Glass microspheres and articles thereof

Abstract

Described herein are glass microspheres and retroreflective articles. In one embodiment, the glass microspheres comprise (a) lanthana, (b) zirconia, (c) alumina, and (d) silica, wherein (i) a weight of (a) is at least 15 wt% and at most 65 wt%; (ii) a weight of (b) is at least 15 wt% and at most 40 wt%; (iii) a weight of (c) is at least 15 wt% and at most 40 wt%; (iv) a weight of (d) is at least 1 wt% and at most 10 wt%; (v) a combined weight of (a), (b), and (c) is at least 90% by weight; and (vi) a combined weight of (a), (b), (c), and (d) is at least 95% by weight.

Description

GLASS MICROSPHERES AND ARTICLES THEREOFTECHNICAL FIELD

[0001] Glass microspheres comprising lantana, zirconia, alumina, and silica are disclosed. Such microspheres can be amorphous, transparent, and have high refractive index, making them especially useful in retroreflective pavement marking articles.SUMMARY

[0002] Glass microspheres are added to pavement markings as retroreflective lenses to enable improved visibility during both wet and dry conditions. There is a desire to identify high refractive index glass microspheres with a more white appearance that are processible at multiple sizes with high quality and having increasing retroreflective brightness with increasing bead size. In some embodiments, it is also desirable to identify glass microspheres which are more cost efficient to make, for example, by having fewer processing steps and / or reduced energy consumption.

[0003] In one aspect, a retroreflective article is described comprising glass microspheres disposed on a surface of the article, wherein the microspheres comprise (a) lanthana, (b) zirconia, (c) alumina, and (d) silica, wherein(i) a weight of (a) is at least 15 wt% and at most 65 wt%;(ii) a weight of (b) is at least 15 wt% and at most 40 wt%;(iii) a weight of (c) is at least 15 wt% and at most 40 wt%;(iv) a weight of (d) is at least 1 wt% and at most 10 wt%;(v) a combined weight of (a), (b), and (c) is at least 90% by weight; and(vi) a combined weight of (a), (b), (c), and (d) is at least 95% by weight.

[0004] In one embodiment, the retroreflective article is a pavement marking or pavement marking tape.

[0005] In another aspect, glass microspheres are described, wherein the glass microspheres comprise (a) lanthana, (b) zirconia, (c) alumina, and (d) silica, wherein(i) a weight of (a) is at least 15 wt% and at most 65 wt%;(ii) a weight of (b) is at least 15 wt% and at most 40 wt%;(iii) a weight of (c) is at least 15 wt% and at most 40 wt%;(iv) a weight of (d) is at least 1 wt% and at most 10 wt%;(v) a combined weight of (a), (b), and (c) is at least 90% by weight; and(vi) a combined weight of (a), (b), (c), and (d) is at least 95% by weight.

[0006] In yet another aspect, a retroreflective article comprising glass microspheres is described, wherein the wherein the glass microspheres comprise (a) lanthana, (b) zirconia, (c) alumina, and (d) silica, wherein(i) a weight of (a) is at least 15 wt% and at most 65 wt%;(ii) a weight of (b) is at least 15 wt% and at most 40 wt%;(iii) a weight of (c) is at least 15 wt% and at most 40 wt%;(iv) a weight of (d) is at least 1 wt% and at most 10 wt%;(v) a combined weight of (a), (b), and (c) is at least 90% by weight; and(vi) a combined weight of (a), (b), (c), and (d) is at least 95% by weight.

[0007] The above summary is not intended to describe each embodiment. The details of one or more embodiments of the invention are also set forth in the description below. Other features, objects, and advantages will be apparent from the description and from the claims.BRIEF DESCRIPTION OF THE DRAWINGS

[0008] FIG. 1 is a cross-sectional view of an illustrative retroreflective element;

[0009] FIG. 2 is a perspective view of an illustrative pavement marking; and

[0010] FIG. 3 is a cross-sectional view of an illustrative pavement marking tape.DETAILED DESCRIPTION

[0011] As used herein, the term “a”, “an”, and “the” are used interchangeably and mean one or more. The term “and / or” is used to indicate one or both stated cases may occur, for example A and / or B includes, (A and B) and (A or B).

[0012] Also herein, recitation of ranges by endpoints includes all numbers subsumed within that range (e.g., 1 to 10 includes 1.4, 1.9, 2.33, 5.75, 9.98, etc.).

[0013] The microspheres of the present disclosure generally comprise lanthana (i.e., I^Os), zirconia (i.e., Z1O2), aluminum oxide (i.e., AI2O3), and silica (i.e., SiO2). This base composition will be referred to herein as "LAZS". Microspheres that include the LAZS base composition will be referred to as "LAZS microspheres."

[0014] The LAZS microspheres of the present disclosure are spherical in nature, meaning that the microspheres have curved edges and / or shapes. In one embodiment, the LAZS microspheres are substantially spherical, which means that when magnified into a two-dimensional image, the LAZS microspheres appear at least substantially circular. A particle will be considered substantially spherical if its outline fits within the intervening space between two, concentric, truly circular outlines differing in diameter from one another by up to about 10% of the diameter of the larger of these outlines.

[0015] The term "solid" refers to microspheres that are not hollow, i.e., free of substantial (e.g., less than 1, 0.5, or even 0.1% by volume) cavities, pores, bubbles, or voids. For use as lens elements, the microspheres are preferably spherical and preferably solid. Solid microspheres are typically more durable than hollow microspheres. Solid microspheres can also focus light more effectively than hollow microspheres, leading to higher retroreflectivity.

