U.S. patent application number 15/707295 was filed with the patent office on 2018-01-11 for method of encapsulating pigment flakes with a metal oxide coating.
The applicant listed for this patent is Viavi Solutions Inc.. Invention is credited to Jianguo FAN, Kelly JANSSEN, Jeffrey James KUNA, Johannes P. SEYDEL, Paula WASHINGTON.
Application Number | 20180009992 15/707295 |
Document ID | / |
Family ID | 57221784 |
Filed Date | 2018-01-11 |
United States Patent
Application |
20180009992 |
Kind Code |
A1 |
KUNA; Jeffrey James ; et
al. |
January 11, 2018 |
METHOD OF ENCAPSULATING PIGMENT FLAKES WITH A METAL OXIDE
COATING
Abstract
A method of encapsulating pigment flakes with a metal oxide
coating is provided. According to the method, pigment flakes are
mixed with a solvent, a metal salt is added to the solvent, and a
reducing agent is added to the solvent, so as to encapsulate the
pigment flakes with a metal oxide coating.
Inventors: |
KUNA; Jeffrey James; (San
Francisco, CA) ; SEYDEL; Johannes P.; (Petaluma,
CA) ; WASHINGTON; Paula; (Sonoma, CA) ;
JANSSEN; Kelly; (Santa Rosa, CA) ; FAN; Jianguo;
(Santa Rosa, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Viavi Solutions Inc. |
Milpitas |
CA |
US |
|
|
Family ID: |
57221784 |
Appl. No.: |
15/707295 |
Filed: |
September 18, 2017 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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14705301 |
May 6, 2015 |
9765222 |
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15707295 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C09C 3/06 20130101; C09C
1/642 20130101; C09C 2200/1058 20130101; C09C 2200/401 20130101;
C09C 3/063 20130101; C09C 1/006 20130101; C01P 2004/03 20130101;
C09C 1/0021 20130101; C09C 2200/1054 20130101 |
International
Class: |
C09C 1/64 20060101
C09C001/64; C09C 1/00 20060101 C09C001/00; C09C 3/06 20060101
C09C003/06 |
Claims
1-15. (canceled)
16. A pigment flake comprising: a metal oxide coating, wherein the
pigment flake is encapsulated with the metal oxide coating by:
mixing pigment flakes with a solvent; adding a metal salt to the
solvent; and adding a reducing agent to the solvent so as to
encapsulate the pigment flakes with the metal oxide coating,
wherein the pigment flakes include the pigment flake, and wherein
the metal salt is a precursor to the metal oxide coating.
17. The pigment flake of claim 16, wherein the metal oxide coating
consists essentially of a metal oxide.
18. The pigment flake of claim 16, wherein the metal salt is a zinc
salt.
19. The pigment flake of claim 16, wherein the metal oxide coating
is a zinc oxide coating.
20. The pigment flake of claim 19, wherein the zinc oxide coating
consists essentially of zinc oxide.
21. The pigment flake of claim 16, wherein the reducing agent is a
hydride reducing agent or a borane complex reducing agent.
22. The pigment flake of claim 16, wherein the reducing agent is
sodium borohydride.
23. The pigment flake of claim 16, wherein the metal salt dissolves
in the solvent to provide metal cations.
24. The pigment flake of claim 23, wherein the reducing agent
reduces the metal cations.
25. The pigment flake of claim 16, wherein the metal oxide coating
fully encapsulates the pigment flake.
26. A pigment flake comprising: a metal oxide coating that
encapsulates the pigment flake, wherein the pigment flake is
encapsulated with the metal oxide coating by: mixing pigment flakes
with a solvent; and adding a reducing agent to the solvent so as to
encapsulate the pigment flakes with the metal oxide coating, and
wherein the pigment flakes include the pigment flake.
27. The pigment flake of claim 26, wherein the pigment flake
further comprises at least one metal layer with at least one
exposed surface.
28. The pigment flake of claim 26, wherein the pigment flakes
further comprises a metal layer having a top surface, a bottom
surface, and at least one side surface.
29. The pigment flake of claim 28, wherein dielectric layers cover
the top surface and the bottom surface of the metal layer and not
the at least one side surface of the metal layer.
