U.S. patent application number 10/636166 was filed with the patent office on 2004-02-12 for composition and method for a coating providing anti-reflective and anti-static properties.
Invention is credited to Park, Sung-Soon, Zheng, Haixing.
Application Number | 20040028819 10/636166 |
Document ID | / |
Family ID | 23561219 |
Filed Date | 2004-02-12 |
United States Patent
Application |
20040028819 |
Kind Code |
A1 |
Park, Sung-Soon ; et
al. |
February 12, 2004 |
Composition and method for a coating providing anti-reflective and
anti-static properties
Abstract
Coating solutions having anti-reflective and anti-static
properties, a coating derived therefrom, a substrate coated with
the coating and methods for their preparation. A coating includes a
sol-gel alkoxide polymeric material and a conductive colloidal
metal compound material.
Inventors: |
Park, Sung-Soon; (Los
Angeles, CA) ; Zheng, Haixing; (Oak Park,
CA) |
Correspondence
Address: |
BLAKELY SOKOLOFF TAYLOR & ZAFMAN
12400 WILSHIRE BOULEVARD, SEVENTH FLOOR
LOS ANGELES
CA
90025
US
|
Family ID: |
23561219 |
Appl. No.: |
10/636166 |
Filed: |
August 6, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10636166 |
Aug 6, 2003 |
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09991765 |
Nov 21, 2001 |
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6638630 |
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09991765 |
Nov 21, 2001 |
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09394987 |
Sep 13, 1999 |
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6372354 |
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Current U.S.
Class: |
427/372.2 |
Current CPC
Class: |
C03C 17/3417 20130101;
G02B 1/111 20130101; Y10T 428/31663 20150401; Y10T 428/31667
20150401; Y10T 428/31612 20150401 |
Class at
Publication: |
427/372.2 |
International
Class: |
B05D 003/02 |
Goverment Interests
[0002] This invention was made with government support under grant
number IR43EY12461-01 awarded by the National Institute of Health.
Claims
What is claimed is:
1. A method for preparing a coating solution comprising: subjecting
a solution of an alkoxide to hydrolyzation and condensation in a
reaction system; aging the solution to form a sol-gel reaction
product; and diluting the reaction product.
2. The method of claim 1, wherein the alkoxide comprises one of the
general formula: M(OR).sub.xwherein M is selected from at least one
of the group consisting of Si, Ti, B, Al, and Zr, wherein R is an
alkyl group having 1-8 carbons and x is the valence state of the
cation.
3. The method of claim 1, wherein the aging comprises 1 to 24 hours
at 50.degree. C.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] The application is a continuation application of co-pending
U.S. patent application Ser. No. 09/991,765, filed Nov. 21, 2001,
which is a continuation application of co-pending U.S. application
Ser. No. 09/394,987, filed Sep. 13, 1999.
BACKGROUND
[0003] 1. Field
[0004] The invention relates to surface coating solutions and
films, and particularly to coatings having anti-reflective and
anti-static properties.
[0005] 2. Related Art
[0006] In optical applications, substrates such as glass and
plastic have been widely used. Plastics, as compared with glass,
are less expensive, more easily formed into complex shapes, lighter
in weight and generally not as brittle. Accordingly, the use of
plastics, such as polycarbonates and acrylic polymers, is
increasing in optical applications. Both glasses and plastics,
however, suffer from reflective losses at the substrate/air
interface, the losses averaging about seven percent of the
transmitted light for the two surfaces. The reflective loss is even
more severe in substrates having high index of refractions (e.g.,
index of refractions on the order of 1.55 or more). While numerous
practical approaches to reducing the reflective losses for glass
substrates have been developed, few low temperature techniques are
available for plastic substrates at low cost.
[0007] Anti-reflective (AR) coatings or films offer one way to
reduce the reflective losses at the substrate/air interface. AR
coatings reduce the reflectance of light from a surface thereby
increasing the light transmittance through the coating/substrate
interface. One challenge to using AR coatings on organic substrates
like plastic is forming such films at temperatures that will not
melt or deform the substrate.
