High-density microspheres and retroreflective articles thereof

Abstract

Described herein are high-density microspheres having a density of at least 5.05 g / cc. These high-density microspheres can be useful in retroreflective pavement marking applications. Disclosed is a retroreflective element including (i) a composite core, wherein the composite core comprises a plurality of first microspheres dispersed in an organic polymer and wherein the composite core has a first density less than 1.7 g / cc; and (ii) a plurality of second microspheres, wherein the second microspheres have a density of at least 5.05 g / cc, wherein the second microspheres are disposed on the perimeter of the composite core to form a retroreflective element having a total density greater than 2.15 g / cc. Also disclosed in a retroreflective article, wherein the plurality of high-density microspheres are partially embedded in a binder.

Description

HIGH-DENSITY MICROSPHERES AND RETROREFLECTIVE ARTICLES THEREOF TECHNICAL FIELD

[0001] Disclosed herein are high-density microspheres having a density of at least 5.05 g / cc, which can be used in retroreflective articles such as retroreflective pavement marking articles. In some instances, the high-density microspheres as disposed on the surface of a composite core made of a polymeric resin and additional microspheres to form a retroreflective element. In other instances, the high-density microspheres are partially embedded in a retroreflective pavement marking tape. SUMMARY

[0002] Pavement or road markings (e.g., paints, tapes, and individually mounted articles) guide and direct motorists and pedestrians traveling along roadways and paths. Pavement or road markings can be used on, for example, roads, highways, parking lots, and recreational trails. Typically, pavement markings form stripes, bars, and markings for the delineation of lanes, crosswalks, parking spaces, symbols, legends, and the like. Paint was a preferred pavement marking for many years but has now been replaced by retroreflective liquid pavement markings which can include retroreflective elements. Retroreflective liquid pavement marking offer significant advantages over paint, such as increased visibility, retroreflectance, improved durability, and temporary, and / or removable marking options. Commercially available retroreflective elements include, for example, All Weather Elements made by 3M Company of St. Paul, MN. Typically, a retroreflective element includes a core adjacent to numerous glass or glass ceramic microspheres that are adhered to the outermost surface of the core by a binder.

[0003] As exemplified in, for example, U.S. Pat. Pub. No.2005 / 0100709 (Bescup et al.), the retroreflective elements are sprinkled onto liquid roadway binder or paint and allowed to dry and cure. Ideally, the retroreflective elements partially embed into the roadway or pavement marking forming markings that are highly visible, day and night, under both wet and dry conditions.

[0004] Ideally, the retroreflective elements should be low cost and high performing. Additionally, since the retroreflective elements are partially embedded into the roadway or pavement it would be advantageous to identify a higher density retroreflective element to enable adequate sinking in typical liquid roadway binder or paint, which can have a density of up to 2.15 g / cc (grams per cubic centimeter).

[0005] In one aspect, a retroreflective element is described comprising: (i) a composite core, wherein the composite core comprises a plurality of first microspheres dispersed in an organic polymer, wherein the first microspheres have a first refractive index and wherein the composite core has a first density less than 1.7 g / cc; and (ii) a plurality of second microspheres having a second refractive index and a second density, wherein the second density is at least 5.05 g / cc, wherein the plurality of second microspheres are at the perimeter of the composite core to form the retroreflective element, wherein the retroreflective element has a total density greater than 2.15 g / cc.

[0006] In some embodiments, the plurality of second microspheres is derived from(a) lanthana, (b) zirconia, and (c) alumina and are substantially free of silica, baria, and titania wherein (i) a weight of (a) that is at least 55 wt% and at most 75 wt% (ii) a weight of (b) that is at least 10 wt% and at most 25 wt%; (iii) a weight of (c) that is at least 5 wt% and at most 20 wt%; and (iv) a combined weight of (a), (b), and (c) is at least 90% by weight.

[0007] In some embodiments, the plurality of second microspheres is derived from (a) lanthana, (b) zirconia, (c) alumina, and (d) silica and are substantially free of baria and titania wherein (i) a weight of (a) that is at least 55 wt% and at most 75 wt% (ii) a weight of (b) that is at least 10 wt% and at most 25 wt%; (iii) a weight of (c) that is at least 5 wt% and at most 20 wt%; (iv) a combined weight of (a), (b), and (c) is at least 90% by weight; and (v) combined weight of (a), (b), (c), and (d) is at least 95% by weight.

[0008] In one aspect, a pavement marking system is described, comprising a binder and a retroreflective element comprising: (i) a composite core, wherein the composite core comprises a plurality of first microspheres dispersed in an organic polymer, wherein the first microspheres have a first refractive index and wherein the composite core has a first density less than 1.7 g / cc; and (ii) a plurality of second microspheres having a second refractive index and a second density, wherein the second density is at least 5.05 g / cc, wherein the plurality of second microspheres are at the perimeter of the composite core to form a retroreflective element, wherein the retroreflective element has a total density greater than 2.15 g / cc.

[0009] In yet another aspect, a pavement marking system is described comprising a binder comprising a plurality of second glass microspheres partially embedded therein wherein the second microspheres have a density of at least 5.05 g / cc.

[0010] 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

[0011] Unless otherwise noted, the schematic drawings below are for illustrative proposes only and are not drawn to scale.

[0012] FIG.1 is a cross-sectional view of an illustrative retroreflective element;

[0013] FIG.2 is a cross-sectional view of an illustrative pavement marking; and

[0014] FIG.3 is a cross-sectional view of an illustrative pavement marking tape. DETAILED DESCRIPTION

[0015] 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).

[0016] 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.).

[0017] When discussing the composition of the microspheres for both the first and second microspheres, the components 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 microspheres.

[0018] The term “retroreflective” as used herein refers to the attribute of reflecting an obliquely incident radiation ray in a direction generally antiparallel to its incident direction such that it returns to the radiation source or the vicinity thereof.

[0019] High-density microspheres are disclosed herein, which can be used in retroreflective articles such as used in pavement markings. One such article is a retroreflective element as shown in FIG.1. Retroreflective element 10 comprises composite core 11 with first microspheres 12 distributed throughout with a plurality of high-density microspheres, referred to herein as second microspheres, 15 disposed at the perimeter of composite core 11. Because the core 11 is a blend of organic polymer 13 and first microspheres 12, the core may be referred to herein as a “composite core.”

