic spheres, and combinations thereof), full ceramming, and combinations thereof. Useful heat treating methods are disclosed, for example, in U.S. Patent No. 6,245,700, which is incorporated herein.

[0057] Microspheres that exhibit X-ray diffraction consistent with the presence of crystalline phases are considered glass-ceramic microspheres. A rough guideline in the art is that materials containing less than about 1% by volume of crystals may not exhibit detectable crystallinity in a typical powder X-ray diffraction measurement. Such materials are often considered "X-ray amorphous" or glass materials, rather than ceramic or glass-ceramic materials. Microspheres that contain crystals detectable by X-ray diffraction measurements (which typically need to be present in amounts of 1% by volume or more for detectability) are considered glass-ceramic microspheres. X-ray diffraction data can be determined as described in the Examples.

[0058] In an exemplary embodiment, the microspheres may be characterized as amorphous microspheres, or in other words "glass microspheres."

[0059] Glass-ceramic microspheres contain one or more crystalline (e.g., nanocrystalline) phases, typically totaling at least 5% by volume. Some glass-ceramic beads formed by quenching molten droplets may contain crystals without secondary heat treatment, but crystallinity is typically developed through heat treatment of amorphous beads. Such crystalline phases may include relatively pure single-component metal oxide phases of titania (e.g., anatase, rutile) and / or zirconia (e.g., baddeleyite). Such crystalline phases may also include relatively pure multicomponent metal oxide phases (e.g., ZrTiO4).

[0060] When initially formed from the melt, beads are formed that are substantially amorphous but may also have some crystallinity. The composition preferably forms clear, transparent glass microspheres when quenched. Upon further heat treatment, the beads can develop crystallinity in the form of a glass-ceramic structure, i.e., a microstructure in which crystals grow from within the initial amorphous structure, thus becoming glass-ceramic beads. Upon heat treatment of the quenched beads, the beads can develop crystallinity in the form of a nanoscale glass-ceramic structure, i.e., a microstructure in which crystals less than about 100 nanometers in size grow from within the initial amorphous structure, thus becoming glass-ceramic beads. A nanoscale glass-ceramic microstructure is a microcrystalline glass-ceramic structure that includes nanoscale crystals. In some embodiments, the (e.g., titania-containing) transparent microbeads are predominantly crystalline (i.e., more than 50% crystalline by volume) immediately after quenching, thus avoiding a heat treatment step. In some embodiments, the microspheres (eg, with low concentrations of titania or no titania) remain amorphous when heat treated to temperatures up to 950°C.

[0061] In some embodiments, the microspheres form a microcrystalline glass-ceramic structure by heat treatment, but remain transparent. For good transparency, it is preferred that the microspheres contain little or no volume fraction of crystals with dimensions greater than about 100 nanometers. Preferably, the microspheres contain less than 20, 10, 15, or 5 volume percent of crystals with dimensions greater than about 100 nanometers. Preferably, the size of the crystals in the crystalline phase is less than about 20 nanometers (0.02 micrometers) in their largest linear dimension. Crystals of this size typically do not scatter visible light effectively, and therefore do not significantly reduce transparency.

[0062] The microspheres typically have a whiteness suitable for use in retroreflective articles, as determined by the test methods described in the Examples. The whiteness index ("WI") is typically at least 50. In some embodiments, the whiteness index is at least 51, 52, 53, 54, 55, 56, 57, 58, 59, or 60. In some embodiments, the whiteness index is 60, 59, 58, 57, 56, or 55 or less.

[0063] The microspheres described herein have sufficient durability. Durability can be determined by a variety of methods. In some embodiments, the brightness retained after sandblasting, as described in the Examples below, indicates the durability of the microspheres and retroreflective articles. In some embodiments, the retroreflective article or microsphere has at least 65, 66, 67, 68, 69, or 70% retained brightness after sandblasting. In some embodiments, the retained brightness is typically about 75% or less. In some embodiments, the retroreflective article or microsphere has at least 4, 5, 6, or 7 (Cd / m) retained brightness after sandblasting. 2 ) / Looks of brightness.