[0008] Low temperature AR coatings are primarily made by vacuum
deposition techniques. In general, there are two types of vacuum
deposition techniques: vacuum evaporation and vacuum sputtering. In
vacuum evaporation, a charge of the material to be evaporated is
placed in a crucible of a refractory material. An electrical
resistance heater heats this charge (either by conduction or by
radiation) after the chamber has been evacuated to about 10.sup.-6
torr. As the temperature of the charge rises, its vapor pressure
rises and a significant evaporation develops. The evaporant then
condenses on the cooler substrate. Vacuum sputtering, on the other
hand, is characterized by a momentum transfer process in which
argon or other ions and atoms from a plasma bombard a target made
of the material to be coated. The collision of the argon atoms and
ions with the surface atoms and molecules of the target knocks off
(sputters) the target material, which then forms a deposit on the
substrate. Both of the vacuum deposition techniques (evaporation
and sputtering) produce high quality AR coatings or films.
Generally, the disadvantages of vacuum deposition are: (1) large
capital expenditure for deposition equipment, (2) possible
temperature build up that can deform or melt a plastic substrate,
(3) restricted substrate size and geometry due to equipment
limitations, and (4) separate coating processes may be required for
each surface.
[0009] U.S. Pat. No. 5,719,705 discloses a transparent multi-layer
AR coating wherein each layer comprises an electrically conductive,
high-refractive index or an electrically-conductive, low-refractive
index material using vacuum deposition techniques: electron beam
reactive evaporation, ion-assisted deposition, and reactive
sputtering of metal targets. The resultant AR coating, in addition
to not attracting dust and other airborne contaminants, and has
hydrophobic properties.
[0010] In addition to vacuum deposition techniques, AR surfaces can
also be formed by chemical modification of a surface by a reactive
plasma at low temperature. Again, this generally requires expensive
equipment, possible heat build up, and size limitations.
[0011] Solutions containing fluorinated organics have also been
deposited on plastic substrates which then exhibited AR properties,
such as an apparatus described in U.S. Pat. No. 5,198,267.
Fluorination processes appear, however, to be limited to
self-developing photographic film applications and do not appear to
be adaptable for large scale applications.
[0012] U.S. Pat. No. 4,966,812 describes the development of a
process for applying a single layer, AR coating to a plastic
substrate via micro-structural tailoring of a sol-gel solution. In
the process, a silicon alkoxide, metal alkoxide or a mixture
thereof is subjected to hydrolyzation then condensation in a
solution to form a sol-gel hydrolyzation, followed by further
condensation to form a polymeric reaction product. The gel formed
from the sol- is reliquified. The reliquified gel is then diluted
to increase its stability and to form the sol-gel AR surface
coating solution. From this solution, a film with a low index of
refraction (e.g., on the order of 1.22) can be deposited on a
plastic substrate without heating or etching. This reliquified gel
contains large polymers in solution resulting in a deposited film
with greater porosity, and hence a lower refractive index. By
careful control of the coating rate, the optimum film thickness can
be obtained.
[0013] U.S. Pat. No. 5,476,717 describes a process to make a single
layer AR coating for a plastic substrate. This single AR coating
layer is formed from colloids of silica in a siloxane binder.
Several other layers have been integrated into the process in order
to enhance the adhesion, abrasion resistance, and hydrophobic
properties. In general, the structure comprises an organic or
inorganic substrate, successively covered by an adhesion-promoting
coating made from a material chosen from among silanes, an
anti-reflection coating of silicon colloidals coated with a
siloxane binder, a coupling agent coating formed from a material
chosen from among the silaxanes, and an anti-abrasive coating of a
fluorine polymer.
[0014] U.S. Pat. No. 5,580,819 describes a composition for
producing durable coatings and a process for preparing a
single-layer AR coating on solid substrates, such as glass,
ceramics, metals, and organic polymeric materials. The coating
composition comprises, in combination, acid-catalyzed hydrolysis
and condensation products of a water-silane monomer mixture and a
film forming amount of a polymer having functional groups selected
from amino, hydroxy and carboxy, hydroxy and amino, amino and
carboxy, and amino, hydroxy and carboxy. The described process
comprises applying the aforesaid coating composition (or an acid
catalyzed sol-gel coating composition), substantially free of
pre-formed oxide sol and water soluble metal salt, to the surface
of a solid substrate, curing the applied coating, and treating the
cured coating with an aqueous electrolytic solution for a time
sufficient to produce a coating having graded porosity which is
anti-reflective over a broad band of the visible spectrum.