[0016] The LAZS microspheres described herein are preferably transparent, meaning that other objects can be seen through the microspheres. Optical transparency can be measured by immersing the microspheres in an oil of approximately the same refractive index as the microspheres and, when viewed under an optical microscope (e.g., at lOOx) the microspheres have the property of transmitting rays of visible light so that bodies beneath the microspheres, such as bodies of the same nature as themicrospheres, can be clearly seen through the microspheres. Although the oil should have an index of refraction approximating that of the microspheres, it should not be so close that the microspheres seem to disappear (as they would in the case of a perfect index match). The outline, periphery, or edges of bodies beneath the microspheres are clearly discernible. In instances where a refractive index oil of approximately the same refractive index of the microspheres is not available or not used, transparency can be determined by the absence of crystalline defects in the microspheres when observed under an optical microscope.

[0017] The LAZS microspheres may be particularly useful as lens elements in retroreflective articles. Articles of the present disclosure share the common feature of comprising the LAZS microspheres described herein disposed on a surface of the article. Thus, at least a portion of the LAZS microspheres and / or reflective elements are exposed on the viewing surface of the article (e.g., pavement marking). The microspheres and / or reflective elements are preferably embedded in a binder or core particle at an average depth ranging from about 30% to about 60% of the microsphere’s diameter.

[0018] The pavement markings described herein comprise a binder. The binder affixes the microspheres or the reflective elements comprising microspheres to a pavement surface. Pavement surfaces are typically substantially solid and include a major portion of inorganic materials. Typically, pavement surfaces include asphalt, concrete, and the like. The binder typically comprises a paint, a thermoplastic material, thermoset material, or other curable material. Common binder materials include polyacrylates, methacrylates, polyolefins, polyurethanes, polyepoxide resins, phenolic resins, and polyesters. For reflective pavement marking paints, the binder may comprise reflective pigment.

[0019] In some embodiments, the retroreflective article is a retroreflective element comprising a core particle comprising the microspheres at least partially embedded in a core. With reference to FIG. 1, retroreflective element 200 comprises LAZS microspheres 117 alone or in combination with higher index microspheres (e.g., having a refractive index of at least 2.20 or greater) 116 partially embedded in the surface of core 202. The core is typically substantially larger than the microspheres. For example, the average core diameter may range from about 0.2 to about 10 millimeters. The core may comprise an inorganic material. Glass-ceramics are useful as a core material. The crystalline phase of the core acts to scatter light resulting in a semi-transparent or opaque appearance. Alternatively, the core may comprise an organic material such as a thermoplastic or bonded resin core, i.e., a crosslinked cured resin such as an epoxy, polyurethanes, alkyds, (methjacrylics, polyesters, phenolics, and the like. Various epoxies, polyurethane, and polyesters are generally described in U.S. Pat. Nos. 3,254,563 (De et al.); 3,418,896 (Rideout) and 3,272,827 (Pesson). The core may be a composite comprising an inorganic particle that is coated with an organic material or a blended composite of inorganic and organic materials as generally described in U.S. Pat. Nos. 2009 / 0291292 (Bescup et al.) and 10,858,496 (Wilding et al.). In both cases, the organic material serves as a binder to affix the microspheres to the outside surface of the core.

[0020] Although the retroreflective elements comprising the LAZS microspheres disclosed herein may be prepared from a non-diffusely reflecting bonded resin core in combination with specularly reflecting microspheres (e.g., vapor coating the microspheres with aluminum), this approach results in less durableretroreflective elements due to the use of metal, which may be susceptible to chemical degradation. Less durable retroreflective elements would also result by vapor coating a metal (e.g., aluminum) onto the core. In some embodiments, the retroreflective elements comprise at least one non-metallic light scattering material dispersed within core. Reflective elements may be made by known processes, such as described in U.S. Pat. Nos. 5,917,652 (Mathers et al.); 5,774,265 (Mathers et al.); and 2005 / 0158461 (Bescup et al.).

[0021] In some aspects, the microspheres and / or retroreflective elements are employed in liquid-applied marking (e.g., pavement) applications. With reference to FIG. 2, the LAZS microspheres 117 and / or reflective elements 200 are sequentially or concurrently dropped onto a liquified binder 10 or compounded within a liquified binder that is provided on pavement surface 20. In other aspects, LAZS microspheres and / or reflective elements are employed in retroreflective sheeting including exposed lens, encapsulated lens, embedded lens, or enclosed lens sheeting. Representative pavement-marking sheet material (tapes) are described in U.S. Pat. Nos. 4,248,932 (Tung et al.); 4,988,555 (Hedblom); 5,227,221 (Hedblom); 5,777,791 (Hedblom); and 6,365,262 (Hedblom).

[0022] Pavement marking tape and sheet material generally includes a backing, a layer of binder material, and a layer of microspheres partially embedded in the layer of binder material. The backing, which is typically of a thickness of less than about 3 millimeters, can be made from various materials, e.g., polymeric films, metal foils, and fiber-based sheets. Suitable polymeric materials include acrylonitrile-butadiene polymers, millable polyurethanes, and neoprene rubber. The backing can also include particulate fillers or skid resistant particles. The binder material can include various materials, e.g., vinyl polymers, polyurethanes, epoxides, and polyesters, optionally with colorants such as inorganic pigments, including specular pigments. The pavement marking sheeting can also include an adhesive, e.g., a pressure sensitive adhesive, a contact adhesive, or a hot melt adhesive, on the bottom of the backing sheet.

[0023] Patterned retroreflective (e.g., pavement) markings advantageously provide vertical surfaces, e.g., defined by protrusions, in which the microspheres are partially embedded. Because the light source usually strikes a pavement marker at high entrance angles, the vertical surfaces, containing embedded microspheres, provide for more effective retroreflection. Vertical surfaces also tend to keep the microspheres out of the water during rainy periods thereby improving retroreflective performance.