30. The pigment flake of claim 28, wherein the metal layer is
formed of aluminum.
31. The pigment flake of claim 29, wherein the dielectric layers
are formed of magnesium fluoride.
32. The pigment flake of claim 26, wherein the pigment flakes
comprise one of three-layer pigment flakes, five-layer pigment
flakes, or seven-layer pigment flakes.
33. The pigment flake of claim 26, wherein one or more of a
transition metal salt, a main-group metal salt, or a rare-earth
metal salt is a precursor to the metal oxide coating.
34. The pigment flake of claim 26, wherein adding the reducing
agent comprises dripping the reducing agent into the solvent.
35. The pigment flake of claim 26, wherein the pigment flake is
encapsulated with the metal oxide coating further by: washing the
pigment flakes with ethanol after adding the reducing agent to the
solvent.
36. An article comprising: a pigment flake including a metal layer,
a first dielectric layer, and a second dielectric; and a metal
oxide coating that encapsulates the pigment flake, wherein the
metal oxide coating has a thickness between about 5 nm and about 20
nm.
37. The article of claim 36, wherein the metal oxide coating is
impermeable to water.
38. The article of claim 36, wherein the pigment flake is
encapsulated with the metal oxide coating by: mixing pigment flakes
with a solvent; and adding a reducing agent to the solvent so as to
encapsulate the pigment flakes with the metal oxide coating, and
wherein the pigment flakes include the pigment flake.
Description
TECHNICAL FIELD
[0001] The present disclosure relates to a method of encapsulating
pigment flakes. More particularly, the present disclosure relates
to a method of encapsulating pigment flakes with a metal oxide
coating.
BACKGROUND
[0002] Many metals, such as aluminum, corrode in the high-pH
aqueous environment typical of water-based paints. Moreover, some
dielectric materials, such as magnesium fluoride, are etched by the
high-pH aqueous environment and/or contain a significant number of
defects promoting environmental attack. Therefore, pigment flakes
may be passivated and/or encapsulated with a metal oxide coating to
inhibit environmental attack in water-based paints. However, an
encapsulating metal oxide coating may not always be conformal,
defect-free, impermeable to water, and inexpensive. In view of the
foregoing, it may be understood that there are significant problems
and shortcomings associated with current solutions and technologies
for encapsulating pigment flakes with a metal oxide coating.
SUMMARY
[0003] Accordingly, an aspect of the present disclosure relates to
a method of encapsulating pigment flakes with a metal oxide
coating, the method comprising: mixing pigment flakes with a
solvent; adding a metal salt to the solvent; and adding a reducing
agent to the solvent, so as to encapsulate the pigment flakes with
a metal oxide coating, wherein the metal salt is a precursor to the
metal oxide coating.
BRIEF DESCRIPTION OF THE DRAWINGS
[0004] Numerous exemplary embodiments will now be described in
greater detail with reference to the accompanying drawings
wherein:
[0005] FIG. 1A is a schematic illustration of a cross-section of an
exemplary embodiment of a pigment flake;
[0006] FIG. 1B is a schematic illustration of a cross section of
the pigment flake of FIG. 1A encapsulated with a metal oxide
coating;
[0007] FIG. 2A is a scanning transmission electron microscope
(STEM) image of a cross section of a ZnO-encapsulated
MgF.sub.2/Al/MgF.sub.2 pigment flake;
[0008] FIG. 2B is a set of energy dispersive X-ray spectroscopic
(EDS) element maps corresponding to the boxed region of FIG. 2A for
carbon, oxygen, fluorine, magnesium, and zinc, respectively;
and
[0009] FIG. 3 is a scanning electron microscope (SEM) image of a
ZnO-encapsulated MgF.sub.2/Al/MgF.sub.2 pigment flake.
DETAILED DESCRIPTION
[0010] In an exemplary embodiment, the present disclosure provides
a method of encapsulating pigment flakes with a metal oxide
coating.
[0011] Conventional methods of encapsulating pigment flakes with a
metal oxide coating have several drawbacks. For example, a sol-gel
process using tetraethyl orthosilicate (TEOS) as a precursor may be
used to encapsulate pigment flakes with a silicon dioxide coating.