[0020] The first microspheres of the composite core can be spherical in nature, meaning that the microspheres have curved edges and / or shapes. In some embodiments, the plurality of first microspheres is substantially spherical, which means that when magnified into a two-dimensional image, the plurality of second microspheres appears 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.

[0021] The term "solid" refers to microspheres that are not hollow, i.e., free of substantial 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.

[0022] The plurality of first microspheres is 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 l00x) 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 the microspheres, 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 ofthe 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.

[0023] The plurality of first microspheres can be derived from any glass or glass ceramic chemistry. For example, the plurality of first microspheres includes at least one or more metal oxides such as SiO2, Al2O3, B2O3, Li2O, Na2O, K2O, MgO, and CaO. To achieve a low-cost retroreflective element, it is preferable to use commodity materials for the composite core and then use a more custom material, such as the plurality of second microspheres disclosed herein to tailor the properties of the retroreflective element. Thus, in some embodiments the plurality of first microspheres is a common glass chemistry, such as those used in float or plate glass.

[0024] In some embodiments, the glass or glass ceramic microspheres in the plurality of first microspheres has a median diameter (or d50) of 30 to 200 micrometers. For example, the plurality of first microspheres has a median diameter of at least 30, 40, 50, 60, or even 70 micrometers. In some embodiments, the plurality of first microspheres has a median diameter of at most 200, 50, 100, 90, or even 80 micrometers. 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.

[0025] The composite core of the retroreflective element contains an organic polymer, meaning a material which comprises macromolecules that are composed of carbon-hydrogen bonds. Suitable organic polymers include thermoplastic or thermoset materials. The organic polymer should be durable, tough, and hard. For example, typically the organic polymer has a hardness greater than the hardness of tire treads. In some embodiments, the organic polymer has a Shore hardness on the D scale of at least 10. For example, the organic polymer has a Shore D hardness of at least 36, 38, 40, 45, or even 50; and at most 68, 66, 64, 60, or even 55. The Shore Hardness may be measured using a durometer such as described in ASTM D 2240-15(2021).

[0026] Generally, the organic polymer of the composite core should be resistant to ultraviolet light, abrasion, salt, water, oil, and chemicals, and have heat deflection or a softening temperature greater than the temperature typically encountered in roadway surfaces. For example, suitable materials include acrylics, methacrylics, polycarbonates, polyurethanes, polyolefins, polyesters, polyvinyl chloride and its copolymers, acid olefin copolymers such as ethylene acrylic acid, ethylene methacrylic acid, acid olefin copolymers neutralized with a base “ionomer” or “ionic copolymer”, and mixtures thereof.

[0027] In some embodiments, the organic polymer is an ionic copolymer. The term “ionic copolymer” as used herein refers to materials that include a fraction of ionized units covalently bonded to a polymer backbone as pendant group moieties. In some embodiments, the ionic copolymer includes no more than 15 mole percent of ionized units covalently bonded to a polymer backbone as pendant group moieties. In some embodiments, the ionic copolymer is a thermoplastic, which allows for desirable processing using an extruder as compared to thermoset or highly crosslinked materials (e.g., epoxy). Some exemplary ionic copolymers include those commercially available under the trade designation SURLYN by DuPont de Nemours. In some embodiments, the ionic copolymer is an ionically cross-linked ethylene methacrylic acid copolymer.

[0028] Without wishing to be bound by theory, it is believed that the ionic copolymer of the composite core forms an ionic bond to microspheres or other additives (e.g., pigments), thereby increasing the strength and durability of the composite. The ionic groups can form bonds or crosslinks in the mixture of the composite core to contribute to toughness, hardness of the material. In particular, the ionic groups will bond to the microspheres. Further, without wishing to be bound by theory, it is believed that the ionic copolymer of the composite core can form an ionic bond to the underlying substrate (i.e., tape, paint), thereby increasing adhesion of composite core to the underlying substrate.

[0029] The composite core may optionally include one or more additional fillers besides the first microspheres. Useful fillers are typically solids that are non-reactive with the other components present. Useful fillers include, for example, crushed quartz, ground or light calcium carbonate (with or without a surface-treatment such as a fatty acid, resin acid, cationic surfactant, or anionic surfactant), magnesium carbonate, sulfates such as barium sulfate, alumina, metals in powder form (e.g., aluminum, zinc and iron), bentonite, kaolin clay, talc, glass particles (e.g., frit or fibers), metal oxide particles, silica particles, ceramic microspheres, hollow polymeric microspheres (such as those available under the trade designation EXPANCEL 551 DE from Akzo Nobel, Duluth, Ga.), hollow glass microspheres (such as those available under the trade designation K37 from 3M Co., St Paul, Minn.), carbonates, metal oxides, silicates (e.g. talc, asbestos, clays, mica), sulfates, silicon dioxide and aluminum trihydrate.

[0030] The filler can also comprise conductive particles (see, for example, U.S. Pat Pub. No. 2003 / 0051807 (Yamaguchi et al.), incorporated herein in its entirety by reference) such as carbon particles or metal particles of silver, copper, nickel, gold, tin, zinc, platinum, palladium, iron, tungsten, molybdenum, solder or the like, or particles prepared by covering the surface of these particles with a conductive coating of a metal or the like. It is also possible to use non-conductive particles of a polymer such as polyethylene, polystyrene, phenol resin, epoxy resin, acryl resin or benzoguanamine resin, or glass microspheres, silica, graphite or a ceramic, whose surfaces have been covered with a conductive coating of a metal or the like.

[0031] In some embodiments, the organic polymer, first microspheres, and additional fillers are mixed to form a relatively homogeneous mixture, wherein fillers and other materials insoluble in the organic polymer are dispersed randomly three-dimensionally throughout the organic polymer. An extruder is suitable for this purpose.

[0032] Skid-resistant particles, if included, can improve dynamic friction between the retroreflective element and a vehicle tire or walker. The skid-resistant particles can be, for example, ceramics such as quartz or aluminum oxide or similar abrasive material. Skid-resistant particles can be included in the organic polymer of the composite core or can be applied to the outer surface of the retroreflective element.

