U.S. patent application number 10/556978 was filed with the patent office on 2006-12-28 for microspheres comprising titania and bismuth oxide.
Invention is credited to Matthew H. Frey.
Application Number | 20060293161 10/556978 |
Document ID | / |
Family ID | 33516933 |
Filed Date | 2006-12-28 |
United States Patent
Application |
20060293161 |
Kind Code |
A1 |
Frey; Matthew H. |
December 28, 2006 |
Microspheres comprising titania and bismuth oxide
Abstract
The present invention relates to microspheres (i.e., beads) that
comprise titania and bismuth oxide. The glass microspheres further
comprise zirconia. The invention also relates to retroreflective
articles, and in particular pavement markings, comprising such
microspheres.
Inventors: |
Frey; Matthew H.; (Cottage
Grove, MN) |
Correspondence
Address: |
3M INNOVATIVE PROPERTIES COMPANY
PO BOX 33427
ST. PAUL
MN
55133-3427
US
|
Family ID: |
33516933 |
Appl. No.: |
10/556978 |
Filed: |
April 20, 2004 |
PCT Filed: |
April 20, 2004 |
PCT NO: |
PCT/US04/12173 |
371 Date: |
November 16, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10458955 |
Jun 11, 2003 |
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10556978 |
Nov 16, 2005 |
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Current U.S.
Class: |
501/33 |
Current CPC
Class: |
C03B 19/1055 20130101;
C04B 2235/3217 20130101; C03B 2201/30 20130101; C03C 3/062
20130101; C03C 3/122 20130101; C04B 2235/9653 20130101; C03B
2201/42 20130101; C04B 2235/3244 20130101; G02B 5/128 20130101;
C03B 2201/60 20130101; C03B 2201/40 20130101; C03C 12/02 20130101;
C04B 35/653 20130101; C04B 35/462 20130101; C03B 19/102 20130101;
C04B 2235/3213 20130101; C04B 2235/3272 20130101; Y10T 428/2982
20150115; C03C 10/00 20130101; C04B 2235/3284 20130101; C04B
2235/3418 20130101; C04B 2235/3298 20130101; C03C 3/127 20130101;
C04B 2235/528 20130101; C04B 2235/3215 20130101; C03B 19/1045
20130101 |
Class at
Publication: |
501/033 |
International
Class: |
C03C 12/00 20060101
C03C012/00; C03C 12/02 20060101 C03C012/02 |
Claims
1. (canceled)
2. The method of claim 21 wherein the microspheres are
transparent.
3. The method of claim 21 wherein the amount of bismuth oxide
ranges up to 30 wt-%.
4. The method of claim 21 wherein the amount of titania ranges up
to about 92 wt-%.
5. The method spheres of claim 21 wherein the microspheres further
comprise at least 10 wt-% alkaline earth metal oxides.
6. (canceled)
7. The method of claim 5 wherein the at least 10 wt-% alkaline
earth metal oxides comprise at least 5 wt-% CaO, at least 5 wt-%
BaO, and optionally at least 5 wt-% SrO.
8. (canceled)
9. The method of claim 21 further comprising up to about 20 wt-%
zinc oxide.
10. The method of claim 21 wherein the microspheres are fused.
11. The method of claim 21 wherein the microspheres have an index
of refraction of greater than 2.2.
12-20. (canceled)
21. A method of producing microspheres comprising: a) providing a
starting composition comprising at least 5 wt-% bismuth oxide and
at least 50 wt-% titania; b) melting that starting compostion to
form molten droplets; c) cooling the molten droplet to form
quenched fused microspheres; and d) heating the quenched fused
microspheres such that microspheres having a glass-ceramic
structure are formed.
22. The method of claim 21 comprising at least 50 wt-% titania, at
least 5 wt-% bismuth oxide and at least 5 wt-% zirconia, based on
the total weight of the microspheres.
23-26. (canceled)
27. The method of claim 22 wherein the amount of titania ranges up
to about 84 wt-%.
28-35. (canceled)
Description
FIELD OF THE INVENTION
[0001] The present invention relates to microspheres (i.e., beads)
that comprise titania and bismuth oxide. The glass microspheres
further comprise zirconia The invention also relates to
retroreflective articles, and in particular pavement markings,
comprising such microspheres.
BACKGROUND OF THE INVENTION
[0002] Transparent glass and glass-ceramic microspheres (i.e.,
beads) are used as optical elements for retroreflective sheeting
and pavement markings. Such microspheres can be produced, for
example, by melting methods. Such melting methods may include
melting a raw material composition in the form of particulate
material. The melted particles can be quenched, in air or water for
example, to give solid beads. Optionally, quenched particles can be
crushed to form particles of a smaller desired size for the final
beads. The crushed particles can be passed through a flame having a
temperature sufficient to melt and spheroidize them. For many raw
material compositions this is a temperature of about 1500.degree.
C. to about 2000.degree. C. Alternatively, the melted raw material
composition can be poured continuously into a jet of high velocity
air. Molten droplets are formed as the jet impinges on the liquid
stream. The velocity of the air and the viscosity of the melt are
adjusted to control the size of the droplets. The molten droplets
are rapidly quenched, in air or water for example, to give solid
beads. Beads formed by such melting methods are normally composed
of a vitreous material that is essentially completely amorphous
(i.e., noncrystalline), and hence, the beads are often referred to
as "vitreous," "amorphous," or simply "glass" beads or
microspheres.
[0003] One exemplary patent that relates to microspheres is U.S.
Pat. No. 6,335,083 (Kasai et al.), that relates to solid, fused
microspheres. In one embodiment, the microspheres contain 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, zirconia and titania is
greater than the content of silica.
[0004] Other exemplary patents include U.S. Pat. Nos. 6,245,700 and
6,461,988 (Budd et al.) that relate to transparent, solid
microspheres that contain 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, wherein the total
content of alumina, zirconia and titania is greater than the
content of silica.
[0005] Although, the microspheres of U.S. Pat. No. 6,335,083 (Kasai
et al.) and U.S. Pat. Nos. 6,245,700 and 6,461,988 (Budd et al.)
exhibit sufficient transparency and mechanical properties for use
as retroreflective lens elements for retroreflective articles such
as pavement markings, industry would find advantage in microsphere
compositions having improved properties and methods of making such
microspheres.
SUMMARY OF THE INVENTION
[0006] In one aspect, the present invention discloses microspheres,
comprising a glass-ceramic structure where the microspheres
comprise at least 5% bismuth oxide and at least 50% titania based
on the total weight of the microspheres. The amount of titania
preferably ranges up to about 92%. The microspheres comprise a
glass-ceramic structure comprising nanoscale crystals. The
nanoscale crystals have dimensions less than about 100 nanometers.
The nanoscale crystals preferably comprise at least 20 volume % of
the microspheres (e.g. at least 50 volume %). The microspheres may
comprise crystals greater than about 100 nanometers in dimension
provided such crystals comprise less than 20 volume % of the
microspheres.
