Transparent microspheres

Details Transparent microspheres Transparent microspheres

1999-07-27

2001-06-12

Group, Karl (Department: 1755)

Compositions: ceramic

Ceramic compositions

Glass compositions, compositions containing glass other than...

C501S003000, C501S033000, C501S010000

Reexamination Certificate

active

06245700

ABSTRACT:

TECHNICAL FIELD
The present invention relates to transparent microspheres (i.e., beads) that preferably include titania plus alumina, zirconia, and/or silica. More particularly, the present invention relates to fused microspheres 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 the 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.
SUMMARY
What is needed are 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.
In one embodiment of the present invention, the 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 particularly 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.
Preferably, the 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.
Yet another preferred embodiment of the present invention includes fused microspheres. As used herein, “fused” microspheres refer to those that are prepared by a melt process, as opposed to a sol-gel process. Such fused microspheres may be completely amorphous (i.e., noncrystalline) or they may have crystalline and noncrystalline regions.
Preferably, the microspheres 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.
The present invention also provides a retroreflective article comprising transparent solid microspheres described above. One preferred article is a pavement marking tape comprising a backing and transparent solid microspheres coated thereon. The present invention also provides a pavement marking comprising transparent solid microspheres described above.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The present invention provides transparent, solid microspheres (i.e., beads) of various compositions containing titania and at least one of alumina, zirconia, and silica. It is preferable that the microspheres exhibit a relatively low liquidus temperature (preferably, no greater than about 1400° C., and more preferably, no greater than about 1300° C.), and form a clear, transparent glass when quenched. Also, preferred microspheres form a microcrystalline glass-ceramic structure via heat treatment yet remain transparent.
Upon initial formation from a melt, typically the beads are substantially amorphous (but can contain some crystallinity); however, upon further heat treatment, the beads can develop crystallinity in the form of a nanoscale glass ceramic microstructure (i.e., microstructure in which a significant volume fraction of crystals less than about 100 nanometers in diameter has grown from within an initially amorphous structure). Surprisingly, even with this crystal formation, the compositions remain transparent. Preferably, the size of the crystals in the crystalline phase is less than about 20 nanometers (0.02 micron) in diameter. Crystals of this size are not typically effective light scatterers, and therefore, do not decrease the transparency significantly.
Typically, fused beads (i.e., those made from a melt process) comprise a dense, atomistically homogeneous glass network from which nanocrystals can nucleate and grow during subsequent heat treatment. Sol-gel beads typically comprise a mixture of amorphous material, such as sintered colloidal silica, and nanocrystalline components, such as zirconia, which crystallize during chemical precursor decomposition or sintering. The remaining amorphous matrix of sol-gel beads tends to be less resistant to further crystallization and opacification than that of fused beads. This is particularly true for alkaline earth containing compositions.
The terms “beads” and “microspheres” are used interchangeably and refer to particles that are substantially, although perhaps not exactly, spherical. The term “solid” refers to beads that are not hollow, i.e., they lack substantial cavities or voids. To be optionally usefu

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