[0033] In the composite core, the plurality of first microspheres is dispersed within the organic polymer. In some embodiments, the first microspheres are uniformly distributed throughout the composite core. Typically, the composite core comprises about 5 to 65 volume % of the first microspheres in the volume of the core composite. For example, in some embodiments, the core composite comprises at least 5, 10,15, 20, 25, 30, or even 35 volume % and at most 65, 60, 55, 50, 45, or even 40 volume % of the first microspheres. Including too high of loading of the first microspheres will impact the mechanical properties of the core composite. In some embodiments, the organic polymer entirely surrounds each first microsphere within the core composite.

[0034] The composite core should have a density less than 1.7 g / cc (grams per cubic centimeter). In some embodiments, the composite core has a density of less than 1.7, 1.65, 1.6, 1.55, or even 1.5 g / cc; and typically has a density of at least 1.2 g / cc. Density may be determined using known techniques under standard conditions such as room temperature (e.g., 23 °C) and atmospheric pressure (e.g., 1 atm), such as taught in the Examples below.

[0035] A plurality of second microspheres (e.g., 15) are disposed around at least a portion of a perimeter of the composite core. The composite core is a three-dimensional body, and therefore “perimeter” means at least a portion of the external surface of the composite core. In some embodiments, second microspheres 15 are disposed around the entire perimeter of the composite core.

[0036] The second microspheres of the present disclosure are of high density, preferably, having a density of at least 5.05 g / cc. The higher density observed in the second microspheres can be achieved by using increased lanthana content.

[0037] In some embodiments, the second microspheres comprise at least 55 wt% lanthana (La2O3). In typical embodiments, the second microspheres comprise no greater than 75, 70, 65, or even 60 wt% of lanthana. In some embodiments, the second microspheres comprise at least 55, 58, 60, or even 62 wt% lanthana. While not bound by theory it is believed that at this concentration lanthana plays an important role as a network former in the manufacture of the glass second microspheres, having a low 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.

[0038] In some embodiments, the plurality of second microspheres generally comprise lanthana (i.e., La2O3), zirconia (i.e., ZrO2), and alumina (i.e., Al2O3).

[0039] In some embodiments, the second microspheres comprise at least 10 wt% zirconia. In some embodiments, the second microspheres comprise at least 10, 12, 14, 15, 16, or even 18 wt% zirconia. In some embodiments, the second microspheres comprise no greater than 25, 24, 23, 22, 21, or even 20 wt% zirconia. Zirconia is present at these concentrations to increase the chemical and mechanical durability, while also contributing to the high refractive index of the microspheres. Higher amounts of zirconia can lead to higher levels of optically detrimental crystallization defects.

[0040] 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 alumina and optionally silica to tailor the refractive index of the second microspheres to a desired range. In some embodiments, the second microspheres comprise at least 5, 8, or even 10 wt% of alumina. In some embodiments, the secondmicrospheres comprise at most 20, 18, 15, or even 12 wt% alumina. In some embodiments, the second 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. While not bound by theory, it is believed alumina and silica take the role of network modifiers in the glass and are needed to lower the melting temperature of the glass to enable one pass flame processing. Concentration of alumina and optionally silica is critical to balance the greater than 1.95 refractive of both zirconia and lanthana. Too little alumina and silica leads to higher refractive index than desired, while too much alumina and silica reduces the refractive index to less useful levels.

[0041] In some embodiments, the total amount of lanthana, zirconia, and alumina in the second microspheres is at least 90, 91, 92, 93, 94, or even 95 wt% of the microspheres. In some embodiments, the total amount of lanthana, zirconia, and alumina is at most 99, 98, 97, or even 96 wt% of the microspheres. In some embodiments, the second microspheres consist of lanthana, zirconia, and alumina.

[0042] In some embodiments, the total amount of lanthana, zirconia, alumina, and silica in the second microspheres is at least 95, 96, or even 97 wt% of the microspheres. In some embodiments, the total amount of lanthana, zirconia, alumina, and silica is at most 99, or even 98 wt% of the microspheres. In some embodiments, the second microspheres consist of lanthana, zirconia, alumina, and silica.

[0043] By forming second microspheres made with at least 90 wt% of lanthana, zirconia, and alumina, and optionally silica, solid microspheres can be formed that are transparent, have high refractive index within the desired operating range, and are significantly crystalline defect free (i.e., less than 10% of the microspheres show visible crystalline defects).

[0044] In some embodiments, the plurality of second microspheres is derived from (a) lanthana, (b) zirconia, and (c) alumina wherein (i) a weight of (a) that is at least 55 wt% and at most 75 wt% (ii) a weight of (b) that is at least 10 wt% and at most 25 wt%; (iii) a weight of (c) that is at least 5 wt% and at most 20 wt%; and (iv) a combined weight of (a), (b), and (c) is at least 90% by weight. In some embodiments, the microspheres are substantially free of silica, baria, and titania.

[0045] In some embodiments, the plurality of second microspheres is derived from (a) lanthana, (b) zirconia, and (c) alumina and (d) silica wherein (i) a weight of (a) that is at least 55 wt% and at most 75 wt% (ii) a weight of (b) that is at least 10 wt% and at most 25 wt%; (iii) a weight of (c) that is at least 5 wt% and at most 20 wt%; (iv) a combined weight of (a), (b), and (c) is at least 90% by weight; and (v) combined weight of (a), (b), (c), and (d) is at least 95% by weight. In some embodiments, the microspheres are substantially free of baria, and titania.

[0046] Silica may or may not be present in the composition of the second microspheres. Silica, in some embodiments, can lower melting temperatures but due to relative high melt viscosity is prone to enabling bubble formation while also lowering the refractive index of the materials. In some embodiments, thesecond microspheres described herein are substantially free of silica (SiO2), meaning silica is present in amounts of less than 1, 0.9, 0.7, 0.5, 0.3, 0.1, or even 0.05 wt% in the microsphere. In some embodiments, silica is not detectable.

[0047] The plurality of second microspheres described herein can be substantially free of baria (e.g., BaO), and titania (e.g., TiO2). 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 air-natural gas flame combustion temperature of 2055 degrees Celsius (°C). 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, as microsphere 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.

[0048] 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.

[0049] The plurality of second 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 plurality of second 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.

[0050] In some embodiments, the plurality of second microspheres comprise alkaline earth oxides, such as SrO, MgO and / or CaO. The amount of alkaline earth oxide(s) (e.g. 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.

[0051] In some embodiments, the plurality of second 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.