[0007] In another aspect, the present invention discloses
microspheres, comprising at least 50% titania, at least 5% bismuth
oxide and at least 5% zirconia, based on the total weight of the
microspheres. The amount of titania may range up to about 84%. Such
microspheres may be glass or glass-ceramic.
[0008] In each of these aspects, the microspheres are preferably
transparent. Further, the amount of bismuth oxide may range up to
30%. The microspheres may comprise at least 10% alkaline earth
metal oxides. The amount of alkaline earth metal oxides typically
ranges up to 35%. The microspheres may comprises up to about 10%
zinc oxide. The microspheres are preferably fused. The microspheres
preferably have an index of refraction of greater than 2.2.
[0009] In other aspects the invention relates to retroreflective
articles comprising a binder and the glass-ceramic and/or glass
microspheres of the invention.
[0010] In another aspect, the invention relates to a
retroreflective element comprising a core (e.g. ceramic) and the
microspheres of the invention partially embedded in the core.
[0011] In another aspect, the invention relates to pavement
markings comprising a binder and the microspheres and/or the
reflective elements.
[0012] In yet another aspect, the invention relates to a method of
producing microspheres comprising providing at least one of the
previously described starting compositions, melting the composition
to form molten droplets, cooling the molten droplet to form
quenched fused microspheres, and heating the quenched fused
microspheres.
DESCRIPTION OF THE DRAWINGS
[0013] FIG. 1 is an X-ray diffraction plot for exemplary glass
beads of the invention.
[0014] FIG. 2 is an X-ray diffraction plot for exemplary
glass-ceramic beads of the invention.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0015] The present invention provides microspheres (i.e., beads) of
various compositions comprising titania (TiO.sub.2) and bismuth
oxide (Bi.sub.2O.sub.3). The glass microspheres further comprise
zirconia (ZrO.sub.2) in an amount of at least 5%. Further, the
glass and glass-ceramic microspheres of the invention preferably
further comprise at least 15% of at least one alkaline earth metal
oxide.
[0016] 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., free of substantial cavities or
voids. For use as lens elements, the beads are preferably spherical
and preferably solid. Solid beads are typically more durable than
hollow beads, particularly when exposed to freeze-thaw cycles.
[0017] The microspheres described herein are preferably
transparent. The term "transparent" means that the beads when
viewed under an optical microscope (e.g., at 100.times.) have the
property of transmitting rays of visible light so that bodies
beneath the beads, such as bodies of the same nature as the beads,
can be clearly seen through the beads when both are immersed in oil
of approximately the same refractive index as the beads. Although
the oil should have an index of refraction approximating that of
the beads, it should not be so close that the beads seem to
disappear (as they would in the case of a perfect index match). The
outline, periphery, or edges of bodies beneath the beads are
clearly discernible. The microspheres described herein are
preferably prepared from a melt process. Microspheres prepared from
a melt process are described herein as "fused." For ease in
manufacturing, it is preferred that the microsphere composition
exhibits a relatively low liquidus temperature, such as less than
about 1700.degree. C., and preferably less than about 1600.degree.
C. Typically the liquidus temperature is less than about
1500.degree. C.
[0018] Upon initial formation from a melt, beads are formed that
are substantially amorphous yet can contain some crystallinity. The
compositions preferably form clear, transparent glass microspheres
when quenched. Upon further heat treatment, the beads can develop
crystallinity in the form of a glass-ceramic structure, i.e.,
microstructure in which crystals have grown from within an
initially amorphous structure, and thus become glass-ceramic beads.
Upon heat treatment of quenched beads, the beads can develop
crystallinity in the form of a nanoscale glass-ceramic structure,
i.e., microstructure in which crystals less than about 100
nanometers in dimension have grown from within an initially
amorphous structure, and thus become glass-ceramic beads. A
nanoscale glass-ceramic microstructure is a microcrystalline
glass-ceramic structure comprising nanoscale crystals. For the
purposes of the present invention, microspheres exhibiting X-ray
diffraction consistent with the presence of a crystalline phase are
considered glass-ceramic microspheres. An approximate guideline in
the field is that materials comprising less than about 1 volume %
crystals may not exhibit detectable crystallinity in typical powder
X-ray diffraction measurements. Such materials are often considered
"X-ray amorphous" or glass materials, rather than ceramic or
glass-ceramic materials. Microspheres comprising crystals that are
detectable by X-ray diffraction measurements typically necessary to
be present in an amount greater than or equal to 1 volume % for
detectability, are considered glass-ceramic microspheres, for the
purposes of the present invention. X-ray diffraction data can be
collected using a Philips Automated Vertical Diffractometer with
Type 150 100 00 Wide Range Goniometer, sealed copper target X-ray
source, proportional detector, variable receiving slits,
0.2.degree. entrance slit, and graphite diffracted beam
monochromator (Philips Electronics Instruments Company, Mahwah,
N.J.), with measurement settings of 45 kV source voltage, 35 mA
source current, 0.04.degree. step size, and 4 second dwell time.
Likewise as used herein "glass microspheres" refers to microspheres
having less than 1 volume % of crystals. Preferably, the
glass-ceramic microspheres comprise greater than 10 volume %
crystals. More preferably, the glass-ceramic microspheres comprise
greater than 25 volume % crystals. Most preferably, the
glass-ceramic microspheres comprise greater than 50 volume %
crystals.
[0019] In preferred embodiments, the microspheres form a
microcrystalline glass-ceramic structure via heat treatment yet
remain transparent. For good transparency, it is preferable that
the microspheres comprise little or no volume fraction of crystals
greater than about 100 nanometers in dimension. Preferably, the
microspheres comprise less than 20 volume % of crystals greater
than about 100 nanometers in dimension, more preferably less than
10 volume %, and most preferably less than about 5 volume %.
Preferably, the size of the crystals in the crystalline phase is
less than about 20 nanometers (0.02 micron) in their largest linear
dimension. Crystals of this size typically do not scatter visible
light effectively and: therefore, do not decrease the transparency
significantly.
[0020] Beads of the present invention are particularly useful as
lens elements in retroreflective articles. Transparent beads
according to the present invention have an index of refraction of
at least about 2.0, typically at least about 2.1, more typically at
least 2.2, preferably at least about 2.3, and more preferably at
least about 2.4. For retroreflective applications in water or a wet
environment, the beads preferably have a high index of refraction.
An advantage of the compositions of the present invention is the
ability to provide microspheres having a relatively higher index of
refraction and thus enhanced wet reflectivity.