[0024] For example, FIG. 3 shows patterned pavement marker 100 containing a (e.g. resilient) polymeric base sheet 102 and a plurality of protrusions 104. For illustrative purposes, only one protrusion 104 has been covered with microspheres and antiskid particles. Polymeric base sheet 102 has front surface 103 from which the protrusions extend, and back surface 105. Polymeric base sheet 102 is typically about 1 millimeter (0.04 inch) thick but may be of other dimensions if desired. Optionally, patterned pavement maker 100 may further comprise scrim 113 and / or adhesive layer 114 on back surface 105. Protrusion 104 has top surface 106, side surfaces 108, and in an illustrative embodiment is about 2 millimeters (0.08 inch) high. Protmsions with other dimensions may be used if desired. As shown, side surfaces 108 meet top surface 106 at rounded top portions 110. Side surfaces 108 preferably form an angle q of about 70° atthe intersection of front surface 103 with lower portion 112 of side surfaces 108. Protrusion 104 is coated with pigment-containing binder layer 115. Embedded in binder layer 115 are a plurality of LAZ S microspheres 117 and a plurality of a second microspheres 116 (e.g., having a higher refractive index than the LAZS microspheres). Optionally, antiskid particles 118 may be embedded on binder layer 115. The microspheres are preferably placed selectively on the side and top surfaces of the protrusions while leaving the valleys between protrusions substantially clear so as to minimize the amount of microspheres, thereby minimizing the manufacturing cost. The microspheres may be placed on any of the side surfaces as well as the top surface of the protrusions to achieve efficient retroreflection.

[0025] Pavement marking sheeting can be made by a variety of known processes. A representative example of such a process includes coating onto a backing sheet a mixture of resin, pigment, and solvent, dropping microspheres 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. U.S. Pat. No. 4,988,541 (Hedblom) discloses a preferred method of making patterned pavement markings. Optionally, a scrim (e.g., woven or nonwoven) and / or an adhesive layer can be attached to the back side of the polymeric base sheet, if desired.

[0026] In some embodied retroreflective articles, two types of microspheres are employed wherein one type are the LAZS microspheres described herein and the second type are "higher index microspheres," having for example a refractive index of at least 2.2, or 2.3 and typically no greater than 2.46 as generally described in U.S. Pat. No. 7,513,941 (Frey et al.). In some aspects, one of the two types of microspheres will be larger. For instance, the LAZS microspheres, having a median diameter ranging in size from 80 to 500 micrometers in diameter may be disposed in combination with larger or smaller microspheres.

[0027] Although FIGS. 1 and 3 illustrate second microsphere 116 being larger than LAZS microsphere 117, in some embodiments, second microsphere 116 may be, on average, smaller than LAZS microsphere 117 or even about the same size.

[0028] In some embodiments, the article comprises a plurality of microspheres, wherein 100% of the microspheres are LAZS microspheres of the present disclosure. In other embodiments, the article comprises a plurality of microspheres, wherein only a portion of the microspheres are the LAZS microspheres of the present disclosure, for example, at least 20, 30, 40, 50, 60, 70, 80, or even 90% of the microspheres in the article are the LAZS microspheres of the present disclosure.

[0029] The binder layer of FIGS. 2 and 3 as well as the core of the retroreflective element depicted in FIG. 1 comprise a light transmissive material so that light entering the retroreflective article is not absorbed but is instead retroreflected by way of scattering or reflection off of pigment particles in the light-transmissive material. Vinyls, acrylics, epoxies, and urethanes are examples of suitable mediums. Urethanes, such as are disclosed in U.S. Pat. No. 4,988,555 (Hedblom) are preferred binder mediums at least for pavement markings. The binder layer typically covers selected portions of the protrusions so that the base sheet remains substantially free of the binder. For ease of coating, the medium will preferably be a liquid with a viscosity of less than 10,000 centipoise at coating temperatures.

[0030] The binder layer of FIGS. 2 and 3 as well as the core of FIG. 1 typically comprise at least one pigment such as a diffuse reflecting or specular reflecting pigment.

[0031] Specular reflecting pigment particles are generally thin and plate-like and are part of the binder layer, the organic core (a core comprising essentially only an organic binder material) of an element, or an organic binder coating on an inorganic particle that together make up a composite core of an element. Light striking the pigment particles is reflected at an angle equal, but opposite to the angle at which it was incident. Suitable examples of specular pigments include pearlescent pigments, mica, and nacreous pigments. Typically, the amount of specular pigment present in the binder layer is at least 15 percent by weight ranging up to 40 or 50 percent by weight. Pearlescent pigment particles are often preferred because of the trueness in color.

[0032] The components of the LAZS microspheres of the present disclosure are described herein in their metal oxide form even though a different metal containing chemical compound such as a hydroxide or carbonate form may be used as a starting material. The metal oxide is the form in which the component batching is controlled in the completely processed glass microspheres as well as retroreflective articles and the form that correctly accounts for the chemical elements and the proportions thereof in the LAZS microspheres. The starting materials used to make the microspheres may include some chemical compounds other than metal oxide, such as organic processing aids that are volatilized during the melting and spheroidizing process.

[0033] Generally, for good dry condition performance in pavement marking, the microspheres have a refractive index greater than 1.80. In some embodiments, the microspheres of the present disclosure have a refractive index of at least 1.81, 1.82, 1.83, 1.84, 1.85, 1.86, 1.87, or even 1.88. In some embodiments, the refractive index of the microspheres of the present disclosure is no greater than 1.95, 1.94, 1.93, 1.92, 1.91, 1.90, 1.89, 1.88, or even 1.87. The index of refraction can be determined by the test method described in the examples. To achieve microspheres with a high refractive index, high amounts of lanthana and zirconia are used.