[0052] 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 Mn2O3can 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).

[0053] In some embodiments, the plurality of second 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 microspheres.

[0054] In some embodiments, the plurality of second microspheres typically have a density of at least 5.05 g / cc (grams per cubic centimeter). The density of the microspheres is typically no greater than 5.6 g / cc. In some embodiments, the density is at least 5.1, 5.2, 5.3, or even 5.4 g / cc. The density of the microspheres may be measured using the method as described in the examples.

[0055] Advantageously, the plurality of second 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. A particle size distribution can be obtained by scatter techniques, reporting d10, d50 and d90 values. In the present disclosure, the plurality of second 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 microns. 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 d10 (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, 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.

[0056] The plurality of second 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. Patent 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.

[0057] 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.

[0058] 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.

[0059] 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.

[0060] Examples of useful volatile organic liquids include methanol, ethanol, isopropyl alcohol, butyl alcohol, heptane, and toluene.

[0061] 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. Patent No.8,701,441 (Kramlich et al.).

[0062] 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, glass-ceramic, glass-bonded ceramic, and crystalline ceramic spheres manufacturing processes and apparatuses such that the combination of time and temperature is sufficient to form a homogenous glass melt.

[0063] 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 2000K (Kelvin), at least about 3000K, or even from about 3000K to about 5000K. The flame can be generated by any suitable fuel and oxidant sources including, e.g., natural gas, hydrogen, oxygen, acetylene, air, and mixtures thereof.

[0064] 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 asufficient 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.

[0065] 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 microspheres.

[0066] In some embodiments, the second microspheres are glass or “amorphous microspheres”. Glass materials may be differentiated from 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. Microspheres comprising crystals that are detectable by X-ray diffraction measurements, typically necessary to be present in an amount greater than or equal to 10 volume% for detectability, are considered glass-ceramic microspheres.

[0067] In some embodiments, the combination of lanthana, alumina, and zirconia and optionally silica, in the amounts disclosed herein can 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.

[0068] Unexpectedly, the second microsphere compositions as disclosed herein result in microspheres that are high index, optically transparent, and show little (<10 %) to no propensity to form crystalline phases without deliberate post heat treatment processing. In the present disclosure, it has been found that the plurality of second microspheres comprising a particular composition as recited herein can be formed in a single pass through the high temperature flame and are substantially free of crystalline defects, without the need to perform additional heat treatment to remove color. This results 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.

[0069] In some embodiments, the plurality of second microspheres has excellent quality as observed by transmitted light optical microscopy. For example, when viewed by optical microscopy, the plurality of second microspheres is transparent, wherein less than 10% of the microspheres have apparent crystallinity large enough to scatter light. In some embodiments, the plurality of second microspheres has low amounts of entrained bubble defects, such as less than 10, 5, 4, 3, 2, or even 1% (defined as thepercentage of microspheres that have a defect). Excessive levels of entrained bubbles must be avoided to maintain required density of the second microspheres to enable a high-density composite core element.

[0070] Notably, the plurality of second microspheres can be of high refractive index and enable articles with sufficiently high retroreflective brightness even in the absence of titania due to the high amounts of lanthana and zirconia present. 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 element and / or the plurality of second microspheres therein have a brightness as measured per the method of the examples of at least 16, 17, 18, 19, 20, 21, 22, 23, or even 24 (Cd / m2) / lux. In some embodiments, the retroreflective brightness of the microspheres and / or retroreflective elements is no greater than 28, 27, 26, 25, 24, 23, 22, 21, 20, 19, or even 18 (Cd / m2) / lux.

[0071] In some embodiments, the plurality of second microspheres has suitable whiteness for use in retroreflective articles as determined by the test method described in the examples. The whiteness index (“WI”) is at least 15. In some embodiments, the whiteness index is at least 15, 18, 20, 22, 25, 30, 35, or even 40. In some embodiments, the whiteness index is no greater than 61, 60, 59, 58, 57, 56, or 55.

[0072] The plurality of second microspheres is spherical in nature. In some embodiments, the plurality of second microspheres is substantially spherical and solid as defined above. Generally, the plurality of second microspheres is preferably transparent.

[0073] The retroreflective elements described herein can be made, manufactured, or formed by any of several methods. Typically, the composite core is formed, and then the second microspheres are applied to the composite core.

[0074] In some embodiments, the composite core is formed by forming small pieces of the polymer, such as disclosed in U.S. Pat. No.5,750,191 (Hachey et al.), the disclosure of which is herein incorporated by reference. The first microspheres are mixed in the organic polymer prior to extrusion.

[0075] In some embodiments, the plurality of second microspheres is secured to the composite core by softening and securing directly to the organic polymer of the composite core. In some embodiments, a softening agent is applied to the composite core and the second microspheres are secured to the softening agent. In some embodiments, an adhesive is applied to the composite core and the second microspheres are secured to the adhesive. In some embodiments, the second microspheres are secured to the composite core by adding the composite core to a mobile bed of second microspheres, such as described in U.S. Pat. No.5,750,191. Typically, the second microspheres are embedded, usually in a monolayer fashion at the perimeter of the composite core. In some embodiments, the second microspheres are embedded to at least 30, 40, 45, 50, or even 51 % of their average diameter at the perimeter of the composite core. In some embodiments, the second microspheres are embedded to most 70, 65, 60, or even 55 % of their average diameter at the perimeter of the composite core.

[0076] Typically, the first microspheres have a refractive index that is different than the refractive index of the second microspheres. In some embodiments, the first microspheres have an average refractive index of at least 1.4, 1.45, 1.5, 1.55, 1.6, 1.65, or even 1.7; and at most 2.0, 1.9, 1.8, 1.7, 1.65, or even1.6. In particular, the plurality of first microspheres having a refractive index ranging between 1.45 and 1.65 such as those glass chemistries commonly use in large scale manufacturing of plate glass and drinkware are preferred to reduce cost of the first microspheres.

[0077] Generally, for good dry condition performance in pavement marking, the second microspheres of high density have a refractive index greater than 1.90. In some embodiments, the second microspheres have a refractive index of at least 1.91, 1.92, 1.93, 1.94, or even 1.95. In some embodiments, the refractive index of the second microspheres of the present disclosure is no greater than 1.97, 1.96, 1.95, or even 1.94. The index of refraction can be determined by the test method described in the examples. In some embodiments, the second microspheres have refractive indices of between about 1.9 and about 1.95. In some embodiments, the second microspheres have a refractive index of between about 1.92 and about 1.94.