However, in general, the silicon dioxide coating formed by this
sol-gel process is porous and must be relatively thick, e.g., 60 nm
to 70 nm in thickness, to provide sufficient protection from
environmental attack. Unfortunately, the large thickness of the
silicon dioxide coating can be detrimental to the optical
performance of the pigment flakes.
[0012] As described in U.S. Pat. No. 6,287,695 to Kaupp et al.,
issued on Sep. 11, 2001, which is incorporated herein by reference
in its entirety, a metal oxide and/or metal hydroxide coating may
be deposited as a passivating protective coating on exposed metal
surfaces of pigment flakes by hydrolysis of a metal salt or a metal
acid ester, where the metal is boron, aluminum, tin, titanium,
vanadium, chromium, molybdenum, zinc, or cerium.
[0013] Such methods may not always be applicable to all types of
pigment flakes. For example, some methods may require a relatively
high-pH environment in which some types of pigment flakes may be
damaged by corrosion and/or etching. Furthermore, such methods may
not always provide an ideal metal oxide coating.
[0014] Unlike conventional methods of encapsulating pigment flakes
with a metal oxide coating, the methods described herein may use a
reducing agent. The reducing agent may generate metal from a metal
salt, which is subsequently oxidized to form the metal oxide
coating on the pigment flakes.
[0015] With reference to FIG. 1A, an exemplary embodiment of a
pigment flake 100 suitable for encapsulation includes a central
metal layer 110 and outer dielectric layers 120. For example, the
pigment flake 100 may be a MgF.sub.2/Al/MgF.sub.2 pigment flake in
which the metal layer 110 is formed of aluminum, and the dielectric
layers 120 are formed of magnesium fluoride (MgF.sub.2). The metal
layer 110 has a top surface, a bottom surface, and at least one
side surface. The dielectric layers 120 cover the top and bottom
surfaces of the metal layer 110, but not the side surface of the
metal layer 110. Accordingly, the side surface of the metal layer
110 is exposed to the environment and susceptible to corrosion.
[0016] Moreover, the dielectric layers 120 themselves are exposed
to the environment. It should be appreciated that the dielectric
layers 120, particularly, when formed of magnesium fluoride, may
contain a significant number of defects which can provide
additional sites of environmental attack on the metal layer 110.
Furthermore, the dielectric layers 120, which are often assumed to
be chemically inert, may themselves be attacked and etched by the
environment. For example, the sol-gel process mentioned heretofore,
which uses tetraethyl orthosilicate (TEOS) as a precursor to form a
silicon dioxide coating, may require a relatively high-pH
environment in which the dielectric layers 120 and the metal layer
110, via its unprotected side surface and via defects in the
dielectric layers 120, may be attacked.
[0017] With reference to FIG. 1B, the methods described herein may
allow the pigment flake 100 to be encapsulated with a thin metal
oxide coating 130 for passivation and protection. Advantageously,
the method may not require a high-pH environment that can damage
the pigment flake 100 through etching and/or corrosion. The metal
oxide coating 130 may fully encapsulate the pigment flake 100 and
may completely cover most or all exposed surfaces of the pigment
flake 100. Preferably, the metal oxide coating 130 may be
continuous over most or all exposed surfaces of the pigment flake.
In particular, the metal oxide coating 130 may cover and protect
the exposed side surface of the metal layer 110, inhibiting
corrosion of the metal layer 110. The metal oxide coating 130 may
also cover and protect the exposed surfaces of the dielectric
layers 120. Accordingly, the encapsulated pigment flake 100 may be
well-suited for use in a water-based paint.
