Compositions: ceramic – Ceramic compositions – Glass compositions – compositions containing glass other than...
Reexamination Certificate
2001-01-18
2002-11-12
Group, Karl (Department: 1755)
Compositions: ceramic
Ceramic compositions
Glass compositions, compositions containing glass other than...
C501S003000, C501S010000, C501S033000
Reexamination Certificate
active
06479417
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to transparent microspheres (i.e., beads) that impart yellow color to retroreflected light. The microspheres preferably comprise titania plus alumina, zirconia, and/or silica and iron oxide, manganese oxide, and mixtures thereof. More particularly, the present invention relates to fused microspheres that impart yellow color to retroreflected light having both transparency and mechanical properties suitable, for example, for lens elements in retroreflective articles.
BACKGROUND
Transparent glass microspheres (i.e., beads) used in reflectors such as reflective sheets and road surface reflectors can be produced by, for example, melting methods. Such melting methods typically include melting a raw material composition in the form of particulate material. The liquid can then be quenched in water, dried, and crushed to form particles of a size desired for the final beads. The crushed particles are then passed through a flame having a temperature sufficient to melt and spheroidize the crushed particles. For most glasses this is a temperature of about 1000° C. to about 1450° C. Alternatively, the liquid can be poured into a jet of high velocity air. Beads are formed directly in the resulting stream. The velocity of the air is adjusted to control the size of the beads. These beads are normally composed of a vitreous material that is completely amorphous (i.e., noncrystalline), and hence, the beads are often referred to as “vitreous,” “amorphous,” or simply “glass” beads or microspheres.
Silica is a common component of glass-forming compositions. Alumina and zirconia have also been used in transparent glass beads to improve mechanical properties such as toughness, hardness, and strength. However, the amount of alumina and zirconia such beads can contain tends to be limited so as to avoid problems arising from crystallization, such as loss of transparency and processing difficulties. The most highly durable compositions comprise primarily alumina, zirconia, and silica with small amounts of modifiers (e.g., alkaline earth oxides). These compositions tend to have very high melting points and require special processing equipment such as a hydrogen flame or plasma torch. Lower melting compositions with high modifier content typically have poorer mechanical properties and provide only modest improvements over conventional glass beads. In addition, compositions with high modifier content (e.g., greater than about 25% by weight) can exhibit poor chemical durability. High durability fused beads described to date generally do not have a desirable refractive index unless the zirconia content is high (e.g., greater than about 55% by weight); however, high zirconia content tends to raise the melting point and the tendency to crystallize, creating difficulty in obtaining transparent microspheres without the use of a plasma gun or other specialized equipment.
Conventional barium titanate-based glass bead compositions are typically based on the compound BaTiO
3
, or a BaO/TiO
2
eutectic, and can contain silica or a higher index oxide such as tin oxide or zinc oxide. They can contain greater than about 55% titania, although lower titania compositions comprising primarily lead oxide are also known. Barium titanate-based glass bead compositions are typically low durability glass beads, contain greater than 20% alkaline earth oxides, have not been or are not readily transformed to transparent beads with significant microcrystallinity, and generally contain little or no alumina and zirconia.
Retroreflective articles, and in particular pavement markings, can be found in several colors around the world. However, the most common colors are white and yellow (or yellow-orange). Achieving white retroreflection at night is obtained by the use of clear microspheres with a pigmented white background material or binder. For yellow retroreflection, clear beads can be used with background materials or binders that are pigmented heavily with yellow colorants.
Alternatively, a preferred approach for achieving yellow retroreflection is to employ yellow glass beads. For example, U.S. Pat. No. 3,294,559 (Searight) relates to glass compositions which have been found particularly suitable for the manufacture of retro-reflective lens elements to be used in illuminating yellow colored or yellow-orange colored objects.
U.S. Pat. No. 5,286,682 (Jacobs) relates to a pavement marking that has yellow-tinted, retroreflective beads partially embedded in a bead-carrier medium. The bead-carrier medium contains 0.5 to 15 volume percent of a light-scattering agent that scatters white light. The pavement marking is able to retroreflect a distinct yellow color at nighttime without using yellow pigments that contain cadmium, chromium and lead.
SUMMARY
The present invention relates to transparent solid beads (i.e., microspheres) having good mechanical properties characteristic of zirconia/alumina/silica (ZAS) compositions having a low modifier content (e.g., no greater than about 25% by weight of an alkaline earth oxide), higher refractive indices, lower melting temperatures, and improved quenching behavior. The transparent solid beads further comprise at least one transition metal such that the beads impart yellow color to retroreflected light. Preferably, the beads also exhibit a yellow ambient-lit color.
In one embodiment of the present invention, the yellow microspheres include titania plus alumina, zirconia, and/or silica in a total content of at least about 75% by weight, based on the total weight of the solid microspheres. The statement that the compositions include “titania plus alumina, zirconia, and/or silica” means that the compositions include titania and at least one of alumina, zirconia, and silica. Preferably, the total content of titania, alumina, and zirconia (which may not all be present in any one composition) is greater than the content of silica (if it is present). In a preferred embodiment, the titania content is at least about 10% by weight, and more preferably, no greater than about 50% by weight, based on the total weight of the solid microspheres.
The preferred concentration of transition metal oxide is somewhat dependent on the choice of colorant. For yellow beads comprising about 1% by weight to about 4% by weight of iron oxide as a transition metal colorant, the beads preferably comprise about 10 to about 70% by weight titania, about 5 to about 35% by weight zirconia, about 5 to about 40% by weight alumina, 0 to about 25% by weight silica, and 0 to about 25% by weight magnesium oxide, calcium oxide, or mixtures thereof. The amount of iron oxide present preferably ranges from about 2.0% by weight to about 4.0% by weight.
For yellow beads comprising about 1% by weight to about 4% by weight of manganese oxide as a transition metal colorant, the beads preferably comprise about 25 to about 70% by weight titania, about 5 to about 20% by weight zirconia, about 5 to about 30% by weight alumina, 0 to about 20% by weight silica, and 0 to about 25% by weight magnesium oxide, calcium oxide, or mixtures thereof. The amount of manganese oxide present preferably ranges from about 1% by weight to about 3% by weight of the total weight of the microspheres and more preferably ranges from greater than 1.0% by weight to less than 2.5% by weight.
Preferably, the yellow microspheres are glass-ceramic microspheres, which preferably have a nanoscale glass-ceramic microstructure. For certain preferred embodiments, the microspheres include a crystalline phase that includes a titanate compound (e.g., calcium titanate), titanium oxide, a zirconate compound, zirconium oxide, or combinations thereof.
The yellow microspheres preferably have an index of refraction of at least about 1.7 and are useful as lens elements in retroreflective articles. In one embodiment, glass-ceramic microspheres have an index of at least about 2.0 and are particularly useful for wet retroreflective articles.
Yet another preferred embodiment of the present invention includes fused, yellow microspheres. As used her
Bailey John E.
Budd Kenton D.
Frey Matthew H.
Kasai Toshihro
Roscoe Stephen B.
3M Innovative Properties Company
Fischer Carolyn A.
Group Karl
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