[0078] Such retroreflective elements, comprising a composite core with second microspheres disposed on the perimeter have been known for good performance in dry conditions and can remain useful for returning light even after portions of the retroreflective element wear away as described in U.S. Pat. No. 10,858,496 (Wilding, et al.) herein incorporated by reference.

[0079] In some embodiments, the difference between the refractive index of the first microsphere and second microsphere is at least 0.30, 0.35, or even 0.40. In some embodiments, the difference in refractive index of the first microsphere to the second microsphere is no more than 0.50.

[0080] In some embodiments, the second microspheres at the perimeter of the composite core are of the same average size as the first microspheres dispersed throughout the composite core. In some embodiments, the second microsphere are of a different average size from the first microspheres. For example, second microspheres may be larger or smaller than first microspheres. If processing the composite core material in an extruder, the first microspheres should be small enough to easily pass through the extruder. In some embodiments, these first microspheres should have an average diameter less than 250 microns. In some embodiments, the first microspheres have the same general shape as the second microspheres. In some embodiments, the first microspheres have of a different composition than the second microspheres.

[0081] In some embodiments, an adhesive is included to bond the second microspheres to the composite core. In some embodiments, an adhesive in included to bond the retroreflective element to a tape or other substrate, such as a roadway surface. Some exemplary adhesive compositions include pressure sensitive adhesives, thermoplastic resin-containing compositions, heat-activated adhesives (i.e., hot melt adhesives), thermoset adhesives, contact adhesives, acrylic adhesives, epoxy adhesives, urethane adhesives, and combinations thereof.

[0082] Other materials can be included within the retroreflective element. These other materials can be added to the organic polymer during manufacturing the composite core, or may be added to the organic polymer prior to manufacturing the composite core. Examples of other materials include pigments, UV (ultra-violet) stabilizers, heat stabilizers, antioxidants, processing aids, and skid-resistant particles, for examples.

[0083] Stabilizing agents improve resistance to UV light or heat resistance to the retroreflective element. Exemplary stabilizing agents include, for example, hindered amine light stabilizers (HALS), phosphonate heat stabilizers, benzophenones, and zinc compounds. Stabilizing agents may be present at levels up to about 5 wt % in the retroreflective element. Some embodiments include one or more plasticizers. In some embodiments, extender resins, often halogenated polymers such as chlorinated paraffins, but also hydrocarbon resins or polystyrenes, are included with the ionic copolymer precursor ingredients, and are miscible with, or form a single phase with, the ionic copolymer.

[0084] The composite core or the retroreflective element of the present disclosure can be any desired color, including, for example, white or yellow. The composite core or the retroreflective element can be colored in any way known in the art, including, for example, inclusion of one or more of organic pigments, inorganic pigments and whitening agents.

[0085] Examples of useful organic pigments include halogenated copper phthalocyanines, aniline Blacks, anthraquinone blacks, benzimidazolones, azo condensations, arylamides, diarylides, disazo condensations, isoindolinones, isoindolines, quinophthalones, anthrapyrimidines, flavanthrones, pyrazolone oranges, perinone oranges, beta-naphthols, BON arylamides, quinacridones, perylenes, anthraquinones, dibromanthrones, pyranthrones, diketopyrrolo-pyrrole pigments (DPP), dioxazine violets, copper and copper-free phthalocyanines, Indanthrones, and the like.

[0086] Examples of useful inorganic pigments include titanium dioxide, zinc oxide, zinc sulphide, lithopone, antimony oxide, barium sulfate, carbon black, graphite, black iron oxide, black micaceous iron oxide, brown iron oxides, metal complex browns, lead chromate, cadmium yellow, yellow oxides, bismuth vanadate, lead molybdate, cadmium red, red iron oxide, prussian blue, ultramarine, cobalt blue, chrome green (Brunswick green), chromium oxide, hydrated chromium oxide, organic metal complexes, lake dye pigments and the like.

[0087] Exemplary whitening agents include, for example, TiO2, barium sulfate, and zinc oxide. In embodiments including TiO2, the composition may include, for example, from about 0.1 or about 0.5 or about 5 wt % to about 5 or about 10 or about 15 wt % TiO2. In some embodiments, the compositions comprise a whitening agent or a yellow organic pigment. In some embodiments, the composition comprises from about 0.5 wt % to about 2.5 wt % of an organic yellow pigment.

[0088] In some embodiments, the resulting retroreflective elements have a mean or average diameter of at least 100, 150, 200, 400, or even 500 micrometers and at most 2000, 1500, 1000, or even 750 micrometers. Typically, the diameter of the retroreflective elements will depend on the size of the first and second microspheres selected.

[0089] In some embodiments, the retroreflective elements are essentially spherical, as described in, for example, U.S. Pat. Nos.5,942,280 (Mathers et al.) and 7,513,941 (Frey at al.), both of which are incorporated herein in their entirety. In some embodiments, the retroreflective elements are non-spherical, as described in, for example, U.S. Pat. No.5,774,265 (Mathers et al) and 9,110,236 (Saito et al.), incorporated by reference herein in its entirety.

[0090] The retroreflective elements disclosed herein can have any desired topography. For example, the elements can be roughly spherical overall, with an outer surface of closely packed second microspheres. In some embodiments, the retroreflective element can include protrusions extending from the core with cavities between adjacent protrusions, such as disclosed in U.S. Pat. No.9,110,236.

[0091] The retroreflective elements disclosed herein can be used with liquid pavement marking. Any known liquid pavement marking can be used with the retroreflective elements described herein. Some exemplary commercially available roadway marking liquid pavement markings capable of use with the retroreflective elements include, for example, Liquid Pavement Marking Series 5000, available from 3M Company, St. Paul, MN; HPS-2, available from Ennis-Flint, Thomasville, N.C.; and LS90, available from Epoplex, Maple Shade, NJ. In some embodiments, the liquid pavement marking includes a colorant. In some embodiments, the liquid pavement marking is white or yellow.