[0021] Although such high index of refraction beads have been
demonstrated in the past, such bead compositions usually contain
relatively high concentrations of PbO, CdO or Bi.sub.2O.sub.3. The
presence of such high concentrations of Bi.sub.2O.sub.3 leads to
undesirable yellow coloration. Also, Bi.sub.2O.sub.3 sources are
generally more expensive than sources of most other metal oxides,
and therefore it is preferred not to manufacture microspheres with
high concentrations of Bi.sub.2O.sub.3. PbO and CdO may be included
to raise the index of refration. However, these component are
typically avoided. Beads of the invention can be made and used in
various sizes. It is uncommon to deliberately form beads smaller
than 10 .mu.m in diameter, though a fraction of beads down to 2
.mu.m or 3 .mu.m in diameter is sometimes formed as a by-product of
manufacturing larger beads. Accordingly, the beads are preferably
at least 20 .mu.m, (e.g. at least 30 .mu.m, at least 40 .mu.m, at
least 50 .mu.m.) Generally, the uses for high index of refraction
beads call for them to be less than about 2 millimeters in
diameter, and most often less than about 1 millimeter in diameter
(e.g. less than 750 .mu.m, less than 500 .mu.m, less than 300
.mu.m).
[0022] The components of the beads are described as oxides, i.e.
the form in which the components are presumed to exist in the
completely processed glass and glass-ceramic beads as well as
retroreflective articles, and the form that correctly accounts for
the chemical elements and the proportions thereof in the beads. The
starting materials used to make the beads may include some chemical
compound other than an oxide, such as a carbonate. Other starting
materials become modified to the oxide form during melting of the
ingredients. Thus, the compositions of the beads of the present
invention are discussed in terms of a theoretical oxide basis.
[0023] The compositions described herein are reported on a
theoretical oxide basis based on the amounts of starting materials
used. These values do not necessarily account for fugitive
materials (e.g., fugitive intermediates) that are volatilized
during the melting and spheroidizing process. Typically, for
example, boria (B.sub.2O.sub.3), alkali metal oxides, and zinc
oxide, are somewhat fugitive. Thus, upon analysis of the finished
bead, as much as 5% of the original amount of boria and/or alkali
metal oxide added to make the final microspheres may be lost during
processing. As is conventional, however, all components of the
final microspheres are calculated based on the amounts of starting
materials and the total weight of the glass forming composition,
and are reported in weight percents of oxides based on a
theoretical basis.
[0024] Microspheres according to the present invention include
titania and bismuth oxide. Both the glass microspheres and
glass-ceramic microspheres of the invention comprise at least 50%,
and more preferably at least 55% titania. The amount of titania in
the glass-ceramic microspheres of the invention ranges up to 92%.
The amount of titania in the glass microspheres of the invention is
slightly less ranging up to 84%. The amount of titania for both the
glass and glass-ceramic microspheres is preferably less than
80%.
[0025] Titania is a high index of refraction metal oxide with a
melting point of 1840.degree. C., and is typically used because of
its optical and electrical properties, but not generally for
hardness or strength. Similar to zirconia, titania is a strong
nucleating agent known to cause crystallization of glass
compositions. Despite its high individual melting point, as a
component in a mixture of certain oxides, titania can lower the
liquidus temperature, while significantly raising the index of
refration of microspheres comprising such mixtures of oxides.
Further, quaternary compositions containing titania are readily
quenched to glasses and controllably crystallized to
glass-ceramics. Hence, compositions of the present invention
comprising titania, bismuth oxide, and optionally zirconia provide
relatively low liquidus temperatures, very high index of refraction
values, high crystallinity when heat-treated appropriately, useful
mechanical properties, and high trasparency.
[0026] Both the glass microspheres and glass-ceramic microspheres
of the invention comprise at least 5% bismuth oxide. Further, the
amount of bismuth oxide ranges up to 30%. In some embodiments, the
amount of bismuth oxide ranges up to 25%. In other embodiments, the
amount of bismuth oxide ranges up to 20% (e.g. 16,%, 17%, 18%,
19%). In some preferred embodiments, the amount of bismuth oxide
ranges up to 15% (e.g. 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13% and
14%).
[0027] The glass microspheres of the invention comprise at least 5%
zirconia; whereas the glass-ceramic microspheres optionally, yet
preferably comprise zirconia. The amount of zirconia for both the
glass microspheres and glass-ceramic microspheres of the invention
ranges up to 30%. Generally, the zirconia contributes chemical and
mechanical durability as well as contributes to the high index of
refraction of the preferred beads. Surprisingly, additions of
zirconia to compositions containing titania in excess of 50% and
bismuth oxide in excess of 5% lead to excellent glass-forming
properties. Also, compositions comprising zirconia and bismuth
oxide, together with titania in excess of 50%, exhibit controlled
crystallization to a glass-ceramic structure with greater than 50
volume % crystallinity and high transparency. Finally, compositions
comprising zirconia and bismuth oxide, together with titania in
excess of 50%, exhibit very high index of refraction values (e.g.,
greater than 2.3) after crystallization.
[0028] In addition, the beads preferably comprise at least one
alkaline earth metal oxide, such as baria (BaO), strontia (SrO),
magnesia (MgO), or calcia (CaO). In certain preferred embodiments,
the total amount of alkaline earth metal oxide(s) is at least 5%
(e.g. 6%, 7%, 8%, 9%, 10%). Further, the total amount of alkaline
earth metal oxide(s) typically ranges up to 35%. Many preferred
embodiments contain less than 20% by weight alkaline earth metal
oxides such as less than 15%. In some embodiments baria and calcia
are included at about equal amount. Further, in some embodiments
the total amount of alkaline earth metal oxide(s) is about twice
that of bismuth oxide and/or zirconia
[0029] In the compositions in which the total content of more than
one component is discussed, the beads may include only one of the
components listed, various combinations of the components listed,
or all of the components listed. For example, if a bead composition
is said to include a total content of baria, strontia, magnesia,
and calcia in an amount of 35% by weight, it can include 35% by
weight of any one of these components or a combination of two,
three or four of these components totaling 35% by weight.
[0030] Alkaline earth modifiers are particularly useful for
lowering the liquidus temperature and for aiding in glass formation
during quenching. With the addition of alkaline earth metal oxides,
the ability to quench to a clear glass is improved, even though the
tendency to crystallize on annealing can be increased. Addition of
magnesia and other alkaline earth metal oxides also can result in
improved crush strength, possibly by controlling crystallization
during the heat treatment step and influencing the resulting
microstructure. Too much alkaline earth metal oxide can result in
poorer mechanical strength or poor chemical resistance to acidic
environments.
[0031] Alternatively, or in addition thereto, the beads of the
invention preferably comprise up to 20% by weight zinc oxide (ZnO).
Typically the amount of zinc oxide is less than about 15% (e.g.
10%). Glass and glass-ceramic beads of the present invention
comprising a minor amount of zinc oxide tend to have the highest
index of refraction values.