[0018] In general, the methods described herein may allow pigment
flakes of any suitable type to be encapsulated with a metal oxide
coating. The pigment flakes may be single-layer or multilayer
pigment flakes. The pigment flakes may be flat or may a incorporate
diffractive structure. Typically, the pigment flakes may be
metal-containing pigment flakes each including at least one metal
layer, such as an aluminum layer, with at least one exposed
surface. Often, the pigment flakes may also each include at least
one dielectric layer, such as a magnesium fluoride layer, with at
least one exposed surface. In some instances, the pigment flakes
may each include a metal layer having a top surface, a bottom
surface, and at least one side surface, and dielectric layers
covering the top and bottom surfaces of the metal layer, but not
the at least one side surface of the metal layer. For example, the
pigment flakes may be three-layer D/M/D pigment flakes, such as
MgF.sub.2/Al/MgF.sub.2 pigment flakes, five-layer M/D/M/D/M pigment
flakes, such as Cr/MgF.sub.2/Al/MgF.sub.2/Cr pigment flakes, or
seven-layer D/M/D/M/D/M/D pigment flakes, such as
MgF.sub.2/Cr/MgF.sub.2/Al/MgF.sub.2/Cr/MgF.sub.2 pigment flakes,
where D is a dielectric layer and M is a metal layer.
Alternatively, the pigment flakes may be all-dielectric pigment
flakes each including at least one dielectric layer, such as a
magnesium fluoride layer, with at least one exposed surface.
[0019] The one or more metal layers may be formed of any suitable
metallic material. The metallic material may be a reflective
metallic material and/or a metallic absorber material. Non-limiting
examples of suitable reflective metallic materials may include
aluminum, silver, copper, gold, platinum, tin, titanium, palladium,
nickel, cobalt, rhodium, niobium, chromium, and compounds,
combinations, or alloys thereof. Non-limiting examples of suitable
metallic absorber materials may include chromium, nickel, aluminum,
silver, copper, palladium, platinum, titanium, vanadium, cobalt,
iron, tin, tungsten, molybdenum, rhodium, niobium, and compounds,
combinations, or alloys thereof. Other various variations may also
be provided.
[0020] The one or more dielectric layers may be formed of any
suitable dielectric material. The dielectric material may be a
high-index dielectric material, having a refractive index of
greater than about 1.65, or a low-index dielectric material, having
a refractive index of less than about 1.65.
[0021] Non-limiting examples of suitable high-index dielectric
materials may include zinc sulfide (ZnS), zinc oxide (ZnO),
zirconium oxide (ZrO.sub.2), titanium dioxide (TiO.sub.2),
diamond-like carbon, indium oxide (In.sub.2O.sub.3), indium tin
oxide (ITO), tantalum pentoxide (Ta.sub.2O.sub.5), cerium oxide
(CeO.sub.2), yttrium oxide (Y.sub.2O.sub.3), europium oxide
(Eu.sub.2O.sub.3), iron oxides such as iron(II,III) oxide
(Fe.sub.3O.sub.4) and iron(III) oxide (Fe.sub.2O.sub.3), hafnium
nitride (HfN), hafnium carbide (HfC), hafnium oxide (HfO.sub.2),
lanthanum oxide (La.sub.2O.sub.3), magnesium oxide (MgO), neodymium
oxide (Nd.sub.2O.sub.3), praseodymium oxide (Pr.sub.6O.sub.11),
samarium oxide (Sm.sub.2O.sub.3), antimony trioxide
(Sb.sub.2O.sub.3), silicon, silicon monoxide (SiO), selenium
trioxide (Se.sub.2O.sub.3), tin oxide (SnO.sub.2), tungsten
trioxide (WO.sub.3), combinations thereof, and the like. Other
examples of suitable high-index dielectric materials include mixed
oxides such as those described in U.S. Pat. No. 5,989,626 to Coombs
et al., issued on Nov. 23, 1999, which is incorporated herein by
reference in its entirety. When the dielectric materials of U.S.
Pat. No. 5,989,626 are used in dielectric layers, they are most
commonly oxidized to their stoichiometric state such as
ZrTiO.sub.4. Non-limiting examples of such mixed oxides may include
zirconium titanium oxide, niobium titanium oxide, combinations
thereof, and the like.