[0034] The microspheres described herein typically comprise at least 15 wt% lanthana (La2O3). In typical embodiments, the microspheres comprise no greater than 65, 64, 63, 62, 61, 60, 59, 58, 55, 50, 45, 40, or even 35 wt% of lanthana. In some embodiments, the microspheres comprise at least 15, 16, 17, 18, 20, 22, 25, 28, 30, or even 32 wt% lanthana. While not bound by theory it is believed that at this concentration, lanthana plays an important role as a network modifier in the glass, lowering the melt viscosity so that the volatile species in the green precursor particles can quickly diffuse out of the molten glass without excess bubble formation. Lanthana also is critical to contributing to the high refractive index needed for optimized retroreflectivity of the resulting microspheres and additionally it limits the development of crystallization compared to titania which is commonly used in high index glasses.

[0035] The microspheres described herein typically comprise at least 15 wt% zirconia (ZrO2). In some embodiments, the microspheres comprise at least 15, 16, 17, 18, 20, 22, 25, 28, or even 30 wt% zirconia. In some embodiments, the microspheres comprise no greater than 40, 39, 38, 35, 34, or even 32 wt% zirconia. Zirconia is present at these concentrations to increase the chemical and mechanical durability ofthe glasses, while also contributing to the high refractive index of the beads. Higher amounts of zirconia can lead to higher levels of optically detrimental crystallization defects.

[0036] Although lanthana and zirconia can lead to high refractive index microspheres, if the refractive index is too high, then the microsphere would not be acceptable for pavement marking applications. Thus, the microspheres described herein typically comprise at least 15 wt% alumina (AI2O3) to tailor the refractive index to a desired range. In typical embodiments, the microspheres comprise no greater than 40, 39, 38, 37, 36, 35, or even 34 wt% of alumina. In some embodiments, the microspheres comprise at least 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, or even 33 wt% alumina. While not bound by theory it is believed alumina takes the role of the network former in the glass and is needed to lower the melting temperature of the glass to enable one pass flame processing. Concentration of alumina is critical to balance the greater than 1.95 refractive of both zirconia and lanthana. Too little alumina leads to higher refractive index than desired while too much alumina reduces the refractive index to less useful levels.

[0037] In some embodiments, the total amount of lanthana, zirconia, and alumina in the microspheres is at least 90, 91, 92, 93, 94, 95, or even 96 wt%. In some embodiments, the total amount of lanthana, zirconia, and alumina is at most 97, 98, or even 99 wt% of the microspheres.

[0038] Silica is incorporated into the LAZS microsphere composition to provide a low melt temperature seed material to accelerate the onset of eutectic melt phase formation in difficult to homogenize glass chemistries, such as those including lanthana, alumina, and zirconia. Care is taken to not include excess silica as it has a negative contribution to refractive index compared to lanthana, alumina, and zirconia. Additionally, the amount of silica present must be limited to reduce the prevalence of entrained bubble defects trapped in the microspheres due to silicas high melt viscosity. In typical embodiments, the microspheres comprise at least 1, 2, 3, 4, 5, or even 6 wt% of silica. In some embodiments, the microspheres comprise at most 7, 8, 9, or even 10 wt% silica.

[0039] By forming microspheres made with at least 95 wt% of lanthana, zirconia, alumina and silica, microspheres can be formed that are transparent, have high refractive index within the desired operating range, and are crystalline defect free. In some embodiments, the microspheres comprise at least 95, 96, 97, 98, or even 99 wt% of lanthana, zirconia, alumina, and silica. In some embodiments, the microspheres consist of only lanthana, zirconia, alumina, and silica.

[0040] The microspheres described herein are substantially free of baria (e.g., BaO), and titania (e.g., TiCh). As used herein, substantially free means that each of these components (i.e., baria, and titania) are present in amounts of less than 5, 4, 3, 2, 1, 0.5, 0.1, or even 0.05 wt% in the microsphere. In some embodiments, baria, and titania are not detectable. Titania is a common component of high refractive index glasses, but suffers from a change of oxidation state in high temperature flame processing. In these embodiments high temperature flame processing is defined as temperatures above the adiabatic airnatural gas flame combustion temperature of 2055 degrees Celsius. Well not being bound theory, it is thought a portion of the titania present reduces in high temperature flame processing and quenches into the Ti3+valence state that then presents as a more optically absorbing, less white glass. Specifically, asmicrosphere diameter increases the absorption present in high temperature flame processed titanate glasses also increases in accordance to Beer-Lambert law leading to a decrease or plateau in usefulness as a retroreflective lens.

[0041] Baria is also a common component in high index glasses but suffers from processing concerns of the raw material most commonly used, barium carbonate, along with emissions concerns from the sulfur dioxide and sulfur trioxide generated from the significant amount of barium sulfate impurities found in many barium compounds useful in glass making.

[0042] The microspheres described herein may comprise other metal oxides than those described above. The total amount of other metal oxides is no greater than 5, 4, 3, 2, or 1 wt% of the microspheres. Such other metal oxides are selected as to not detract from the (e.g., brightness) properties of the LAZS microspheres. Other metal oxides may be selected for their benefits as glass network modifiers with the purpose of lowering the melting point and lowering the melt viscosity of the material, leading to easier processing. The addition must be limited to avoid excess bubble entrainment in the microspheres due to metal oxide volatilization in high temperature flame processing. Suitable other metal oxides include for example LiO2, Na2O, K2O, ZnO, and B2O3.

[0043] In some embodiments, the microspheres comprise alkaline earth oxides, such as SrO, MgO, and CaO. The amount of alkaline earth oxide(s) (e.g., SrO, MgO, CaO, or the sum thereof) is typically less than 5, 4, 3, 2, or even 1 wt% of the microspheres. In some embodiments, the amount of alkaline earth oxide(s) is at least 0.5, 1, 2, 3, or even 4 wt% and at most 4.5 or even 5 wt% of the microspheres.