[0064] The microspheres described herein (e.g., in retroreflective articles) have sufficient crush resistance. Crush resistance can be determined by the method described in U.S. Pat. No. 4,772,511. Using this procedure, the beads can exhibit a median crush resistance of at least about 700, 750, 800, 850, 900, 950, or 1000 MPa. In some embodiments, the crush resistance is no greater than 1800, 1700, 1600, 1500, 1400, 1300, or 1200 MPa. EXAMPLES

[0065] Unless otherwise stated or readily apparent from the context, all parts, percentages, ratios, etc. in the examples and elsewhere in the specification are by weight.

[0066] The following abbreviations are used in the Examples section: Cd = candela, m = meter, cm = centimeter. [Table 1]

[0067] Test Method Refractive Index Procedure: The refractive index of the microspheres was measured according to T. Yamaguchi, "Refractive Index Measurement of High Refractive Index Beads," Applied Optics Volume 14, Number 5, pages 1111-1115 (1975). The data is presented in Table 4. Examples with a "-" reported were not transparent (i.e., crystalline) as made, and therefore refractive index data could not be generated.

[0068] Brightness Procedure: Dry white patch brightness values ​​were determined using a retroreflectometer (Road Vista 932 obtained from Road Vista LLC, San Diego, CA). The device directs white light onto a flat monolayer of glass microspheres partially submerged in a diffuse white backing (3M 7000-109-3 Patch Brightness Tape) with an incidence angle set relative to the normal of the partially submerged monolayer. The diffuse white backing without the monolayer of glass microspheres had a whiteness index of 86 when measured according to the color procedure described below. The photodetector of the retroreflectometer measured the brightness (Cd / m 2 Retroreflective luminance, or patch luminance, is measured at a fixed divergence angle relative to the angle of incidence (observer angle) in units of 1000 nm / lux. The results presented in Table 4 were measured at a 0.2° observer angle and a -4° incidence angle setting. Retroreflective luminance measurements were made for the purpose of comparing luminance between glass microspheres of different chemistries. The 0.2° observer angle data generated for this measurement is in accordance with ASTM E1709.

[0069] Mechanical Durability Procedure: Sandblast Abrasive Wear Testing was performed. Test patches were prepared by embedding the glass microspheres to be tested into 1.5 inch (3.8 cm) circular patches of 6 mil (0.15 mm) thick ethylene methacrylic acid (EMAA) film. The EMAA film was opaque white and was loaded with pigments to act as diffuse reflectors so that the patch retroreflective brightness could be measured both initially and after sandblasting. The patches were prepared by taking a 1.5 inch (3.8 cm) circular piece of EMAA film and attaching it to the center of a 3 inch (7.6 cm) x 3 inch (7.6 cm) x 24 ga (0.607 mm) aluminum plate using double-sided adhesive tape. To embed the glass microspheres, the microsphere samples were first preheated to 120°C in an oven. The patch was then heated in the oven at 120°C for 60 seconds, then quickly removed and placed on an insulating mat. The preheated microsphere sample was immediately poured onto the patch to allow the glass microspheres to adhere to the surface. The excess glass microspheres were then poured off the patch, leaving a monolayer. The glass microspheres were further embedded into the EMAA film by placing the patch in a 130°C oven for approximately 2-2.5 minutes. The soak time was adjusted to achieve a target microsphere embedment level of 30-40%. Embedment levels below this range may result in loss of glass microspheres, while embedment levels above 50% will affect the retroreflective brightness results. The initial dry retroreflective brightness of the prepared patches was then measured according to the brightness procedure.

[0070] To perform the sandblasting test, 200.0 grams of AO Fastblast #46 (35 / 50) mesh Al2O3 blasting media (Washington Mills Inc., North Grafton, MA, product # 22650046) was weighed into the cup. The sample patch / aluminum plate was securely mounted in a holder perpendicular to and aligned with the sandblasting gun at a distance of 20.0 inches (51 cm) from the nozzle exit of the gun. The ceramic nozzle of the gun is 0.196 inches (0.498 cm) inside diameter by 2.3 inches (5.8 cm) long. An air flow of 1.50 standard cubic feet per minute (42.5 standard liters per minute) was metered to the gun using a Brooks Model 1307D08F1A1Z33 rotameter with a compressed air supply at 60.0 pounds per square inch (413.7 kilopascals) gauge pressure. The Al2O3 blasting media in a cup was then poured into the gun's feed inlet funnel all at once and the media was aspirated into the gun's air stream for approximately 10-15 seconds.