[0022] Non-limiting examples of suitable low-index dielectric
materials may include silicon dioxide (SiO.sub.2), aluminum oxide
(Al.sub.2O.sub.3), metal fluorides such as magnesium fluoride
(MgF.sub.2), aluminum fluoride (AlF.sub.3), cerium fluoride
(CeF.sub.3), lanthanum fluoride (LaF.sub.3), sodium aluminum
fluorides (e.g., Na.sub.3AlF.sub.6 or Na.sub.5Al.sub.3F.sub.14),
neodymium fluoride (NdF.sub.3), samarium fluoride (SmF.sub.3),
barium fluoride (BaF.sub.2), calcium fluoride (CaF.sub.2), lithium
fluoride (LiF), combinations thereof, and the like. Other examples
of suitable low-index dielectric materials may include organic
monomers and polymers, including alkenes such as dienes, acrylates
(e.g., methacrylate), perfluoroalkenes, polytetrafluoroethylene
(Teflon), fluorinated ethylene propylene (FEP), combinations
thereof, and the like.
[0023] The pigment flakes may be fabricated by any suitable method.
Typically, the pigment flakes are fabricated by depositing a
single-layer or multilayer film on a substrate, stripping the film
from the substrate, and grinding the resulting product. Of course,
the fabrication method may include different steps or additional
steps, e.g., steps to remove impurities, such as sodium chloride,
or steps to create break lines in the film.
[0024] Examples of suitable pigment flakes, as well as methods of
fabricating such pigment flakes, are disclosed in U.S. Pat. No.
6,013,370 to Coulter et al., issued on Jan. 11, 2000, in U.S. Pat.
No. 6,157,489 to Bradley, Jr. et al., issued on Dec. 5, 2000, in
U.S. Pat. No. 6,692,830 to Argoitia et al., issued on Feb. 17,
2004, and in U.S. Pat. No. 6,841,238 to Argoitia et al., issued on
Jan. 11, 2005, all of which are incorporated herein by reference in
their entireties. Other examples of suitable pigment flakes may
include SpectraFlair.RTM. and ChromaFlair.RTM. pigment flakes sold
by JDS Uniphase Corporation.
[0025] In general, the methods described herein may be performed on
the as-fabricated pigment flakes without any pretreatment. The
methods may allow the pigment flakes to be encapsulated with a
metal oxide coating while minimizing damage to the pigment flakes
through etching and/or corrosion.
[0026] The metal oxide coating may be a transition metal oxide
coating, such as a zinc oxide coating and/or a zirconium oxide
coating, a main-group metal oxide coating, such as a tin oxide
coating, a rare-earth metal oxide coating, such as a cerium oxide
coating, or a mixture thereof. In some embodiments, the metal oxide
coating may be a zinc oxide coating. The metal oxide coating
comprises a metal oxide, such as zinc oxide (ZnO), zirconium oxide
(ZrO.sub.2), tin oxide (SnO.sub.2), or cerium oxide (CeO.sub.2),
but may also comprise impurities such as water, hydroxyl groups, or
alkoxyl groups. In some embodiments, the metal oxide coating may
consist essentially of the metal oxide. In some embodiments, the
metal oxide coating may consist essentially of zinc oxide. The
encapsulated pigment flakes may, typically, comprise about 5 wt %
to about 15 wt % metal oxide coating, preferably, about 8 wt %
metal oxide coating. Other various ratios may also be provided.
[0027] In some embodiments, the metal oxide coating may be a thin
layer, typically, having a thickness of about 5 nm to about 20 nm,
preferably, having a thickness of about 10 nm to about 15 nm, that
provides passivation and protection. Alternatively, the metal oxide
coating may be a thicker layer, typically, having a thickness of
about 20 nm to about 300 nm, that contributes to the optical design
of the pigment flakes. The metal oxide coating may fully
encapsulate the individual pigment flakes and/or completely cover
most or all exposed surfaces of the individual pigment flakes. In
particular, when the pigment flakes are metal-containing pigment
flakes each including at least one metal layer with at least one
exposed surface, the metal oxide coating may cover and protect the
exposed surface of the metal layer, inhibiting corrosion of the
metal layer. When the pigment flakes each include at least one
dielectric layer with at least one exposed surface, the metal oxide
coating may cover and protect the exposed surface of the dielectric
layer. In some embodiments, the metal oxide coating may be
continuous over most or all exposed surfaces of the individual
pigment flakes. Specifically, the metal oxide coating may be
substantially free of defects and may be impermeable to water.