[0032] The microspheres may optionally include minor amounts (e.g.
each typically less than 5%) of oxides of elements such as lithium
(Li.sub.2O), sodium (Na.sub.2O), potassium (K.sub.2O), aluminum
(Al.sub.2O.sub.3), silicon (SiO.sub.2), yttrium (Y.sub.2O.sub.3),
tin (SnO.sub.2), boron (B.sub.2O.sub.3), halfnium (HfO.sub.2),
germanium (GeO.sub.2), phosphorous (P.sub.2O.sub.5), antimony
(Sb.sub.2O.sub.5), molybdenum (MoO.sub.3), tungsten (WO.sub.3) and
combinations thereof. Typically, the total amount of such inorganic
oxide is less than 10%, although more may be present provided the
presence thereof does not detrimentally impact the desired
properties of the beads (e.g. index of refraction).
[0033] Specific metal oxides may be included for the improvement of
mechanical properties. For example, one or metal oxides selected
from the following group, typically in amounts of up to 5% each,
can improve mechanical properties; SiO.sub.2, Al.sub.2O.sub.3,
HfO.sub.2, La.sub.2O.sub.3, and Y.sub.2O.sub.3. For some such metal
oxides, Al.sub.2O.sub.3 and SiO.sub.2 for example, higher
concentrations will tend to decrease the index of refraction
undesirably.
[0034] The glass-ceramic microspheres of the invention comprise one
or more crystalline phases, typically totaling at least 5 volume %.
Crystallinity is typically developed though heat-treatment of
amorphous beads, although some glass-ceramic beads according to the
invention and formed by quenching molten droplets may contain
crystals without secondary heat treatment. Such a crystalline phase
or phases may include relatively pure single-component metal oxide
phases of titania (e.g., anatase, rutile), zirconia (e.g.,
baddeleyite), and/or bismuth oxide (e.g., bismite). Also, such a
crystalline phase or phases may include relatively pure
multicomponent metal oxide phases (e.g., Bi.sub.4Ti.sub.3O.sub.12,
ZrTiO.sub.4). Such a crystalline phase or phases may include
crystalline solid solutions that are isostructural with relatively
pure single-component or multicomponent metal oxide phases.
Finally, such crystalline phase or phases may include at least one
heretofore unreported crystalline phase, in terms of crystal
structure and/or composition. The compositions exhibit controlled
crystallization characteristics such that they remain transparent
following heat treatments.
[0035] Colorants can also be included in the beads of the present
invention. Such colorants include, for example, CeO.sub.2,
Fe.sub.2O.sub.3, CoO, Cr.sub.2O.sub.3, NiO, CuO, MnO.sub.2,
V.sub.2O.sub.5 and the like. Typically, the beads of the present
invention include no more than about 5% by weight (e.g. 1%, 2%, 3%,
4%/o) colorant, based on the total weight of the beads (theoretical
oxide basis). Also, rare earth elements, such as praseodymium,
neodymium, europium, erbium, thulium, ytterbium may optionally be
included for fluorescence. Preferably, the microspheres are
substantially free of lead oxide (PbO) and cadmim oxide (CdO).
[0036] Microspheres according to the invention can be prepared by
conventional processes as, for example, disclosed in U.S. Pat. No.
3,493,403 (Tung et al). In one useful process, the starting
materials are measured out in particulate form, each starting
material being preferably about 0.01 .mu.m to about 50 .mu.m in
size, and intimately mixed together. The starting raw materials
include compounds that form oxides upon melting or heat treatment.
These can include oxides, (e.g. titania, bismite, optional
zirconia, and optional alkaline earth metal oxide(s)), hydroxides,
acid chlorides, chlorides, nitrates, carboxylates, sulfates,
alkoxides, and the like, and the various combinations thereof.
Moreover, multicomponent metal oxides such as calcium titanate
(CaTiO.sub.3) and barium titanate (BaTiO.sub.3) can also be
used.
[0037] The oxide mixture is melted in a gas-fired or electrical
furnace until all the staring materials are in liquid form. The
liquid batch can be poured into a jet of high-velocity air. Beads
of the desired size are formed directly in the resulting stream.
The velocity of the air is adjusted in this method to cause a
proportion of the beads formed to have the desired dimensions.
Typically, such compositions have a sufficiently low viscosity and
high surface tension.
[0038] Melting of the starting materials is typically achieved by
heating at a temperature within a range of about 1500.degree. C. to
about 1900.degree. C., and often at a temperature of, for example,
of about 1700.degree. C. A direct heating method using a
hydrogen-oxygen burner or acetylene-oxygen burner, or an oven
heating method using an arc image oven, solar oven, graphite oven
or zirconia oven, can be used to melt the starting materials.
[0039] Alternatively, the liquid is quenched in water, dried, and
crushed to form particles of a size desired for the final beads.
The crushed particles can be screened to assure that they are in
the proper range of sizes. The crushed particles can then be passed
through a flame having a temperature sufficient to remelt and
spheroidize the particles.
[0040] In a preferred method, the starting materials are first
formed into larger feed particles. The feed particles are fed
directly into a burner, such as a hydrogen-oxygen burner or an
acetylene-oxygen burner or a methane-air burner, and then quenched
in water (e.g., in the form of a water curtain or water bath). Feed
particles may be formed by melting and grinding, agglomerating, or
sintering the starting materials. Agglomerated particles of up to
about 2000 .mu.m in size (the length of the largest dimension) can
be used, although particles of up to about 500 .mu.m in size are
preferred. The agglomerated particles can be made by a variety of
well known methods, such as by mixing with water, spray drying,
pelletizing, and the like. The starting material, particularly if
in the form of agglomerates, can be classified for better control
of the particle size of the resultant beads. Whether agglomerated
or not, the starting material may be fed into the burner with the
burner flame in a horizontal orientation. Typically, the feed
particles are fed into the flame at its base. This horizontal
orientation is desired because it can produce very high yields
(e.g., 100%) of spherical particles of the desired level of
transparency.
[0041] The procedure for cooling the molten droplets can involve
air cooling or rapid cooling. Rapid cooling is achieved by, for
example, dropping the molten droplets of starting material into a
cooling medium such as water or cooling oil. In addition, a method
can be used in which the molten droplets are sprayed into a gas
such as air or argon. The resultant quenched fused beads are
typically sufficiently transparent for use as lens elements in
retroreflective articles. For certain embodiments, they are also
sufficiently hard, strong, and tough for direct use in
retroreflective articles. Typically, however, a subsequent heat
treating step is desired to improve their mechanical properties.
Also, heat treatment and crystallization lead to increases in index
of refraction.
[0042] In a preferred embodiment, a bead precursor can be formed
and subsequently heated. As used herein, a "bead precursor" refers
to the material formed into the shape of a bead by melting and
cooling a bead starting composition. This bead precursor is also
referred to herein as a quenched fused bead, and may be suitable
for use without further processing if the mechanical properties and
transparency are of desirable levels. The bead precursor is formed
by melting a starting composition containing prescribed amounts of
raw materials (e.g., titanium raw material, bismuth raw material,
optional raw materials), forming molten droplets of a predetermined
particle size, and cooling those molten droplets. The starting
composition is prepared so that the resulting bead precursor
contains the desired raw materials in predetermined proportions.