[0015] U.S. Pat. No. 4,361,598 describes the use of sol-gel
techniques to deposit two layer AR SiO.sub.2/TiO.sub.2 coatings
onto solar cells and stainless steel or silicon ribbon. The
refractive index range attainable using mixtures of these solutions
is 1.4-2.4. The refractive index required for AR film on plastics
is generally about 1.22 and cannot be achieved using the method
described without the introduction of porosity. In addition, the
method described requires heat treatments considerably higher than
the typical upper temperature limits of plastics. Refractive index
control is achieved by compositional control, firing temperature
(300.degree. C.-600.degree. C.) and the firing atmosphere.
[0016] U.S. Pat. No. 5,858,526 primarily describes a method to make
a two-layer AR coating having a high reflective index by a sol-gel
solution process. Metal oxide colloids are coated with a polyvinyl
material and rendered soluble in water-containing molecular
solvents. The coating consists of a half wave-thick
zirconia-polyvinyl pyrolidone layer of 1.72 refractive index and a
quarter wave-thick porous silica-siloxane layer of 1.26 index. Both
layers are centered at 600-nm wavelength. In general, this two
layer AR coating is not abrasion resistant, and the AR coating also
has poor adhesion. To achieve a moderate abrasion-resistance and
hydrophobic behavior, the coating is overcoated with a very thin
layer of a lubricating material. This lubricating material slightly
permeates the porous silica layer underneath and hence increases
the refractive index value a bit from 1.26 up to 1.30. In addition,
several adhesion promoting layers containing organosilanes
generally must be deposited between the layers to promote
adhesion.
SUMMARY
[0017] Coating compositions having anti-reflective and anti-static
properties, a coating derived therefrom, a substrate coated with
the coating and methods for their preparation are described. In one
aspect, a coating in accordance with the invention comprises a
multilayer or composite layer of a sol-gel alkoxide polymeric layer
and a conductive colloidal metal compound layer over a hardcoat or
scratch-resistant layer on a substrate.
[0018] Objects, advantages, and novel features of this invention
will be apparent to those skilled in the art upon examination of
the following detailed description and accompanying drawings
learned in the practice of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] The accompanying figures, which are incorporated in and form
part of the specification, illustrate an embodiment of the
invention and, together with the description, serve to explain the
principles of the invention. However, these figures, as well as the
following detailed description and examples, are given by way of
illustration only and thus are not intended to limit the
invention.
[0020] FIG. 1 shows a composite coating on a substrate in
accordance with an embodiment of the invention.
[0021] FIG. 2 shows reflectance of an embodiment of the composite
coating of the invention versus wavelength measured from 450-750
nanometers.
DETAILED DESCRIPTION OF THE INVENTION
[0022] According to the invention, there is provided a method for
the preparation and the low temperature application of a composite
coating or film having anti-reflective (AR) and anti-static
properties on a substrate, a method for making conductive
crystalline colloidal solution and using the resulting solution,
and a method for tailoring the growth of polymers in solution in
order to deposit a low reflective index sol-gel film as part of the
multi-layer coating.
[0023] In general, sol-gel is a process that uses alkoxides,
M(OR).sub.x, as oxide glass precursors, where M is at least one of
silicon (Si), titanium (Ti), boron (B), aluminum (Al), zirconium
(Zr) or other ceramic types of metals; R is an alkyl group, having,
for example, 1-8 carbon atoms; and x is equal to the valence state
of the cation. The contemplated alkoxides, when mixed with water
and a catalyst in an alcohol solution, undergo hydrolysis and
condensation reactions to form a polymeric network. The polymer,
which continues to crosslink until a gel is formed, expels its
solvent, and upon heating, densifies to form a glass. Glass
coatings can be applied using sol-gel technology. Processing
temperatures to form a glass by sol-gel techniques are typically
much lower than conventional glass formation via melting of metal
oxides.