[0092] Any known process for including or applying retroreflective elements to a liquid pavement marking composition may be used to include or apply the retroreflective elements described herein to a roadway marking or liquid pavement marking. For example, the methods described in the following patents may be used: U.S. Pat. Nos.3,935,158 (Watanabe) and 5,774,265 (Mathers et al.), both of which are incorporated in their entirety herein.

[0093] The disclosed retroreflective elements can be used with any substrate to make a pavement marking tape. For example, single or multilayers of materials comprising a resilient polymeric base sheet, a binder layer, optical elements, and optionally a scrim and / or adhesive layer are commonly used to make pavement marking tapes, as described in U.S. Pat. Nos.4,988,541 (Hedblom) and 5,777,791 (Hedblom).

[0094] The pavement markings described herein comprise a binder. The binder affixes 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.

[0095] Ideally, these retroreflective elements are dropped into a paint for road markings. Ideally, these retroreflective elements should be of a sufficient density to properly embed in the binder.

[0096] In some aspects, the retroreflective elements are employed in liquid-applied marking (e.g. pavement) applications. Shown in Fig.2 is a schematic of retroreflective elements 20 of the present disclosure, embedded into binder 28, which is disposed atop surface (e.g., pavement surface) 29. The reflective elements of the present disclosure can be sequentially or concurrently dropped onto a liquified binder or compounded within a liquified binder that is provided on a pavement surface.

[0097] Although Fig.1 illustrates second microsphere 15 being a slightly smaller size as compared to the first microsphere 12, in some embodiments, second microsphere may be, on average, larger than first microsphere or even of similar size.

[0098] The binder layer of FIG.2 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. Patent No.4,988,555 (Hedblom) are one preferred binder medium at least for pavement markings. For ease of coating, the medium will preferably be a liquid with a viscosity of less than 10,000 centipoise at coating temperatures. Another preferred binder medium is a hot melt applied thermoplastic pavement marking such as are disclosed in U.S. Patent No.11,702,804 (Rajendran et al). Hot melt applied thermoplastic binder mediums can reach high filler concentrations and see rapid viscosity increase upon cooling which can lead to undesirable levels of retroreflective element embedment if the retroreflective element density is too low.

[0099] The binder layer of FIG.2 as well as the composite core of FIG.1 typically comprise at least one pigment such as a diffuse reflecting or specular reflecting pigment.

[0100] 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.

[0101] In addition to being used in retroreflective elements as described above, in some embodiments, the second microspheres disclosed herein may be used in a pavement marking tape as shown in Fig.3. Pavement marking tape and sheet material generally includes a backing, a layer of binder material, and a layer of second microspheres of the present disclosure 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.

[0102] Patterned retroreflective (e.g., pavement) markings advantageously provide vertical surfaces, e.g., defined by protrusions, in which the second microspheres of the present disclosure are partially embedded. Because the light source usually strikes a pavement marker at high entrance angles, the vertical surfaces, containing embedded second microspheres, provide for more effectiveretroreflection. Vertical surfaces also tend to keep the second microspheres out of the water during rainy periods thereby improving retroreflective performance.

[0103] 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 second microspheres of the present disclosure and antiskid particles. Base sheet 102 has front surface 103 from which the protrusions extend, and back surface 105. Base sheet 102 is typically about 1 millimeter (0.04 inch) thick but may be of other dimensions if desired. Optionally, 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. Protrusions 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° at the 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 second microspheres of the present disclosure 117 and a plurality of third microspheres 116 (e.g., having a lower refractive index than the second microspheres). Optionally, antiskid particles 118 may be embedded on binder layer 115. The second microspheres of the present disclosure 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 second microspheres, thereby minimizing the manufacturing cost. The second microspheres may be placed on any of the side surfaces as well as the top surface of the protrusions to achieve efficient retroreflection.

[0104] 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 second 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. EXAMPLES

[0105] 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.

[0106] The following abbreviations are used in the Example Section: cd = candela, cm = centimeter, g = gram, m = meter, min = minute, mL = milliliter, mm = millimeter, nm = nanometer, µm= micrometer, ˚C = degrees Celsius, and wt% = weight percent.Table 1. Materials List Abbreviation Material Source Cell-gum Sodium Obtained from Hercules Incorporated, Aqualon carboxymethylcellulose Division, Wilmington, DE, under trade (Cell Gum) designation “7ULC” Dispersant Sodium polymethacrylate Obtained from Geo Specialty Chemicals, Ambler, solution PA, under solution trade designation “DAXAD 30” ZrO2Zirconium (IV) Oxide Obtained from Z–- Tech LLC, Bow, NH, under trade designation “CF PLUS- HM” Al2O3Aluminum (III) Oxide Obtained from Alcoa Chemicals, Point Comfort, TX, under trade designation “A16- SG” SiO2Silicon (IV) Oxide Obtained from Vitro Minerals, Jackson, TN, under the trade designation “RS-50” La(OH)3Lanthanum Hydroxide Obtained from Pacific Industrial Development Corporation, Ann Arbor, MI EMAA Ionomer of ethylene acid Obtainable from DuPont de Nemours, Inc. ionomer copolymer Wilmington, DE, under the trade designation “SURLYN 9120” Pearlescent Weather resistant mica- Obtainable from Merck KGaA, Darmstadt, pigment based pearl luster Germany, under the trade designation “IRIODIN pigment 9119 Polar White WR” Glass Solid glass spheres with a Obtainable from Potters Industries LLC, Malvern, Microspheres mean particle size of PA, under the trade designation “SPHERIGLASS I 71micrometers and a A-2530” reported refractive index of 1.51

[0107] Test Methods

[0108] Index of Refraction: Index of refraction of the microspheres was measured according to T. Yamaguchi, “Refractive Index Measurement of High Refractive Index Microspheres” Applied Optics Volume 14, Number 5, pages 1111-1115 (1975).

[0109] Brightness: Dry white patch brightness values were determined using a retroreflectometer (Delta GRX, obtained from Force Technology, Brøndby, Denmark). Patch brightness refers to the coefficient of retroreflection (RA) determined using a retroluminometer. The device directs white light onto a planar monolayer of microspheres disposed on a white backing material at a fixed entrance angle to the normal to the monolayer. The white backing 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 TiO2pigment. 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).

[0110] 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) is placed between the spectrophotometer light source and the cup holding the sample to be measured.

[0111] 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.

[0112] 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 with isolated 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.

[0113] 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 d10, d50 and d90 values.