[0044] In some embodiments, the microspheres comprise additional rare earth oxides, such as CeO2, Gd2O3, and Y2O3. The amount of rare earth oxide(s) (e.g., CeO2, Gd2O3, and Y2O3, or the sum thereof) is typically less than 5, 4, 3, 2, or even 1 wt% of the microspheres. In some embodiments, the amount of rare earth oxide(s) is at least 0.5, 1, 2, 3, or even 4 wt% and at most 4.5 or even 5 wt% of the microspheres.

[0045] Small amounts of metals (or metal oxides) may be present due to impurities found in the raw materials. In some embodiments, additional metals (or metal oxides) may be added to adjust the physical properties. For example, transition metal oxide colorants such as Fe2O3, CoO, NiO, CuO, Cr2O3, and MirO; can be added in small amounts (<5 wt%) to impart retroreflective colors in the as formed microspheres. Rare earth elements such as europium may also be added for color or fluorescence. Preferably, the microspheres are substantially free of lead oxide (PbO) and cadmium oxide (CdO).

[0046] In some embodiments, the microspheres comprise little or no other metal oxides. In this embodiment, the amount of other metal oxides is no greater than 5, 4, 3, 2, 1 or 0.5 wt% of the LAZS microspheres.

[0047] In some embodiments, the LAZS microspheres typically have a density of at least 4.0 g / cc (grams per cubic centimeter). The density of the microspheres is typically no greater than 5.1 g / cc. In some embodiments, the density is at least 4.0, 4.1, 4.2, 4.3, 4.4, 4.5, 4.6, 4.7, or even 4.8 g / cc. The density of the microspheres may be measured using the method as described in the examples.

[0048] Advantageously, the microspheres of the present disclosure can be made large in size via high temperature flame processing. Microspheres can be made and used in various sizes where retroreflective brightness of a monolayer of microspheres in a reflective binder increases with microsphere diameter. It is uncommon to deliberately form microspheres smaller than 10 micrometers in diameter, though a fraction of microspheres down to 2 micrometers or 3 micrometers in diameter is sometimes formed as a by-product of manufacturing larger microspheres. The particle size of the microspheres can be determined based on techniques known in the art, for example, microscopy, electrical impedance, or light scattering techniques. A particle size distribution can be obtained by scatter techniques, reporting dlO, d50 and d90 values. In the present disclosure, the plurality of LAZS microspheres has a d50 when measured using a light scattering technique of at least 80, 90, 100, 110, 120, 130, 140, 150, 160, 170, 180, 190, or even 200 micrometers. Typically, the microspheres have a d50 no greater than 1 or even 2 millimeters. More commonly the microspheres have a median diameter (d50) no greater than 750, 500, 400, or 300 micrometers. The d50 measurement, or median, is where 50 percent by volume of microspheres in the distribution are smaller than the indicated size diameter. In some embodiments, the dlO (where 10 percent by volume of microspheres fall below this diameter value) of the particle size distribution as measured as described in the examples, is typically at least 65, 70, 80, 90, 100, 110, 120, 130 or even 140 micrometers. In some embodiments, the d90 (where 90 percent by volume of microspheres fall below this diameter value) of the particle size distribution is no greater than 95, 100, 125, 150, 175, 200, 225, 250, 275, or even 300 micrometers. For the purposes of the present disclosure, the median size by volume is determined by laser light diffraction by dispersing the microspheres in deaerated, deionized water.

[0049] The LAZS microspheres described herein can be prepared by any suitable method. In some embodiments, molded precursor green particles were prepared from slurries by following the general teachings of U.S. Pat. No. 8,701,441 (Kramlich et. al), incorporated herein by reference. To form glass microspheres, molded precursor green particles were fed into a methane / oxygen torch flame (i.e., flame former) thereby generating glass microspheres.

[0050] The glass precursor composition used to form the spheres includes glass precursor particles and optionally at least one liquid of water, volatile organic liquid, and fugitive binder, i.e., a binder that dissipates during the elevated temperature processing used in forming 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 includes glass precursor particles and water and is in the form of a slurry.

[0051] The liquid (e.g., water) is typically present in the glass precursor composition in an amount of at least 5, 10, 15, 20, 25, or even 30% by weight of the slurry composition. In some embodiments, the liquid is present in an amount no greater than 50, 40 or 35% by weight of the slurry composition.

[0052] Aqueous-based glass precursor compositions can include other additives including, e.g., hydrocolloids (e.g., xanthan, maltodextrin, galactomannan and tragacanth) polysaccharides, natural gums (e.g., gum Arabic), starch derivates, surfactants (e.g., cationic, anionic, nonionic, and zwitterionic)including, e.g., sodium lauryl sulfate polysorbate, and sodium 2-ethylhexyl sulfate, and combinations thereof.

[0053] Examples of useful volatile organic liquids include methanol, ethanol, isopropyl alcohol, butyl alcohol, heptane, and toluene.

[0054] Useful fugitive binders include water soluble and water dispersible binders including, e.g., dextrin, starch, cellulose, hydroxyethylcellulose, hydroxypropylcellulose, carboxyethylcellulose, carboxymethylcellulose, carragenan, scleroglycan, xanthan gum, guar gum, hydroxypropylguar gum and combinations thereof. Other suitable binders are described in U.S. Pat. No. 8,701,441.

[0055] The molded microparticles are then passed through a flame or other source of sufficient thermal energy (e.g., a gas-fired furnace or an electrical furnace) to form molten glass droplets. Any suitable sphere forming process and apparatus can be used including, e.g., glass sphere manufacturing processes and apparatuses such that the combination of time and temperature is sufficient to form a homogenous glass melt.