[0071] The damaged patch was removed and then measured again for dry patch retroreflective brightness according to the Brightness Procedure. The percentage of brightness retained was reported as provided in Table 5 by dividing the brightness after sandblasting by the brightness of the original dry patch.

[0072] Color Procedure: A Colorflex spectrophotometer (obtained from HunterLab, Reston, VA) configured with a C element 2-degree observer was used to measure the whiteness index and color of the glass microspheres in accordance with ASTM E313 "Standard Practice for Calculating Yellowness and Whiteness Indices from Instrumentally Measured Color Coordinates." The sample to be measured was prepared by filling a glass clear-bottomed sample cup with at least 0.25 inches (0.64 cm) of glass microspheres. A 420 nanometer UV filter (1.25 inch UV port insert D02-1010-618 obtained from HunterLab, Reston, VA) was placed between the spectrophotometer light source and the cup holding the sample to be measured. The color data is reported in Table 6.

[0073] Crystallinity Procedure: To determine whether a sample of glass microspheres was crystalline or amorphous, an X-ray diffraction (XRD) spectrum of the glass microsphere bed was obtained using an X-ray diffractometer (MiniFlex 600, obtained from Rigaku Americas Corporation, The Woodlands, TX). Microspheres that were mostly amorphous exhibited a broad diffuse spectrum with no distinct narrow peaks. Microspheres with both crystalline and amorphous phases exhibited a broad diffuse peak with the presence of an isolated distinct peak. Microspheres that were completely crystalline exhibited distinct peaks in the measured X-ray spectrum with no broad diffuse peaks present.

[0074] Particle size distribution procedure: Glass microsphere particle size distribution was determined using a Mastersizer 3000 particle size analyzer equipped with a Hydro MV module obtained from Malvern Panalytical, Worcestershire, United Kingdom. An aqueous dispersion of microspheres was produced by the instrument and then the light scattering pattern was measured to determine the particle size distribution of the sample. The reported data is the size at which a given volume fraction of particles is below a reference threshold. For example, for a sample with a d10 of 100 micrometers (microns, mm), 10% of the volume of the sample has a particle size less than 100 microns. The particle size distribution results are presented in Table 7.

[0075] Density Procedure: Glass microsphere density was determined using an AccuPyc II 1345 Gas Displacement Pycnometry System obtained from Micromeritics Instrument Corporation, Norcross, GA. Approximately 20 grams of sample was loaded into the sample cup and a standard density analysis of the instrument was performed. Particle size distribution and density are reported in Table 7.

[0076] Examples EX-1 to EX-13 and Comparative Examples CE-1 to CE-3 To prepare Examples EX-1-EX-13 and Comparative Examples CE-1-CE-3, homogenous slurries of mixed metal oxide powders were prepared by adding the materials in the amounts shown in Table 2 below. First, the Cel-Gum was added very slowly to water in a 1200 mL stainless steel mixing jar and completely dissolved with vigorous high shear Cowles blade mixing for at least 10 minutes. Next, the dispersant was added and mixed for at least 5 minutes. The SiO2 powder was then added very slowly over a period of 30 minutes to give the dispersant time to act and prevent clumping. The remaining powders were then added slowly and mixed for an additional 30 minutes. The mixture was then transferred to a 1 liter alumina reinforced grinding jar (obtained under the trade designation "ROALAX" from US Stoneware, East Palestine, Ohio) by filling the jar halfway with 1 cm cylindrical alumina media (obtained under the trade designation "BURUNDUM" from US Stoneware) and ball milled at 170 revolutions per minute for 24 hours to create a homogenous suspension (slurry).

[0077] Shaped precursor green particles were prepared from the slurry following the general teachings of US Pat. No. 8,701,441 (Kramlich et al.), which is incorporated herein by reference.