[0114] Comparative Example 1 (CE1)

[0115] Microspheres having a refractive index of 1.9 can be isolated from reflective elements available under the trade designation “3M Connected Roads All Weather Elements Series Dry White” (3M Co., Maplewood, MN). This product comprises a 1.9 refractive index beads partially embedded on the perimeter of a composite core comprising 1.5 refractive index beads. The reflective elements where heated for 30 minutes in a muffle furnace set to 600° C to burn off the binder and any organics, leaving just the microspheres, which can be cooled to room temperature. A vibratory separation apparatus such as described in U.S. Pat. No.4,624,370 A (Danner et al.) can then be used to separate the two different refractive indices microspheres based on density. The higher density microspheres can be collected and used below.

[0116] Examples 1-8 (Ex.1-8)

[0117] To prepare examples Ex.1 through Ex.8, 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).

[0118] Molded precursor green particles were made from the slurry by following the general teachings of U.S. Patent No.8,701,441 (Kramlich et. al.), which is incorporated herein by reference. Mold cavities were chosen to achieve desired microsphere d50 sizes.

[0119] 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 CH4at 7.5 standard liters per minute (SLPM), O2at 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.

[0120] 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 oversized particle defects. The cell-gum and dispersant are volatilized during flame forming and are not present in the final microspheres.

[0121] 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 Amount (g) used to make Slurry Theoretical % Wt of metal oxide Ex. La(OH)3Al2O3ZrO2SiO2La2O3Al2O3ZrO2SiO21 553.2 132.2 164.6 0.0 61.3 17.2 21.5 0.0 2 544.9 130.2 162.2 12.7 60.3 16.9 21.1 1.7 3 536.0 128.1 159.5 26.4 59.2 16.6 20.7 3.4 4 536.0 128.1 159.5 26.4 59.2 16.6 20.7 3.4 5 536.0 128.1 159.5 26.4 59.2 16.6 20.7 3.4 6 536.0 128.1 159.5 26.4 59.2 16.6 20.7 3.4 7 634.2 57.3 111.6 46.9 71.4 7.6 14.8 6.2 8 533.2 85.9 175.9 55.0 58.9 11.1 22.8 7.1

[0122] The microspheres from each of the samples was 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. All samples (Ex.1-Ex.8) were shown to be amorphous (i.e., glass) when tested by XRD. The samples also were tested for refractive index, particle size, brightness, whiteness and density and the results are shown in Table 3. Table 3 Ex. Refractive d10 d50 D90 Brightness ndex (µm) (µm) (cd / m2Whiteness Density I (µm) ) / lux Index (g / cc) CE1 1.89 65 83 97 14.2 -13 3.53 1 1.94 98 115 137 21.2 20 5.22 2 1.93 109 121 138 23.7 28 5.16 3 1.92 101 120 143 20.3 31 5.10 4 1.92 129 144 161 21.4 37 5.10 5 1.92 107 123 141 20.6 42 5.10 6 1.92 60 83 109 19.8 49 5.10 7 1.94 115 138 165 23.4 21 5.43 8 1.92 115 142 172 20.2 30 5.17

[0123] Prophetic Examples

[0124] Composite cores could be made using a twin-screw extrusion compounding process by compounding 35 wt % EMAA ionomer (59.4 Vol.%), 20 wt % Pearlescent pigment (10.97 Vol. %), and 45 wt % Glass Microsphere I (29.63 Vol. %). The material can be formed into a cylinder with a 1.5 mm diameter and 1.5 mm length using a pelletizer. The composite core would have a density of 1.65 g / cc based on the volume fraction and density of the input raw materials. Retroreflective elements (CE2, Ex. 9-16) could be created by coating the composite core with second microspheres from CE 1 and Ex.1-8. The second microspheres from CE 1 and EX.1-8 are first heated in a fluidized bath at approximately 350°C. Then room temperature composite cores in a mesh basket are introduced into the fluidized bath containing a surplus of the designated second microspheres and allowed to sit for approximately 5 seconds before being removed from the fluidized bed. During the fluidized bath exposure of the composite cores, the heated second microspheres melt the surface of the composite core and tack into the composite core upon cooling.

[0125] The surface area of the composite core which can be coated by the second microspheres was estimated to be 10.60 mm2. The mass of second microspheres (i.e., CE 1, Ex.1 to Ex.8) per composite core was calculated assuming a normal distribution of second microspheres packing into a monolayer with approximately 84% of the surface area of the composite core covered with second microspheres. Microsphere cross sectional area and volumes were calculated from the measured d50 microsphere diameter. The surface microsphere mass (e.g., CE1 and Ex.1-8) per retroreflective element is calculated from the number of second microspheres required to coat the surface of the retroreflective element and their measured volumes and densities. By combining the mass and volume of the second microspheres with the mass and volume of the composite core the prophetic element densities ofprophetic samples CE2 and PE1-PE8 were calculated and reported in Table 4 below, assuming a 50% sink of the second microspheres into the composite core. Also reported in table 4 below is the density and d50 particle size for each type of microsphere. Table 4 Prophetic Second microspheres on surface Final Product Samples Type Density d50 Density (g / cc) (g / cc) (µm) CE2 CE 1 3.53 83 1.95 PE1 Ex.1 5.22 115 2.38 PE2 Ex.2 5.16 121 2.40 PE3 Ex.3 5.10 120 2.38 PE4 Ex.4 5.10 144 2.49 PE5 Ex.5 5.10 123 2.40 PE6 Ex.6 5.10 83 2.19 PE7 Ex.7 5.43 138 2.54 PE8 Ex.8 5.17 142 2.50

[0126] 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 element comprising: (i) a composite core, wherein the composite core comprises a plurality of first microspheres dispersed in an organic polymer, wherein the first microspheres have a first refractive index and wherein the composite core has a first density less than 1.7 g / cc; and (ii) a plurality of second microspheres having a second refractive index and a second density, wherein the second density is at least 5.05 g / cc, and wherein the plurality of second microspheres are at the perimeter of the composite core to form the retroreflective element wherein the retroreflective element has a total density greater than 2.15 g / cc.

2. The retroreflective element of claim 1, wherein the plurality of second microspheres are partially embedded at the perimeter of the composite core.

3. The retroreflective element of claim 2, wherein the plurality of second microspheres are embedded at least 30 % to at most 75% of their diameter.