[0056] In one useful method, the molded microparticles are in the form of a free-flowing powder and the passing involves allowing the free-flowing powder to be dispersed in a flame. The flame has a temperature sufficient to transform, e.g., fuse, the glass precursors present in the molded microparticle into a homogenous state. The flame temperature is selected to be suitable for melting and fusing the molded microparticles into glass droplets. Useful flame temperatures are at least about 2000 K (Kelvin), 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 sources including, e.g., natural gas, hydrogen, oxygen, acetylene, air, and mixtures thereof.

[0057] The duration of the molded microparticles in the flame is referred to as “residence time.” The residence time is selected to achieve spheres having a desired property(s). Variables that impact the residence time include, e.g., flame velocity, flame size, flame shape, flame temperature, molded microparticle volume, the composition of the molded microparticle, the density of the molded microparticle, and the density of the sphere. The molten droplets can be maintained in the flame for a sufficient period of time to transform the molten droplets into spheres through any suitable mechanism including, e.g., directing gas currents under the molten droplets, allowing the molten droplets to fall freely through the heating zone, and combinations thereof.

[0058] The fused glass droplets form spheroids, which are then quenched to form spheres. Various quenching methods are suitable including, e.g., air cooling (e.g., by free falling through a space a sufficient distance), rapid cooling and combinations thereof. A useful rapid cooling method includes allowing the spheroids to continue their free fall through a cooling zone or into a cooling medium, e.g., water, oil or a combination thereof. Alternately or in addition, a gas (e.g., air or argon) can be sprayed into the free-falling stream of fused spheroids causing the spheroids to accelerate and cool forming solid, transparent glass microspheres.

[0059] In typical embodiments, the LAZS microspheres of the present disclosure may be characterized as glass microspheres or in other words "amorphous microspheres". Glass materials may be differentiatedfrom crystalline materials by X-ray diffraction, where microspheres that are majority amorphous show broad diffuse spectra with no defined narrow peaks and are referred to as glass. Microspheres that have both crystalline and amorphous phases will have broad diffuse peaks with isolated defined peaks also present and are referred to as glass-ceramics. Microspheres that are fully crystalline (or ceramic) have distinct peaks in the measured x-ray spectra with no presence of broad diffuse peaks. An approximate guideline in the field is that a glass material comprises less than about 1 volume% crystals in typical powder X-ray diffraction measurements.

[0060] The combination of lanthana, alumina, zirconia, and silica in the amounts disclosed herein provide amorphous particles with little to no crystalline (e.g., nanocrystalline) phases. Compositions that contain too much alumina and zirconia can lead to crystallization of the material, leading to loss of transparency and processing difficulties.

[0061] Unexpectedly, the particular microsphere compositions as disclosed herein result in microspheres that are high index, optically transparent, and show little (<1 vol.%) to no propensity to form crystalline phases without deliberate post heat treatment processing. In the present disclosure, it has been found that the LAZS microspheres comprising a particular composition as recited herein can be formed in a single pass through the high temperature flame and are exceptionally free of crystalline defects, without the need to perform additional heat treatment to remove color. Subsequent heat treatments are disclosed in U.S. Patent. No. 6,2457,00 (Budd et al.). The compositions of the present disclosure can result in large, transparent, and white amorphous microspheres. As used herein, “amorphous” refers to material that lack long range crystal structure as determined by X-ray diffraction as described in the example section.

[0062] The LAZS microspheres of the present disclosure have excellent quality as observed by transmitted light optical microscopy. For example, when viewed by optical microscopy as described in the example section, the LAZS microspheres as disclosed herein are transparent wherein less than 1% of the microspheres have apparent crystallinity large enough to scatter light. In some embodiments, the plurality of LAZS microspheres of the present disclosure have low amounts of entrained bubble defects such as less than 40, 30, 20, 10, or even 5 %.

[0063] Notably, the microspheres of the present disclosure are of high refractive index and enable articles with sufficiently high retroreflective brightness even in the absence of titania in the glass composition due to the high amounts of lanthana and zirconia present in the composition. The retroreflective brightness (or retroreflectance) of the microspheres and / or retroreflective elements for an entrance angle of -4° and a 0.2° observation angle (as determined according to the test method of the examples). In some embodiments, the retroreflective article or microspheres have a brightness as measured per the method of the examples of at least 8, 9, 10, 12, 14, 16, 18, or even 19 (Cd / m2) / lux. In some embodiments, the retroreflective brightness of the microspheres and / or retroreflective elements is no greater than 25, 20, 19, 18, 17, 16, 15, 14, 13, 12, 11, or even 10 (Cd / m2) / lux.

[0064] The LAZS microspheres typically have suitable whiteness for use in retroreflective articles as determined by the test method described in the examples. Ideally, microspheres having a higher whiteness value are desirable as it results in whiter microspheres which provide improved differentiationfrom a black or grey road surface when used as a pavement marking. Typically, the whiteness index (“WI”) should be at least 20. In some embodiments, the whiteness index of the LAZS microspheres is at least 20, 22, 25, 30, 35, or even 40. In some embodiments, the whiteness index of the LAZS microspheres is no greater than 65, 64, 63, 62, 61, 60, 59, 58, 57, 56, or 55.EXAMPLES

[0065] Unless otherwise noted, all parts, percentages, ratios, etc. in the examples and the rest of the specification are by weight, and all reagents used in the examples were obtained, or are available, from general chemical suppliers such as, for example, Sigma- Aldrich Company, Saint Louis, Missouri, or may be synthesized by conventional methods.

[0066] The following abbreviations are used in the Example Section: cd = candela, cm = centimeter, g = gram, m = meter, min = minute, mL = milliliter, mm = millimeter, pm = micrometer, nm = nanometer, °C = degrees Celsius, and wt% = weight percent.Table 1. Materials List

[0067] Test Methods

[0068] Index of Refraction: Index of refraction 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).