The particle size of the molten droplets is normally within the
range of about 10 microns (.mu.m) to about 2,000 .mu.m. The
particle size of the bead precursors as well as the particle size
of the final transparent fused beads can be controlled with the
particle size of the molten droplets.
[0043] Thus, in certain preferred embodiments, a bead precursor
(i.e., quenched fused bead) is subsequently heated. Preferably,
this heating step is carried out at a temperature below the melting
point of the bead precursor. Typically, this temperature is at
least about 750.degree. C. Preferably, it is about 850.degree. C.
to about 1000.degree. C., provided it does not exceed the melting
point of the bead precursor. If the heating temperature of the bead
precursor is too low, the effect of increasing the index of
refraction or the mechanical properties of the resulting beads will
be insufficient. Conversely, if the heating temperature is too
high, bead transparency can be diminished due to light scattering
from large crystals. Although there are no particular limitations
on the time of this heating step to increase index of refraction,
develop crystallinity, and/or improve mechanical properties,
heating for at least about 1 minute is normally sufficient, and
heating should preferably be performed for about 5 minutes to about
100 minutes. In addition, preheating (e.g., for about 1 hour) at a
temperature within the range of about 600.degree. C. to about
800.degree. C. before heat treatment may be advantageous because it
can further increase the transparency and mechanical properties of
the beads.
[0044] The latter method of preheating is also suitable for growing
fine crystal phases in a uniformly dispersed state within a phase.
A crystal phase containing oxides of zirconium, titanium, etc., can
also form in compositions containing high levels of zirconia or
titania upon forming the beads from the melt (i.e., without
subsequent heating). Significantly, the crystal phases are more
readily formed (either directly from the melt or upon subsequent
heat treatment) by including high combined concentrations of
titania and zirconia (e.g., combined concentration greater than
80%).
[0045] Microspheres made from a melt process are characterized as
"fused." Fully vitreous fused microspheres comprise a dense, solid,
atomistically homogeneous glass network from which nanocrystals can
nucleate and grow during subsequent heat treatment. As an
alternative to melt processes, however, the microspheres of the
invention may be prepared by a sol gel technique, such as described
in U.S. Pat. No. 4,564,556 (Lange). Sol-gel beads typically
comprise a mixture of amorphous material, such as sintered
colloidal silica, and nanocrystalline components, such as zirconia,
that crystallize during chemical precursor decomposition or
sintering.
[0046] The crush strength values of the beads of the invention can
be determined according to the test procedure described in U.S.
Pat. No. 4,772,511 (Wood). Using this procedure, the beads
demonstrate a crush strength of preferably at least about 350 MPa,
more preferably at least about 700 MPa
[0047] The durability of the beads of the invention can be
demonstrated by exposing them to a compressed air driven stream of
sand according to the test procedure described in U.S. Pat. No.
4,758,469 (Lange). Using this procedure, the beads are resistant to
fracture, chipping, and abrasion, as evidenced by retention of
about 30% to about 60% of their original reflected brightness.
[0048] Transparent (preferably, fused) beads according to the
present invention are suitable for use for jewelry, abrasives,
abrasion resistant coating coating as well as a wide variety of
retroreflective articles. In some aspects, the beads are employed
directly. Alternatively, or in combination with beads, reflective
elements comprising a core (e.g. ceramic, polymeric) and beads of
the present invention partially embedded in the core, such as
described in U.S. Pat. Nos., 5,774,265 and 3,964,821, may be
employed.
[0049] Reflective articles of the invention share the common
feature of comprising the inventive beads and/or reflective element
comprising such beads at least partially embedded in a binder
material.
[0050] In some aspects, the beads and/or reflective elements are
employed in liquid (e.g. pavement) marking applications wherein the
beads and/or reflective elements are sequentially or concurrently
dropped-on a liquified binder or compounded within a liquified
binder. The liquidified binder may be a traffic paint such as
described in U.S. Pat. No. 3,645,933; U.S. Pat. No. 6,132,132; and
U.S. Pat. No. 6,376,574. Other binder materials include
thermoplastics such as described in U.S. Pat. No. 3,036,928; U.S.
Patent No. 3,523,029; and U.S. Pat. No. 3,499,857; as well as
two-part reactive binders including epoxies such as described in
U.S. Pat. Nos. 3,046,851 and 4,721,743, and polyureas such as
described in U.S. Pat. No. 6,166,106.
[0051] In other aspects, beads and/or reflective elements are
employed in retroreflective sheeting including exposed lens,
encapsulated lens, embedded lens, or enclosed lens sheeting.
Representative pavement-marking sheet material (tapes) as described
in U.S. Pat. No. 4,248,932 (Tung et al.), U.S. Pat. No. 4,988,555
(Hedblom); U.S. Pat. No. 5,227,221 (Hedblom); U.S. Pat. No;
5,777,791 (Hedblom); and U.S. Pat. No. 6,365,262 (Hedblom). In
addition to pavement-marking sheet material, sheeting useful for
retroreflective signs may incorporate microspheres of the present
invention.
[0052] Pavement marking sheet material generally includes a
backing, a layer of binder material, and a layer of beads partially
embedded in the layer of binder material. The backing, which is
typically of a thickness of less than about 3 mm, 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.
[0053] Pavement marking sheetings 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 beads according to the present invention 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.
[0054] Other applications for retroreflective sheeting or liquid
marking materials incorporating beads of the invention include
graphics and signing in marine settings, where wet reflectivity is
desired. For example, applied graphics and signing on motorized
watercraft, floating markers, or stationary shoreline structures
may incorporate beads of the invention.
EXAMPLES
[0055] The following provides an explanation of the present
invention with reference to its examples and comparative examples.
Furthermore, it should be understood that the present invention is
no way limited to these examples. All percentages are in weight
percents, based on the total weight of the compositions, unless
otherwise specified.