[0024] An embodiment of a low reflective index SiO.sub.2 layer is
made by sol-gel derived SiO.sub.2 sols. The silicon alkoxides
undergo hydrolyzation and condensation. More particularly, the
SiO.sub.2 sols are formed from a reaction system that comprises a
silicon alkoxide mixed with water and a catalyst in an alcohol
solution. In one example, the alkoxides are represented by formula
Si(OR).sub.4; where R is an alkyl group having 1-8 carbon atoms,
and more preferably having 1-4 carbon atoms. One polymeric reaction
product is formed from monomeric alkoxides of
tetraethylorthosilicate (TEOS), in an alcohol solution also
containing water and an acid catalyst. An SiO.sub.2 coating
solution suitable for use in an embodiment of an AR coating on a
substrate is prepared by diluting the sol with alcohol solvents. A
typical dilution is a solution such that the weight percent of the
sol is 0.5 to 5.0 percent. A thin glass film or layer having a
thickness in the sub-micron range can be deposited by spinning,
spraying or dipping. The film has a reflective index in the range
of 1.20-1.45.
[0025] One embodiment of a coating according to the invention
involves combining the polymerized alkoxide layer with a high
reflective index layer on a substrate. The high index layer is made
from a high reflective index conductive crystalline colloidal
solution. In one aspect, the high index crystalline colloidal
solution is made by dispersion of conductive crystalline powders
into organic liquids.
[0026] In one embodiment, the conductive powders of the colloidal
solution include, but are not limited to, particles of Indium Tin
Oxide (ITO), Indium Oxide (10), Tin Oxide (TO), Antimony Tin Oxide
(ATO), and Titanium Oxide (TiO.sub.x). Other suitable powders
includes, but are not limited to, metal nitrides (e.g., titanium
nitride), metal carbides (e.g., titanium carbide), and metal
fluorides (e.g., titanium fluoride). The powders are made by either
vacuum processes or wet processes as known in the art. The powders
are typically made by a grinding process to a particle size
suitable for forming a transparent film or layer. The particle
size, in one embodiment, is less than one micron and on the order
of one hundred to several hundred nanometers. Particle sizes on the
order of 120 to 145 nanometers have been shown to be suitable in
films without producing streaking or opaqueness.
[0027] The colloidal solutions are typically prepared by grinding
and dispersing the conductive powders into organic solvents mixed
with dispersing agents. The dispersing agent is suitably soluble in
alcohols. A suitable dispersing agent is chosen from an amide,
including but not limited to, dimethylformamide (DMF). In one
embodiment, the respective proportions of the oxide particles and
dispersing agent are, by weight, between approximately 20 and 50
percent based on the solvent.
[0028] The colloidal solutions are prepared by mechanical mixing of
the conductive powders in the dispersing agent containing solvent
solution. The mechanical mixing methods include, but are not
limited to, grinding and shaking. During grinding of these powders,
unpaired electrons of carbonyl (--C.dbd.O) of amide are believed to
chemically bond with surface metal of the grounded powder. An acid,
such as nitric acid, is then added into the surface modified
particles solution. It is believed that the proton (H.sup.+) of the
acid bonds with the nitrogen of the amide which is bonded to the
oxide particles. This proton bond results in particles with
positive surface charge. The particles repel each other and form
stable colloids.
[0029] For use as a colloid coating solution, the colloids are
diluted by mixing with alcohol solvents to approximately 1.0 to
15.0 percent by weight of the solution.
[0030] One embodiment of a method for preparing a surface coating
composition according to the invention comprises several features.
The coating composition of the invention may be introduced on an
organic (e.g., plastic), glass or other substrate. The coating
composition may be introduced directly onto a base surface of the
substrate or over an adhesion or hardcoat composition. A typical
hardcoat composition for an organic substrate is polysiloxane. The
introduction of a hardcoat onto a substrate surface is known in the
art.
[0031] The diluted high index colloidal solution is introduced as a
first layer over a hardcoat overlying a surface of a substrate or
onto a bare surface of a substrate. In one example, the colloidal
solution is introduced by a spinning process. Spinning continues
for about one minute to form a stable high index first layer having
a thickness on the order of 500-1500 .ANG.. The diluted sol-gel low
index coating solution is then overcoated on the high index first
layer by spinning. Spinning continues for about one minute to form
a stable low index second layer having a thickness on the order of
500-1000 .ANG.. The substrate, which is now coated with a first
layer of high index metal compound and a second layer of low index
SiO.sub.2, is baked at a temperature of 50 to 100.degree. C. for
about one hour.