Also, the metal oxide coating may be substantially uniform and
conformal to the individual pigment flakes.
[0028] The metal oxide coating may be formed by wet-chemical
methods, which are, typically, carried out in a single container,
i.e., as one-pot reactions. According to the methods, the pigment
flakes may be mixed with a solvent, a metal salt may be added to
the solvent, and a reducing agent may be added to the solvent, so
as to encapsulate the pigment flakes with a metal oxide coating.
Advantageously, the methods may not require the addition of a
strong base. The order of the method steps may be varied and, in
some instances, the method steps may be carried out simultaneously.
For example, the methods may be carried out in a continuous flow
reactor where the pigment, metal salt, reducing agent, and solvent
may be mixed, the reaction allowed to proceed, and the resulting
encapsulated pigment flakes may be filtered and washed, in a
continuous fashion.
[0029] In an exemplary embodiment, the pigment flakes may be mixed
with the solvent, typically, at a concentration of about 10 g/L to
about 300 g/L. Optionally, the pigment flakes may be dispersed in
the solvent by adding a cosolvent to the solvent or by adding a
surfactant to the solvent, typically, at a concentration of about 1
mM to about 30 mM. The metal salt may be dissolved in the solvent,
typically, at a concentration of about 1 mM to about 100 mM,
preferably, at a concentration of about 20 mM to about 100 mM,
forming a solution. The reducing agent may be introduced into the
solution, typically, in an amount of about 1.5 moles to about 25
moles per mole of metal cation. Consequently, the metal oxide
coating may be deposited from the solution onto the pigment
flakes.
[0030] The metal salt, which is a precursor to the metal oxide
coating, may serve as a metal source. In general, the metal salt
may be soluble in the solvent used and may dissolve in the solvent
to provide metal cations. The metal salt may be a transition metal
salt, such as a zinc salt and/or a zirconium salt, a main-group
metal salt, such as a tin salt, a rare-earth metal salt, such as a
cerium salt, or a mixture thereof. In some embodiments, the metal
salt may be a zinc salt, which may serve as a metal source for a
zinc oxide coating. Also, the metal salt may include an anion that
does not react with the pigment flakes. Typically, the metal salt,
which may or may not be a hydrate, may be a metal mineral acid
salt, such as a metal nitrate, a metal sulfate, a metal phosphate,
and/or a metal chloride, a metal organic acid salt, such as a metal
acetate, or a mixture thereof. For example, the metal salt may be
zinc nitrate hexahydrate (Zn(NO.sub.3).sub.2.6H.sub.2O).
[0031] In general, the solvent may be a polar solvent that
dissolves the metal salt. Typically, the solvent may be water, an
alcohol, such as methanol, ethanol, isopropanol, ethylene glycol,
and/or butyl cellosolve (ethylene glycol butyl ether), an ester,
such as ethyl acetate, or a mixture thereof. In some embodiments,
the solvent may be ethanol. Optionally, a cosolvent, such as butyl
cellosolve, or a surfactant may be added to the solvent to
facilitate dispersion of the pigment flakes in the solvent.
[0032] In a preferred embodiment, a surfactant may be added to the
solvent to facilitate dispersion of the pigment flakes in the
solvent. Typically, the surfactant may be a carboxylic acid, such
as benzoic acid, octanoic acid, or hexadecanoic acid.
Advantageously, because such carboxylate-containing surfactants
have an affinity for the metal oxide coating, their use may lead to
a smoother coating. Such surfactants may also remain on the surface
of the encapsulated pigment flakes, rendering the encapsulated
pigment flakes hydrophobic, which can lead to improved leafing of
the pigment flakes.