Example 1
[0056] 16g of zirconium oxide (commercially available from Z-TECH
division of Carpenter Engineering Products, (Bow, N.H.) under the
trade designation "CF-PLUS-HM"), 130 g of titanium oxide
(commercially available from Fisher Scientific (Fair Lawn, N.J.)
under the trade designation "T315-500"), 18 g of bismuth trioxide
(commercially available from Fisher Scientific (Fair Lawn, N.J.)
under the trade designation "B-339"), 23.4 g of barium carbonate
(commercially available from Chemical Products Corporation
(Cartersville, Ga.) under the trade designation "Type S"), and 32.1
g of calcium carbonate (commercially available from Akrochem
Corporation (Akron, Ohio) under the trade designation "Hubercarb
Q325") were combined in a porcelain jar mill with 350 g of water
and 1600 g of 1 cm diameter zirconium oxide milling media The
ingredients were combined in appropriate proportions for the
preparation of beads with the following base composition:
[0057] 65% TiO.sub.2
[0058] 9% Bi.sub.2O.sub.3
[0059] 9% BaO
[0060] 9% CaO
[0061] 8% Zro.sub.2
[0062] The slurry was milled for 24 hours and then dried overnight
at 100.degree. C. to yield a mixed powder cake with the components
homogeneously distributed. After grinding with a mortar and pestle,
dried and sized particles (<250 microns diameter) were fed into
the flame of a hydrogen/oxygen torch (commercially available from
Bethlehem Apparatus Company, Hellertown Pa. under the trade
designation "Bethlehem Bench Burner PM2D Model-B"), referred to as
"Bethlehem burner" hereinafter. The Bethlehem burner delivered
hydrogen and oxygen at the following rates, standard liters per
minute (SLPM): TABLE-US-00001 Hydrogen Oxygen Inner ring 8.0 3.0
Outer ring 23.0 9.8 Total 31.0 12.8
The particles were melted by the flame and transported to a water
quenching vessel. The quenched particles were dried and then passed
through the flame of the Bethlehem burner a second time, where they
were melted again and transported to the water quenching vessel.
The beads were collected and examined using an optical microscope.
They measured about 40 to 250 microns in diameter and a majority of
the beads were clear and substantially free of defects (e.g.,
optically visible inclusions, bubbles). The measured index of
refraction for the beads is given in Table I. The index of
refraction can be measured by the Becke method, which is disclosed
in F. Donald Bloss, "An Introduction to the Methods of Optical
Crystallography," Holt, Rinehart and Winston, N.Y., pp. 47-55
(1961), the disclosure of which is incorporated herein by
reference.
Example 2
[0063] Following the procedure set forth in example 1, a batch of
beads was prepared using the following raw materials: 12 g of
zirconium oxide (commercially available from Z-TECH division of
Carpenter Engineering Products, (Bow, N.H.) under the trade
designation "CF-PLUS-HM"), 140 g of titanium oxide (commercially
available from Fisher Scientific (Fair Lawn, N.J.) under the trade
designation "T315-500"), 16 g of bismuth trioxide (commercially
available from Fisher Scientific (Fair Lawn, N.J.) under the trade
designation "B-339"), 20.6 g of barium carbonate (commercially
available from Chemical Products Corporation (Cartersville, Ga.)
under the trade designation "Type S"), and 28.6 g of calcium
carbonate (commercially available from Airochem Corporation (Akron,
Ohio) under the trade designation "Hubercarb Q325"). The
ingredients were combined in appropriate proportions for the
preparation of beads with the following base composition:
[0064] 70% TiO.sub.2
[0065] 8% Bi.sub.2O.sub.3
[0066] 8% BaO
[0067] 8% CaO
[0068] 6% ZrO.sub.2
[0069] The beads were examined using an optical microscope. They
measured about 40 to 250 microns in diameter and a majority of the
beads were clear and substantially free of defects (e.g., optically
visible inclusions, bubbles). The measured index of refraction for
the beads is given in Table I.
Example 3
[0070] Following the procedure set forth in example 1, a batch of
beads was prepared using the following raw materials: 24 g of
zirconium oxide (commercially available from Z-TECH division of
Carpenter Engineering Products, (Bow, N.H.) under the trade
designation "CF-PLUS-HM"), 126 g of titanium oxide (commercially
available from Fisher Scientific (Fair Lawn, N.J.) under the trade
designation "T315-500"), 18 g of bismuth trioxide (commercially
available from Fisher Scientific (Fair Lawn, N.J.) under the trade
designation "B-339"), 20.6 g of barium carbonate (commercially
available from Chemical Products Corporation (Cartersville, Ga.)
under the trade designation "Type S"), and 28.6 g of calcium
carbonate (commercially available from Airochem Corporation (Akron,
Ohio) under the trade designation "Hubercarb Q325"). The
ingredients were combined in appropriate proportions for the
preparation of beads with the following base composition:
[0071] 63% TiO.sub.2
[0072] 12% ZrO.sub.2
[0073] 9% Bi.sub.2O.sub.3
[0074] 8% BaO
[0075] 8% CaO
[0076] The beads were examined using an optical microscope. They
measured about 40 to 250 microns in diameter and a majority of the
beads were clear and substantially free of defects (e.g., optically
visible inclusions, bubbles). The measured index of refraction for
the beads is given in Table I.
Example 4
[0077] Following the procedure set forth in Example 1, a batch of
beads was prepared using the following raw materials: 20 g of
zirconium oxide (commercially available from Z-TECH division of
Carpenter Engineering Products, (Bow, N.H.) under the trade
designation "CF-PLUS-HM"), 120 g of titanium oxide (commercially
available from Fisher Scientific (Fair Lawn, N.J.) under the trade
designation "T315-500"), 20 g of bismuth trioxide (commercially
available from Fisher Scientific (Fair Lawn, N.J.) under the trade
designation "B-339"), 25.7 g of barium carbonate (commercially
available from Chemical Products Corporation (Cartersville, Ga.)
under the trade designation "Type S"), and 35.7 g of calcium
carbonate (commercially available from Akrochem Corporation (Akron,
Ohio) under the trade designation "Hubercarb Q325"). The
ingredients were combined in appropriate proportions for the
preparation of beads with the following base composition:
[0078] 60% TiO.sub.2
[0079] 10% ZrO.sub.2
[0080] 10% Bi.sub.2O.sub.3
[0081] 10% BaO
[0082] 10% CaO
[0083] The beads were examined using an optical microscope. They
measured about 40 to 250 microns in diameter and a majority of the
beads were clear and substantially free of defects (e.g., optically
visible inclusions, bubbles). X-ray diffraction analysis confirmed
that the beads were quenched in the amorphous state. FIG. 1 is a
plot of the X-ray diffraction data. The measured index of
refraction for the beads is given in Table I. A crush strength
value was determined for the microspheres, following the test
procedure described in U.S. Pat. No. 4,772,511 (Wood). The
microspheres measured 540 MPa in crush strength.