4. The retroreflective element of any one of the previous claims, wherein the plurality of second microspheres comprise amorphous microspheres.

5. The retroreflective element of any one of the previous claims, wherein the plurality of second microspheres has a refractive index of at least 1.90 and less than 1.

97.

6. The retroreflective element of any one of the previous claims, wherein the plurality of second microspheres has a refractive index of at most 1.

95.

7. The retroreflective element of any one of the previous claims, wherein the plurality of second microspheres has a median diameter of greater than 80 micrometers.

8. The retroreflective element of any one of the previous claims, wherein the plurality of second microspheres are transparent.

9. The retroreflective element of any one of the previous claims, wherein the plurality of second microspheres has a whiteness index of at least 18.

10. The retroreflective element of any one of the previous claims, wherein the plurality of second microspheres comprises at least 55 wt% lanthana.

11. The retroreflective element of any one of the previous claims, wherein the plurality of second microspheres comprise at least 10 wt% zirconia.

12. The retroreflective element of any one of the previous claims, wherein the plurality of second microspheres comprise at least 5 wt% alumina.

13. The retroreflective element of any one of the previous claims, wherein the plurality of second microspheres comprise at least 1 wt% silica.

14. The retroreflective element of any one of the previous claims, wherein the plurality of second microspheres is derived from (a) lanthana, (b) zirconia, (c) alumina, and (d) silica and are substantially free of titania and baria wherein (i) a weight of (a) that is at least 55 wt% and at most 75 wt% (ii) a weight of (b) that is at least 10 wt% and at most 25 wt%; (iii) a weight of (c) that is at least 5 wt% and at most 20 wt%; (iv) a combined weight of (a), (b), and (c) is at least 90% by weight; and (v) a combined weight of (a), (b), (c), and (d) is at least 95% by weight.

15. The retroreflective element of anyone claims 1-12, wherein the plurality of second microspheres comprise less than 1 wt% silica.

16. The retroreflective element of claim 15, wherein the plurality of second microspheres is derived from (a) lanthana, (b) zirconia, and (c) alumina, and are substantially free of silica, titania, and baria wherein (i) a weight of (a) that is at least 55 wt% and at most 75 wt% (ii) a weight of (b) that is at least 10 wt% and at most 25 wt%; (iii) a weight of (c) that is at least 5 wt% and at most 20 wt%; and (iv) a combined weight of (a), (b), and (c) is at least 90% by weight.

17. The retroreflective element of any one of the previous claims, wherein the plurality of second microspheres further comprises one or more other oxides in an amount no greater than 5 wt%.

18. The retroreflective element of any one of the previous claims, wherein the plurality of second microspheres has a density of at least 5.1 and less than 5.6 g / cc.

19. The retroreflective element of any one of the previous claims, wherein the first refractive index ranges from at least 1.45 to at most 1.65.

20. The retroreflective element of any one of the previous claims, wherein the organic polymer is an acrylic, methacrylic, polycarbonates, polyurethanes, polyolefins, polyesters, polyvinyl chloride and its copolymers, acid olefin copolymers, and mixtures thereof.

21. The retroreflective element of any one of the previous claims, wherein the organic polymer is an ionic copolymer.

22. The retroreflective element of any one of the previous claims, wherein the composite core comprises at least 5 volume % and at most 65 volume % of the plurality of first microspheres.

23. The retroreflective element of any one of the previous claims, wherein the plurality of second microspheres is at the entire perimeter of the composite core.

24. A retroreflective article comprising a monolayer of retroreflective elements according to any one of the previous claims partially embedded in a binder.

25. The retroreflective article of claim 24, wherein the binder is a paint, a thermoplastic material, a thermoset material, or mixtures thereof.

26. The retroreflective article of any one of claims 24-25, wherein the retroreflective article is a pavement marking or pavement marking tape.

27. A retroreflective article comprising a monolayer of glass microspheres partially embedded in a binder, wherein the glass microspheres have a density of at least 5.05 g / cc.

28. The retroreflective article of claim 27, wherein the glass microspheres are derived from (a) lanthana, (b) zirconia, (c) alumina, and (d) silica and are substantially free of titania and baria wherein (i) a weight of (a) that is at least 55 wt% and at most 75 wt% (ii) a weight of (b) that is at least 10 wt% and at most 25 wt%; (iii) a weight of (c) that is at least 5 wt% and at most 20 wt%; (iv) a combined weight of (a), (b), and (c) is at least 90% by weight; and (v) combined weight of (a), (b), (c), and (d) is at least 95% by weight.

29. The retroreflective article of claim 27, wherein the glass microspheres are derived from (a) lanthana, (b) zirconia, and (c) alumina and are substantially free of silica, baria, and titania wherein (i) a weight of (a) that is at least 55 wt% and at most 75 wt% (ii) a weight of (b) that is at least 10 wt% and at most 25 wt%; (iii) a weight of (c) that is at least 5 wt% and at most 20 wt%;(iv) a combined weight of (a), (b), and (c) is at least 90% by weight.

30. The retroreflective article of any one of claims 27-29, wherein the retroreflective article is a pavement marking or pavement marking tape.

31. A microsphere derived from (a) lanthana, (b) zirconia, (c) alumina, and (d) silica and are substantially free of titania and baria wherein (i) a weight of (a) that is at least 55 wt% and at most 75 wt% (ii) a weight of (b) that is at least 10 wt% and at most 25 wt%; (iii) a weight of (c) that is at least 5 wt% and at most 20 wt%; (iv) a combined weight of (a), (b), and (c) is at least 90% by weight; and (v) combined weight of (a), (b), (c), and (d) is at least 95% by weight.

32. A microsphere derived from (a) lanthana, (b) zirconia, and (c) alumina and are substantially free of silica, baria, and titania wherein (i) a weight of (a) that is at least 55 wt% and at most 75 wt% (ii) a weight of (b) that is at least 10 wt% and at most 25 wt%; (iii) a weight of (c) that is at least 5 wt% and at most 20 wt%; (iv) a combined weight of (a), (b), and (c) is at least 90% by weight.

33. The microsphere of any one of claims 31-32, wherein the microsphere has a density of at least 5.05 g / cc.

34. The microsphere of any one of claims 31-33, wherein the microsphere is a glass.

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