[0069] Brightness: Dry white patch brightness values were determined using a retroreflectometer (Delta GRX, obtained from Force Technology, Brondby, Denmark). Patch brightness refers to the coefficient of retroreflection (RA) determined using a retroluminometer. The device directs white light onto a planarmonolayer of microspheres disposed on a diffuse white scattering material at a fixed entrance angle to the normal to the monolayer. The diffuse white scattering material comprises a transparent acrylic copolymer pressure sensitive adhesive having a refractive index in the range of 1.46 to 1.53 and about 17.2 wt % of TiOi pigment. The coefficient of retroreflection is measured by a photodetector at a fixed divergence angle to the entrance angle (observation angle) in units of (cd / m2) / lux. Data reported herein were measured at -4° entrance angle and 0.2° observation angle. Retroreflective brightness measurements were made for the purpose of comparison of brightness between microspheres of different composition. The 0.2-degree observation angle data generated for this measurement is conforming with ASTM E1709-16 (reapproved 2022).

[0070] Whiteness Index: Whiteness index of the microspheres was measured using a spectrophotometer (ColorFlex spectrophotometer obtained from HunterLab, Reston, VA) configured with a C element 2- degree observer and D65 / 2 illuminator conforming with ASTM E313-20 “Standard Practice for Calculating Yellowness and Whiteness Indices from Instrumentally Measured Color Coordinates”. The sample to be measured was prepared by filling a sample cup with a glass transparent bottom with at least 0.25 inches (6.35 millimeters) of microspheres. A 420-nanometer UV filter (1.25-inch (31.75-millimeter) 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.

[0071] Density: Glass microsphere density was determined by using gas displacement pycnometry system (AccuPyc II 1345 obtained from Micromeritics Instrument Corporation Norcross, GA). Approximately 20 grams of microspheres was loaded into the sample cup and the instrument’s standard density analysis was performed.

[0072] Glass Quality: To determine the glass forming quality of a sample of microspheres, optical microscopy was performed using a digital microscope (VHX-5000 obtained from Keyence Corporation, Minnetonka, MN). A monolayer of microspheres was deposited onto a piece of clear tape leaving some adhesive uncovered which was then used to fix the microsphere-coated tape to a glass slide. The slide was then analyzed on the microscope using the transparent light analysis mode. The microspheres were visually examined via the microscope and were categorized into three different groups: 1-Transparent Defect Free are microspheres with no visible included bubbles or presence of crystalline defects, 2- Transparent Bubbles are microspheres with visually apparent internal bubble defects and 3-Crystalline Defects are microspheres that are either fully opaque to transparent light with signs of a rough irregular crystallized surfaces or partially transparent microspheres with signs of opaque internal crystallization. For a given sample, the number of microspheres in each category was determined and the percentage is reported in Table 4.

[0073] X-ray Diffraction: To determine whether a sample of microspheres is crystalline, mixed phase, or amorphous x-ray diffraction (XRD) spectra of the microspheres were obtained using an x-ray diffractometer (MiniFlex 600, obtained from Rigaku Americas Corporation, The Woodlands, TX). Microspheres that are completely amorphous show broad diffuse spectra with no defined narrow peaks. Microspheres that have both crystalline and amorphous phases will have broad diffuse peaks withisolated defined peaks also present. Microspheres that are fully crystalline have distinct peaks in the measured x-ray spectra with no presence of broad diffuse peaks.

[0074] Particle Size: Glass microsphere particle size distribution was determined using Mastersizer 3000 particle size analyzer with Hydro MV module obtained from Malvern Panalytical, Worcestershire, United Kingdom. An aqueous dispersion of microspheres is generated by the device and then light scattering patterns are measured to determine the particle size distribution of the sample. Reported are dlO, d50 and d90 values.

[0075] Examples 1-30 (Ex. 1-30) and Comparative Examples 1-5 (CE1-CE5)

[0076] To prepare examples Ex. 1 through Ex. 30 and CE1 through CE5, homogeneous slurries of inorganic powders were prepared by adding materials in the amounts indicated in Table 2 as follows. First, 10.6 g of Cell-gum was added to 326.9 g of deionized water in a 1200 mL stainless steel mixing jar very slowly and fully dissolving with aggressive high shear Cowles blade mixing for at least 10 min. Next, 12.0 g Dispersant was added and mixed for at least 5 min. Then, the inorganic powders in the amounts designated in Table 2 were added slowly to the mixing jar and mixed for 30 minutes. The mixture was then transferred to a 1 -liter high alumina grinding jar (obtained from U.S. Stoneware East Palestine, OH, under the trade designation “ROALAX”) with 1-cm cylindrical alumina media (obtained from U.S. Stoneware under the trade designation “BURUNDUM”) half filling the jar to be ball milled for at least 24 hours at 170 revolutions per minute to make a homogeneous aqueous suspension (slurry).

[0077] Molded precursor green particles were made from the slurry by following the general teachings of U.S. Pat. No. 8,701,441. Mold cavities were chosen to achieve desired microsphere d50 sizes.

[0078] To form glass microspheres, molded precursor green particles were fed into a methane / oxygen torch flame (i.e., flame former). The flame former used to melt the particles, thereby generating glass microspheres, was a Bethlehem bench burner (obtained from Bethlehem Apparatus Co., Hellertown, PA, under the trade designation “CHAMPION”), which produces an oxygen enriched methane flame. The burner is oriented vertically with the burner face on the bottom and feed inlet on the top. The burner has a center channel for feeding particles directly through the burner face. The gas flow rates were CH4 at 7.5 standard liters per minute (SLPM), O2 at 15 SLPM, and 1 SLPM of argon push gas to prevent backfire and to move the particles through the center channel of the flame former.

[0079] The particles were fed into the center channel of 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 to 3 g / min. The flame formed microspheres were then screened through a 212- micron sieve to remove any microspheres with size defects. The cell-gum and dispersant are volatilized during flame forming and are not present in the final microspheres.