[0078] To form glass microspheres, the shaped precursor green particles were fed into a methane / oxygen torch flame (i.e., flame former). The flame former used to melt the particles and thereby produce the glass microspheres was a Bethlehem bench burner, obtained under the trade designation "Champion" from Bethlehem Apparatus Co., Hellertown, PA, producing an oxygen-enriched methane flame. The gas flow rates were 7.5 standard liters per minute (SLPM) of CH4, 15 SLPM of O2, and 1 SLPM of argon push gas. The particles were fed into the flame former via an FMC Syntron Magnetic Feeder (Model FTO-C) feeder, obtained from Syntron Material Handling Saltillo, MS, limiting the feed rate to approximately 2.5-3 grams per minute. The flame-formed microspheres were then sieved at approximately 212 microns to remove size defects. The cell-gum and dispersant volatilize during flame formation and are not present in the final microspheres. [Table 2] [Table 3] [Table 4] [Table 5]

[0079] The microspheres of EX-11 and EX-13 were heat treated at a rate of 10°C / min to 950°C and held at 950°C for 1 hour. The brightness and refractive index were evaluated as described above. The results are as follows: [Table 6] [Table 7] [Table 8]

[0080] The crush resistance of the exemplary beads was determined using an apparatus having two parallel plates made of a very hard, non-deformable material (e.g., sapphire or tungsten carbide) according to the test procedure described in U.S. Patent No. 4,772,511 (Wood). A single microsphere of known diameter was placed on the lower plate and The top plate was lowered until the microsphere broke. Crush resistance is the force exerted on the microsphere at break divided by the cross-sectional area of ​​the microsphere (71'r2). Ten microspheres of a given composition were tested and the average result was reported as the crush resistance of the composition. [Table 9]

Claims

1. 1. A retroreflective article comprising microspheres disposed on a surface of the article, the microspheres comprising: at least 40% by weight of alumina; at least 35 wt. % zirconia; and silica.

2. The retroreflective article of claim 1 , wherein the article is a pavement marking or pavement marking tape.

3. 3. The retroreflective article of claim 1, wherein the retroreflective article is a retroreflective element comprising a core particle comprising the microspheres at least partially embedded in the core.

4. 3. The retroreflective article of claim 1 or 2, wherein the microspheres comprise amorphous microspheres and / or comprise nanocrystalline glass-ceramic microspheres.

5. The retroreflective article of claim 1 or 2, wherein the microspheres are transparent.

6. The retroreflective article of claim 1 or 2, wherein the microspheres comprise at least 40, 45, or 50 weight percent zirconia.

7. 3. The retroreflective article of claim 1 or 2, wherein the microspheres comprise no more than 20, 15, 10, or 5 weight percent silica.

8. 3. The retroreflective article of claim 1 or 2, wherein the zirconia, alumina, and silica together comprise at least 80, 85, 90, 95, or 100% by weight of the microspheres.

9. 3. The retroreflective article of claim 1, wherein the microspheres further comprise one or more other oxides in an amount of 10% by weight or less.

10. 3. The retroreflective article of claim 1 or 2, wherein the microspheres have a refractive index of at least 1.72, 1.73, 1.74, 1.75, 1.76, 1.77, 1.78, 1.79, 1.80, 1.81, 1.82, 1.83, 1.84, or 1.

85.

11. The retroreflective article or microspheres are (A) At an observation angle of 0.2 degrees and an incidence angle of -4 degrees, the Cd / m ２ ) / Lux brightness, (a) having at least 65 or 70% retained brightness after sandblasting; (c) having a brightness of at least 4, 5, 6, or 7 (Cd / m 2 ) / lux after sandblasting; (D) a whiteness index of at least 50, 55, or 60; Satisfy one or more of the following: The retroreflective article according to claim 1 or 2.

12. The retroreflective article of claim 1 or 2, wherein the microspheres have a density of at least 3.4 or 3.

5.

13. at least 40% by weight of alumina; at least 35 wt. % zirconia; and a microsphere comprising silica.

14. Microspheres comprising at least 80% by weight of a combination of alumina, zirconia and silica and having a density of at least 3.

4.

15. Microspheres comprising at least 80% by weight of a combination of alumina, zirconia and silica and having a median crush strength of at least 700 MPa.

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