[0033] The reducing agent may be a hydride reducing agent, such as
sodium borohydride (NaBH.sub.4) or lithium aluminum hydride
(LiAlH.sub.4), or a borane complex reducing agent, such as borane
tert-butylamine complex ((CH.sub.3).sub.3CNH.sub.2.BH.sub.3). In
some embodiments, the reducing agent may be sodium borohydride. In
such embodiments, the reaction may be carried out at room
temperature, and sodium borohydride may be dripped into the
reaction mixture, i.e., the solvent containing the pigment flakes
and the metal salt, to control the reaction rate. The total
reaction time may, typically, be less than about 24 h, preferably,
less than about 1 h. In some instances, the reaction may be
complete by the time that the addition of the reducing agent is
complete. In other embodiments, the reaction conditions and
reaction time may be adjusted to compensate for the reactivity of
the reducing agent. For example, when a less reactive borane
complex is used as the reducing agent, the borane complex may be
added all at once to the reaction mixture, the reaction mixture may
be heated, and the reaction time may be increased.
[0034] It should be appreciated that the use of a reducing agent
may be advantageous in producing a metal oxide coating providing
effective and complete encapsulation. It is believed that the
reducing agent may reduce the metal cation of the metal salt,
forming a thin metal coating on the pigment flakes as an
intermediate, and that the metal coating may then be oxidized in
situ, forming the metal oxide coating. For example, the solvent,
trace water in the solvent, and/or atmospheric oxygen may act as
the oxidizing agent. It is thought that the intermediate metal
coating may serve as an adhesion-promoting layer that facilitates
further coating growth. As the reducing agent is added, the pigment
flakes have been observed to become darker and less reflective for
a time, before largely regaining their original appearance. This
color change may be associated with a change from an absorbing
metal coating to a transparent metal oxide coating.
[0035] Once the desired encapsulating metal oxide coating has been
formed on the pigment flakes, the encapsulated pigment flakes may
be washed, typically, several times with ethanol, to remove excess
metal oxide and other byproducts. The encapsulated pigment flakes
may then be separated from the solvent by vacuum filtration,
cyclonic separation, or centrifugation. Optionally, the
encapsulated pigment flakes may also be dried, typically, in air at
a temperature of about 80.degree. C. to about 150.degree. C.,
alternatively, under vacuum, to remove any remaining solvent.
However, baking or calcination at higher temperatures may not be
required.
[0036] To further illustrate the present invention, the following
examples are provided.
[0037] In a first example, MgF.sub.2/Al/MgF.sub.2 pigment flakes,
known as SpectraFlair.RTM. Bright Silver, were encapsulated with a
zinc oxide coating according to the method described heretofore. A
filtered solution of sodium borohydride (0.9 g) in ethanol (200
proof, 20 mL) was dripped into a mixture of the
MgF.sub.2/Al/MgF.sub.2 pigment flakes (1 g) and zinc nitrate
hexahydrate (0.31 g) in ethanol (200 proof, 50 mL) at room
temperature. The mixture was stirred for 1 h. The supernatant was
then decanted, and the ZnO-encapsulated MgF.sub.2/Al/MgF.sub.2
pigment flakes were washed several times with ethanol (200 proof)
and filtered by vacuum. The ZnO-encapsulated MgF.sub.2/Al/MgF.sub.2
pigment flakes were then dried at about 80.degree. C. in air.
Elemental analysis of the zinc oxide coating by X-ray photoelectron
spectroscopy (XPS) indicated that it consisted essentially of zinc
and oxygen, with carbon-containing impurities. Notably, the
measured amounts of magnesium and fluorine were insignificant,
indicating coverage of the magnesium fluoride layers by the zinc
oxide coating.
[0038] A scanning transmission electron microscope (STEM) image of
a cross section of a ZnO-encapsulated MgF.sub.2/Al/MgF.sub.2
pigment flake is shown in FIG. 2A. Energy dispersive X-ray
spectroscopic (EDS) element maps corresponding to the boxed region
of FIG. 2A for carbon 210, oxygen 220, fluorine 230, magnesium 240,
and zinc 250 are shown in FIG. 2B. A magnesium fluoride layer is
visible, surrounded by the embedding medium used to prepare the
cross section. Notably, there is a large concentration of zinc
along the edge of the pigment flake, corresponding to the zinc
oxide coating. It is apparent that the zinc oxide formed a thin
coating on the surface of the pigment flake and did not react with
or diffuse into the underlying pigment flake. The zinc oxide
coating has a thickness of about 10 nm to about 15 nm.