Example 5
[0084] Following the procedure set forth in example 1, a batch of
beads was prepared using the following raw materials: 14 g of
zirconium oxide (commercially available from Z-TECH division of
Carpenter Engineering Products, (Bow, N.H.) under the trade
designation "CF-PLUS-HM"), 130 g of titanium oxide (commercially
available from Kerr McGee (Oklahoma City, Okla.) under the trade
designation "Kemira 110"), 24 g of bismuth trioxide (commercially
available from Fisher Scientific (Fair Lawn, N.J.) under the trade
designation "B-339"), 20.6 g of barium carbonate (commercially
available from Chemical Products Corporation (Cartersville, Ga.)
under the trade designation "Type S"), and 28.6 g of calcium
carbonate (commercially available from Akrochem Corporation (Akron,
Ohio) under the trade designation "Hubercarb Q325"). The
ingredients were combined in appropriate proportions for the
preparation of beads with the following base composition:
[0085] 65% TiO.sub.2
[0086] 12% Bi.sub.2O.sub.3
[0087] 8% BaO
[0088] 8% CaO
[0089] 7% ZrO.sub.2
[0090] The beads were examined using an optical microscope. They
measured about 40 to 250 microns in diameter and a majority of the
beads were clear and substantially free of defects (e.g., optically
visible inclusions, bubbles). The measured index of refraction for
the beads is given in Table L.
Example 6
[0091] Following the procedure set forth in example 1, a batch of
beads was prepared using the following raw materials: 16 g of
zirconium oxide (commercially available from Z-TECH division of
Carpenter Engineering Products, (Bow, N.H.) under the trade
designation "CF-PLUS-HM"), 126 g of titanium oxide (commercially
available from Kerr McGee (Oklahoma City, Okla.) under the trade
designation "Kemira 110"), 16 g of bismuth trioxide (commercially
available from Fisher Scientific (Fair Lawn, N.J.) under the trade
designation "B-339"), 20.6 g of barium carbonate (commercially
available from Chemical Products Corporation (Cartersville, Ga.)
under the trade designation "Type S"), and 19.7 g of calcium
carbonate (commercially available from Akrochem Corporation (Akron,
Ohio) under the trade designation "Hubercarb Q325"), 5 g of
aluminum oxide (commercially available from from ALCOA Industrial
Chemicals, (Pittsburgh, Pa.) under the trade designation "16SG"),
and 10 g of wollastonite ((CaSiO.sub.3) powder, commercially from
R.T. Vanderbilt (Norwalk, Conn.) under the trade designation
"Vansil W-30"). The ingredients were combined in appropriate
proportions for the preparation of beads with the following base
composition:
[0092] 63% TiO.sub.2
[0093] 8% ZrO.sub.2
[0094] 8% Bi.sub.2O.sub.3
[0095] 8% BaO
[0096] 8% CaO
[0097] 2.5 wt % Al.sub.2O.sub.3
[0098] 2.5 wt % SiO.sub.2
[0099] The beads were examined using an optical microscope. They
measured about 40 to 250 microns in diameter and a majority of the
beads were clear and substantially free of defects (e.g., optically
visible inclusions, bubbles). The measured index of refraction for
the beads is given in Table I.
Example 7
[0100] Following the procedure set forth in example 1, a batch of
beads was prepared using the following raw materials: 20 g of
zirconium oxide (commercially available from Z-TECH division of
Carpenter Engineering Products, (Bow, N.H.) under the trade
designation "CF-PLUS-HM"), 120 g of titanium oxide (commercially
available from Kerr McGee (Oklahoma City, Okla.) under the trade
designation "Kemira 110"), 20 g of bismuth trioxide (commercially
available from Fisher Scientific (Fair Lawn, N.J.) under the trade
designation "B-339"), 19.3 g of barium carbonate (commercially
available from Chemical Products Corporation (Cartersville, Ga.)
under the trade designation "Type S"), 26.8 g of calcium carbonate
(commercially available from Akrochem Corporation (Akron, Ohio)
under the trade designation "Hubercarb Q325"), 10 g of zinc oxide
(commercially available from EM Science (Cherry Hill, N.J.) under
the trade designation "ZX0090-1"). The ingredients were combined in
appropriate proportions for the preparation of beads with the
following base composition:
[0101] 60% TiO.sub.2
[0102] 10% Zro.sub.2
[0103] 10% Bi.sub.2O.sub.3
[0104] 7.5% BaO
[0105] 7.5% CaO
[0106] 5% ZnO
[0107] The beads were examined using an optical microscope. They
measured about 40 to 250 microns in diameter and a majority of the
beads were clear and substantially free of defects (e.g., optically
visible inclusions, bubbles). The measured index of refraction for
the beads is given in Table I.
Example 8
[0108] Following the procedure set forth in example 1, a batch of
beads was prepared using the following raw materials: 19.4 g of
zirconium oxide (commercially available from Z-TECH division of
Carpenter Engineering Products, (Bow, N.H.) under the trade
designation "CF-PLUS-HM"), 116.4 g of titanium oxide (commercially
available from Kerr McGee (Oklahoma City, Okla.) under the trade
designation "Kemira 110"), 19.4 g of bismuth trioxide (commercially
available from Fisher Scientific (Fair Lawn, N.J.) under the trade
designation "B-339"), 25.0 g of barium carbonate (commercially
available from Chemical Products Corporation (Cartersville, Ga.)
under the trade designation "Type S"), 34.6 g of calcium carbonate
(commercially available from Akrochem Corporation (Akron, Ohio)
under the trade designation "Hubercarb Q325"), and 30.4 g of ferric
nitrate ((Fe(NO.sub.3).sub.3-9H.sub.2O) commercially available from
Fisher (Fair Lawn, N.J.) under the trade designation "1110-500").
The ingredients were combined in appropriate proportions for the
preparation of beads with the following base composition:
[0109] 58.2% TiO.sub.2
[0110] 9.7% ZrO.sub.2
[0111] 9.7% Bi.sub.2O.sub.3
[0112] 9.7% BaO
[0113] 9.7% CaO
[0114] 3% Fe.sub.2O.sub.3
[0115] The beads were black in appearance. They measured about 40
to 250 microns. The quenched black glass beads were placed in an
alumina crucible and heated at a rate of 10.degree. C./minute to
850.degree. C., held at 850.degree. C. for 1 hr, and then allowed
to cool slowly with the furnace through natural dissipation of heat
into the environment. The beads were removed from the furnace after
cooling back to room temperature. After heat treatment, the beads
were converted from a black appearance to a yellow appearance. The
measured index of refraction of the beads is given in Table I.
Example 9
[0116] Beads were prepared according to Example 1. The quenched
beads were placed in an alumina crucible and heated at a rate of
10.degree. C./minute to 850.degree. C., held at 850.degree. C. for
1 hr, and then allowed to cool slowly with the furnace through
natural dissipation of heat into the environment. The beads were
removed from the furnace after cooling back to room temperature.
After heat treatment, the beads remained substantially clear when
viewed using an optical microscope. The measured index of
refraction of the beads is given in. Table I.
Example 10
[0117] Beads were prepared according to Example 2. The quenched
beads were placed in an alumina crucible and heated at a rate of
10.degree. C./minute to 850.degree. C., held at 850.degree. C. for
1 hr, and then allowed to cool slowly with the furnace through
natural dissipation of heat into the environment. The beads were
removed from the furnace after cooling back to room temperature.
