Stock material or miscellaneous articles – Structurally defined web or sheet – Continuous and nonuniform or irregular surface on layer or...
Reexamination Certificate
1998-04-17
2002-01-01
Kelly, Cynthia H. (Department: 1774)
Stock material or miscellaneous articles
Structurally defined web or sheet
Continuous and nonuniform or irregular surface on layer or...
C428S148000, C428S149000, C428S325000, C428S329000, C428S330000, C428S331000, C428S402000, C106S419000, C106S442000, C106S444000
Reexamination Certificate
active
06335083
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field
The present invention relates to transparent beads that include alumina and/or zirconia, optionally silica, and optionally alkaline earth metals. More particularly, the present invention relates to fused beads having both transparency and mechanical properties suitable for lens elements. In addition, the present invention also relates to methods for producing such transparent beads.
2. Background
Transparent glass beads (i.e., microspheres) used in reflectors such as reflective sheets and road surface reflectors can be produced by, for example, melting methods. Melting methods used to produce beads 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 are completely amorphous (i.e., noncrystalline), and hence, are often referred to as “vitreous,” “amorphous,” or simply “glass” beads. 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. Examples of beads containing silica, alumina, and/or zirconia formed using the melting methods of the prior art are described in the following documents.
Glass fibers or beads containing 40-65% by weight silica, 1-10% by weight alumina, 1-10% by weight zirconia, and 25-60% by weight calcia are disclosed in Japanese Unexamined Patent Publication No. 51-55428.
Glass fibers containing 45-65% by weight silica, 0-5% by weight alumina, and 0-24% by weight zirconia are disclosed in Japanese Unexamined Patent Publication (Kokai) No. 53-22513.
Glass beads containing 42-52% by weight silica, 10-23% by weight alumina, 1-8% by weight zirconia, and 10-25% by weight calcia are disclosed in Japanese Unexamined Patent Publication (Kokai) No.53-102325.
Glass beads containing 42.5-60% by weight silica, 5-20% by weight alumina, 0-5% by weight zirconia, and 1-15% by weight calcia are disclosed in Japanese Unexamined Patent Publication (Kokai) No. 61-270235.
Glass beads containing 45-55% by weight titania, 0-20% by weight barium oxide, 0-15% by weight zirconia, and 0-20% by weight zinc oxide are disclosed in Japanese Unexamined Patent Publication (Kokai) No. 53-88815.
Glass beads containing 28-48% by weight silica, 5-20% by weight alumina, 0-5% by weight zirconia, and 20-45% by weight lead oxide are disclosed in Japanese Unexamined Patent Publication (Kokai) No. 55-126548.
Glass beads containing 35-55% by weight silica, 15-35% by weight alumina, 2-12% by weight titania, 6% by weight or less zirconia, 0.5-10% by weight boron oxide, and 0-20% by weight calcia are disclosed in Japanese Unexamined Patent Publication Kokai) No. 56-41852.
Glass beads containing 28-65% by weight silica, 1-15% by weight alumina, 10-45% by weight zinc oxide, and 5-25% by weight boron oxide are disclosed in Japanese Unexamined Patent Publication (Kokai) Nos. 60-215549 and 61-68349.
Low melting point, crystallized glass containing 4.5-34% by weight silica, 17-42% by weight alumina, and 13.5-40% by weight boron oxide is disclosed in Japanese Unexamined Patent Publication (Kokai) Nos. 55-20254 and 55-20256.
Hollow glass beads containing 40-59% by weight silica, 0-13% by weight alumina, 640% by weight zirconia (or titania), and 5-25% by weight calcia are disclosed in Japanese Unexamined Patent Publication (Kokai) No. 5-85771.
Bead glass containing 30-50% by weight silica, 2-15% by weight alumina, 2-15% by weight zirconia, 10% by weight or less calcia, 0-15% by weight titania and 2-12% by weight boron oxide is disclosed in Japanese Unexamined Patent Publication (Kokai) No. 55-126547.
Beads with higher levels of alumina and/or zirconia can also be formed by melting methods, but tend to be crystalline or partly crystalline. Such beads can have correspondingly higher hardness, strength, and/or toughness. They have been used as grinding media and similar applications. For example, refractory spheroidal particles or pellets containing up to 60% by weight silica, up to 22% by weight alumina, and 45-75% by weight zirconia are disclosed in U.S. Pat. No. 2,924,533 (McMullen). These refractory particles include crystalline zirconia, with or without crystalline mullite (a stable form of aluminum silicate), embedded in a siliceous glassy matrix. The crystals (i.e., region of material which has boundaries inside of which the well-defined periodic structure is continuous) of zirconia have a maximum length of around 2 microns. However, such beads are generally opaque due to light scattering from grain boundaries (i.e., boundaries between variably oriented crystals) and boundaries between amorphous and crystalline phases. Highly opaque bodies and glazes result when high index crystals such as zirconia are present in a low index matrix such as fused silica. Maximum opacity results when the size of the high index crystals is near the wavelength of light, for example, about 0.5 micron to 1.0 micron.
Accordingly, it is difficult to efficiently and reproducibly produce glass beads in high yields having a high zirconia and/or high alumina content (e.g., greater than about 40% by weight) that are transparent. This is particularly true using standard melt processes.
Sol-gel techniques have been used to produce transparent beads containing silica, alumina, and/or zirconia. Transparency is achieved because the crystal or grain sizes are typically very small (e.g., less than about 0.1 micron). For example, U.S. Pat. No. 4,564,556 (Lange), discloses solid, transparent, nonvitreous, ceramic beads containing at least one metal oxide phase, such as silica, alumina, and zirconia. Generally, sol-gel techniques involve converting a soluble precursor, colloidal dispersion, sol, aquasol, or hydrosol of a metal oxide (or precursor thereof) to a gel, in which the mobility of the components is restrained. The gelling step is typically followed by drying and then firing to obtain a ceramic, nonvitreous material. Such sol-gel methods produce good quality ceramic beads; however, the processing costs can be quite high.
An object of the present invention is to provide transparent fused beads having alumina and/or zirconia, optionally silica, and optionally alkaline earth metal oxides, that also have good mechanical properties. A further object of the invention is to provide crystalline or partially crystalline fused beads having high transparency and good retroreflectivity. Another object of the present invention is to provide methods for producing these transparent fused beads.
SUMMARY OF THE INVENTION
The present invention provides transparent solid, fused beads (i.e., microspheres). In one embodiment, the microspheres include alumina, zirconia, and silica in a total content of at least about 70% by weight, based on the total weight of the solid, fused microspheres, wherein the total content of alumina and zirconia is greater than the content of silica, and further wherein the microspheres have an index of refraction of at least about 1.6 and are useful as lens elements.
In another embodiment, transparent solid, fused microspheres comprise: alumina, zirconia, and silica in a total content of at least about 70% by weight, based on the total weight of the solid, fused microspheres, wherein: (i) there is no greater than about 35% by weight silic
Budd Kenton D.
Kasai Toshihiro
Laird James A.
Lieder Stephen L.
Matsumoto Kenji
3M Innovative Properties Company
Fischer Carolyn A.
Garrett Dawn L.
Kelly Cynthia H.
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