[0080] Shown in Table 2 is the theoretical weight % of the metal (reported as metal oxide form) in the various microsphere samples. Loss on ignition of carbonates and hydroxides bound to the desired oxides is accounted for in Table 2.Table 2

[0081] The microspheres from each of the samples were tested for their physical properties following the test methods described above. The results from Index of Refraction, Brightness, Whiteness Index, and density are shown in Table 3. Examples CE1 and CE2 were found to be crystalline, while the rest of the samples (CE3-CE5 and Ex. 1-Ex. 30) were shown to be amorphous when tested by XRD. The samples also were tested for Glass Quality, and particle size and the results are shown in Table 4.Table 3nm = not measured

[0082] In Table 3, set CE3-CE5, set Ex. 4-6, set Ex. 9-11, set Ex. 13-15, set Ex. 17-19, and set Ex. 24-26 have the same composition within each set, but have differing median particle size. As shown in Table 3, the brightness is higher for the compositions of sets according to the present disclosure versus the set of comparative examples (CE3-CE5). For set CE3-CE5, the brightness is similar across the various particle sizes, whereas for the sets according to the present disclosure, Ex. 4-6, Ex. 9-11, Ex. 13-15, Ex. 17-19, and Ex. 24-26, there is a trend of increasing brightness as the median particle size increases.Table 4

[0083] Foreseeable modifications and alterations of this invention will be apparent to those skilled in the art without departing from the scope and spirit of this invention. This invention should not be restricted to the embodiments that are set forth in this application for illustrative purposes. To the extent that there is any conflict or discrepancy between this specification as written and the disclosure in any document mentioned or incorporated by reference herein, this specification as written will prevail.

Claims

What is claimed is:

1. A retroreflective article comprising glass microspheres disposed on a surface of the article wherein the glass microspheres comprise (a) lanthana, (b) zirconia, (c) alumina, and (d) silica, wherein(i) a weight of (a) is at least 15 wt% and at most 65 wt%;(ii) a weight of (b) is at least 15 wt% and at most 40 wt%;(iii) a weight of (c) is at least 15 wt% and at most 40 wt%;(iv) a weight of (d) is at least 1 wt% and at most 10 wt%;(v) a combined weight of (a), (b), and (c) is at least 90% by weight; and(vi) a combined weight of (a), (b), (c), and (d) is at least 95% by weight.

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

3. The retroreflective article of any one of the previous claims, wherein the retroreflective article is a retroreflective element comprising a core particle comprising the glass microspheres at least partially embedded in a core.

4. The retroreflective article of any one of the previous claims, wherein the glass microspheres comprise less than 1% crystalline defects.

5. The retroreflective article of any one of the claims, wherein the glass microspheres have a refractive index of at least 1.80 and less than 1.95.

6. The retroreflective article of any one of the claims, wherein the glass microspheres have a refractive index of at least 1.85.

7. The retroreflective article of any one of the claims, wherein the glass microspheres have a median diameter (d50) of at least 80 micrometers.

8. The retroreflective article of any one of the claims, wherein the glass microspheres are transparent.

9. The retroreflective article of any one of the previous claims, wherein the glass microspheres have a whiteness index of at least 20.

10. The retroreflective article of any one of the previous claims, wherein the glass microspheres have a density of at least 4.0 g / cc and less than 5.1 g / cc.

11. The retroreflective article of any one of the previous claims, wherein the glass microspheres have a brightness of at least 8 (cd / m2) / lux.

12. The retroreflective article of any one of the previous claims, wherein the glass microspheres comprise at least 30 wt% lanthana.

13. The retroreflective article of any one of the previous claims, wherein the glass microspheres comprise at least 27 wt% zirconia.

14. The retroreflective article of any one of the previous claims, wherein the glass microspheres comprise at least 25 wt% alumina.

15. The retroreflective article of any one of the previous claims, wherein the glass microspheres further comprise one or more other oxides in an amount no greater than 3 wt%.

16. Glass microspheres comprising: (a) lanthana, (b) zirconia, (c) alumina, and (d) silica, wherein(i) a weight of (a) is at least 15 wt% and at most 65 wt%;(ii) a weight of (b) is at least 15 wt% and at most 40 wt%;(iii) a weight of (c) is at least 15 wt% and at most 40 wt%;(iv) a weight of (d) is at least 1 wt% and at most 10 wt%;(v) a combined weight of (a), (b), and (c) is at least 90% by weight; and(vi) a combined weight of (a), (b), (c), and (d) is at least 95% by weight.

17. The glass microspheres of claim 16, wherein the glass microspheres have a refractive index of at least 1.80 and less than 1.95.

18. The glass microspheres of any one of claims 16-17, wherein the glass microspheres have a median diameter (d50) of greater than 80 micrometers.

19. The glass microspheres of any one of claims 16-18, wherein the glass microspheres have a whiteness index of at least 20.

20. The glass microspheres of any one of claims 16-19, wherein the glass microspheres have a density of at least 4.0 and less than 5.1.

21. The glass microspheres of any one of claims 16-20, wherein the glass microspheres have a brightness of at least 8 (cd / m2) / lux.

22. The glass microspheres of any one of claims 16-21, wherein the glass microspheres comprise at least 30 wt% lanthana.

23. The glass microspheres of any one of claims 16-22, wherein the glass microspheres comprise at least 27 wt% zirconia.

24. The glass microspheres of any one of claims 16-23, wherein the glass microspheres comprise at least 25 wt% alumina.

25. The glass microspheres of any one of claims 16-24, wherein the glass microspheres further comprise one or more other oxides in an amount no greater than 3 wt%.

26. A retroreflective article comprising glass microspheres according to any one of claims 16-25 disposed on a surface of the retroreflective article.

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