After heat treatment, the beads remained substantially clear when
viewed using an optical microscope. The measured index of
refraction of the beads is given in Table I.
Example 11
[0118] Beads were prepared according to Example 3. The quenched
beads were placed in an alumina crucible and heated at a rate of
10.degree. C./minute to 850.degree. C., held at 850.degree. C. for
1 hr, and then allowed to cool slowly with the furnace through
natural dissipation of heat into the environment. The beads were
removed from the furnace after cooling back to room temperature.
After heat treatment, the beads remained substantially clear when
viewed using an optical microscope. The measured index of
refraction of the beads is given in Table I.
Example 12
[0119] Beads were prepared according to Example 4. The quenched
glass beads were placed in an alumina crucible and heated at a rate
of 10.degree. C./minute to 850.degree. C., held at 850.degree. C.
for 1 hr, and then allowed to cool slowly with the furnace through
natural dissipation of heat into the environment. The beads were
removed from the furnace after cooling back to room temperature.
After heat treatment, the beads remained substantially clear when
viewed using an optical microscope. X-ray diffraction analysis
confirmed that the heat-treated beads were substantially
crystalline. FIG. 2 is a plot of the X-ray diffraction data. The
volume percentage crystallinity was estimated to be 60-70%. The
measured index of refraction of the beads is given in Table I.
Example 13
[0120] Beads were prepared according to Example 5. The quenched
beads were placed in an alumina crucible and heated at a rate of
10.degree. C./minute to 850.degree. C., held at 850.degree. C. for
1 hr, and then allowed to cool slowly with the furnace through
natural dissipation of heat into the environment. The beads were
removed from the furnace after cooling back to room temperature.
After heat treatment, the beads remained substantially clear when
viewed using an optical microscope. The measured index of
refraction of the beads is given in Table I.
Example 14
[0121] Beads were prepared according to Example 6. The quenched
beads were placed in an alumina crucible and heated at a rate of
10.degree. C./minute to 850.degree. C., held at 850.degree. C. for
1 hr, and then allowed to cool slowly with the furnace through
natural dissipation of heat into the environment. The beads were
removed from the furnace after cooling back to room temperature.
After heat treatment, the beads remained substantially clear when
viewed using an optical microscope. The measured index of
refraction of the beads is given in Table I.
Example 15
[0122] Beads were prepared according to Example 7. The quenched
beads were placed in an alumina crucible and heated at a rate of
10.degree. C./minute to 850.degree. C., held at 850.degree. C. for
1 hr, and then allowed to cool slowly with the furnace through
natural dissipation of heat into the environment. The beads were
removed from the furnace after cooling back to room temperature.
After heat treatment, the beads remained substantially clear when
viewed using an optical microscope. The measured index of
refraction of the beads is given in Table I.
Example 16
[0123] Following the procedure set forth in example 1, a batch of
beads was prepared using the following raw materials: 120 g of
titanium oxide (commercially available from KRONOS, (Cranbury,
N.J.) under the trade designation "KRONOS 1000"), 26.6 g of bismuth
trioxide (commercially available from Fisher Scientific (Fair Lawn,
N.J.) under the trade designation "B-339"), 34.5 g of barium
carbonate (commercially available from Chemical Products
Corporation (Cartersville, Ga.) under the trade designation "Type
S"), and 47.5 g of calcium carbonate (commercially available from
Akrochem Corporation (Akron, Ohio) under the trade designation
"Hubercarb Q325"). The ingredients were combined in appropriate
proportions for the preparation of beads with the following base
composition:
[0124] 60% TiO.sub.2
[0125] 13.3% Bi.sub.2O.sub.3
[0126] 13.3% BaO
[0127] 13.3% CaO
[0128] The beads were examined using an optical microscope. They
measured about 40 to 250 microns in diameter and a majority of the
beads were clear and substantially free of defects (e.g., optically
visible inclusions, bubbles). The quenched beads were placed in an
alumina crucible and heated at a rate of 10.degree. C./minute to
765.degree. C., held at 765.degree. C. for 30 min, and then allowed
to cool slowly with the furnace through natural dissipation of heat
into the environment. The beads were removed from the furnace after
cooling back to room temperature. After heat treatment, the beads
remained substantially clear when viewed using an optical
microscope. X-ray diffraction confirmed the presence of at least
one crystalline phase, with a total crystalline phase concentration
of about 10 volume %. The measured index of refraction of the beads
is given in Table I.
Example 17
[0129] Following the procedure set forth in example 1, a batch of
beads was prepared using the following raw materials: 126 g of
titanium oxide (commercially available from Aldrich Chemical
Company (Milwaukee, Wis.) under the trade designation "24,857-6"),
28 g of bismuth trioxide (commercially available from Fisher
Scientific (Fair Lawn, N.J.) under the trade designation "B-339"),
20.6 g of barium carbonate (commercially available from Chemical
Products Corporation (Cartersville, Ga.) under the trade
designation "Type S"), 28.6 g of calcium carbonate (commercially
available from Akrochem Corporation (Akron, Ohio) under the trade
designation "Hubercarb Q325"), and 19.9 g of strontium carbonate
(commercially available from Aldrich Chemical Company (Milwaukee,
Wis.) under the trade designation "28,983-3"). The ingredients were
combined in appropriate proportions for the preparation of beads
with the following base composition:
[0130] 63% TiO.sub.2
[0131] 14% Bi.sub.2O.sub.3
[0132] 8% BaO
[0133] 8% CaO
[0134] 7% SrO
[0135] The beads were examined using an optical microscope. They
measured about 40 to 250 micros in diameter and a majority of the
beads were clear and substantially free of defects (e.g., optically
visible inclusions, bubbles). The quenched beads were placed in an
alumina crucible and heated at a rate of 10.degree. C./minute to
750.degree. C., held at 750.degree. C. for 1 hr, and then allowed
to cool slowly with the furnace through natural dissipation of heat
into the environment. The beads were removed from the furnace after
cooling back to room temperature. After heat treatment, the beads
remained substantially clear when viewed using an optical
microscope. X-ray diffraction confirmed the presence of at least
one crystalline phase, with a total crystalline phase concentration
of about 10 volume %. The measured index of refraction of the beads
is given in Table I. TABLE-US-00002 TABLE I Example Index of
Refraction 1 2.28 2 2.33 3 2.31 4 2.28 5 2.32 6 2.26 7 2.32 8 2.39
9 2.39 10 2.41 11 2.34 12 2.37 13 2.43 14 2.35 15 2.43 16 2.24 17
2.30
[0136] The complete disclosures of all patents, patent documents,
and publications are incorporated herein by reference as if
individually incorporated. It will be appreciated by those skilled
in the art that various modifications can be made to the above
described embodiments of the invention without departing from the
essential nature thereof. The invention is intended to encompass
all such modifications within the scope of the appended claims.
* * * * *