[0039] A scanning electron microscope (SEM) image of a
ZnO-encapsulated MgF.sub.2/Al/MgF.sub.2 pigment flake is shown in
FIG. 3. An edge of the encapsulated pigment flake is visible.
Notably, the zinc oxide coating completely covers all exposed
surfaces of the pigment flake, and the underlying structure of the
pigment flake is not visible. The roughness and defects in the
magnesium fluoride layers and the exposed surface of the aluminum
layer are covered and hidden by the zinc oxide coating.
[0040] In a second example, diffractive MgF.sub.2/Al/MgF.sub.2
pigment flakes, known as SpectraFlair.RTM. Silver 1500, were
encapsulated with a zinc oxide coating according to the method
described heretofore. A filtered solution of sodium borohydride
(0.9 g) in ethanol (200 proof, 20 mL) was dripped into a mixture of
the diffractive MgF.sub.2/Al/MgF.sub.2 pigment flakes (1 g) and
zinc nitrate hexahydrate (0.31 g) in ethanol (200 proof, 50 mL) at
room temperature. The mixture was stirred for 1 h. The supernatant
was then decanted, and the ZnO-encapsulated diffractive
MgF.sub.2/Al/MgF.sub.2 pigment flakes were washed several times
with ethanol (200 proof) and filtered by vacuum. The
ZnO-encapsulated diffractive MgF.sub.2/Al/MgF.sub.2 pigment flakes
were then dried at about 80.degree. C. in air.
[0041] In a third example, Cr/MgF.sub.2/Al/MgF.sub.2/Cr pigment
flakes, known as ChromaFlair.RTM., were encapsulated with a zinc
oxide coating according to the method described heretofore. A
filtered solution of sodium borohydride (0.9 g) in ethanol (200
proof, 20 mL) was dripped into a mixture of the
Cr/MgF.sub.2/Al/MgF.sub.2/Cr pigment flakes (1 g) and zinc nitrate
hexahydrate (0.31 g) in ethanol (200 proof, 50 mL) at room
temperature. The mixture was stirred for 1 h. The supernatant was
then decanted, and the ZnO-encapsulated
Cr/MgF.sub.2/Al/MgF.sub.2/Cr pigment flakes were washed several
times with ethanol (200 proof) and filtered by vacuum. The
ZnO-encapsulated Cr/MgF.sub.2/Al/MgF.sub.2/Cr pigment flakes were
then dried at about 80.degree. C. in air.
[0042] In a fourth example,
MgF.sub.2/Cr/MgF.sub.2/Al/MgF.sub.2/Cr/MgF.sub.2 pigment flakes
were encapsulated with a zinc oxide coating according to the method
described heretofore. A filtered solution of sodium borohydride
(0.9 g) in ethanol (200 proof, 20 mL) was dripped into a mixture of
the MgF.sub.2/Cr/MgF.sub.2/Al/MgF.sub.2/Cr/MgF.sub.2 pigment flakes
(1 g) and zinc nitrate hexahydrate (0.31 g) in ethanol (200 proof,
50 mL) at room temperature. The mixture was stirred for 1 h. The
supernatant was then decanted, and the ZnO-encapsulated
MgF.sub.2/Cr/MgF.sub.2/Al/MgF.sub.2/Cr/MgF.sub.2 pigment flakes
were washed several times with ethanol (200 proof) and filtered by
vacuum. The ZnO-encapsulated
MgF.sub.2/Cr/MgF.sub.2/Al/MgF.sub.2/Cr/MgF.sub.2 pigment flakes
were then dried at about 80.degree. C. in air.
[0043] The present disclosure is not to be limited in scope by the
specific embodiments described herein. Indeed, other various
embodiments of and modifications to the present disclosure, in
addition to those described herein, will be apparent to those of
ordinary skill in the art from the foregoing description and
accompanying drawings. Thus, such other embodiments and
modifications are intended to fall within the scope of the present
disclosure. Further, although the present disclosure has been
described herein in the context of a particular implementation in a
particular environment for a particular purpose, those of ordinary
skill in the art will recognize that its usefulness is not limited
thereto and that the present disclosure may be beneficially
implemented in any number of environments for any number of
purposes.
* * * * *