[0032] As noted above, in one embodiment, the composite coating is
introduced over a hardcoat layer on, for example, a plastic
substrate. The introduction of the first layer (e.g., high index
ITO material) and the second layer (e.g., low index SiO.sub.2
material) by a process such as spinning tends to embed the first
layer material and possibly the second layer material in the
hardcoat layer. This embedding renders durability to the composite
coating of the invention.
[0033] The colloidal coating solution that forms the first layer
contains volatile alcohol solvents that evaporate during spinning.
Although the capillary force of the solvents between particles
tends to form compact structure layers, the evaporation of the
solvents leaves behind open or closed pores in the spin-coated
first layer. It is believed that open pores of the first layer will
be filled with the second layer material (e.g., SiO.sub.2) during
spinning. During baking of a coated organic substrate, the solvents
in the two layers evaporate. Gels (e.g., SiO.sub.2 gels) in the
second layer contact each other to form a continuous -M-O-M-O-
network by polycondensation. Also M-OR groups of the gel, which
have permeated in the lower metal compound layer, react,
representatively, with the --OH, --NH, --CH, or --F groups of the
metal compound particles. This results in good chemical bonding of
the gels to the metal oxide particles. In the example where the
first layer is a metal oxide layer material of ITO and the second
layer material is SiO.sub.2, a typical reaction may be illustrated
as follows:
ITO particle-OH+(OR)Si--O--Si--O-.fwdarw.ITO
particle-O--Si--O--Si--O-+ROH
[0034] The continuous --Si--O--Si--O-- network of the SiO.sub.2
gels in the low index SiO.sub.2 layer and in the high index ITO
layer contacts the ITO layer vertically and horizontally. This
results in a compact microstructure of a composite of the high
index ITO material and the low index SiO.sub.2 material although
the composite is dried at the low temperature of 50 to 100.degree.
C. In one respect, the low temperature formation of the composite
coating on a substrate is achieved by the following: (1) the
densification of low index SiO.sub.2 layer through polycondensation
of SiO.sub.2 sols, and (2) the permeation of SiO.sub.2 sols into
the high index ITO layer and the chemical bonding between SiO.sub.2
gels and ITO particles.
[0035] FIG. 1 illustrates an embodiment of a composite coating of
the invention over an organic substrate, such as a plastic
substrate. FIG. 1 shows hardcoat layer 11 overlying a surface of
substrate 10 having a thickness of approximately 1-10 .mu.m.
Overlying hardcoat layer 11 is first layer 12 of, for example, a
high index metal oxide colloid. Overlying first layer 12 is second
layer 13 of, for example, a low index alkoxide polymer. It is to be
appreciated that the individual layers do not necessarily overly
one another with a distinct interface between each layer. Instead,
as noted above particularly with, spin operations, second layer
material fills pores in first layer 12 and first layer material,
and possibly second layer material, is embedded in hardcoat layer
11.
[0036] By controlling the thickness of the individual layers of the
composite coating of the invention, a desired AR property of the
coating may be achieved. In one embodiment, second layer 13 acts as
an interference layer that modifies the path difference of light
incident on substrate 10. In general, the thickness of second layer
13 may be determined for destructive interference by the
relationship: 1 d = 4 n 0
[0037] where d represents the thickness of second layer 13,
.lambda. represents the wavelength where zero reflectivity is
desired (i.e., complete destructive interference), and no
represents refractive index of second layer 13. As illustrated in
FIG. 1, ideally the anti-reflectance of the multi-layer is achieved
when light waves 20 and 25 are completely out of phase.
[0038] The composite coating of the invention is formed from
coating solutions comprising a high index conductive compound
colloid solution and a sol-gel low index oxide sol solution. The
coating is useful for coating organic substrates in that the
deposited film acquires a desired refractive index and thickness
without having to heat treat the deposited coating at a high
temperature, or heat treat the film followed by an etching
step.
[0039] An article of the invention comprises, in one embodiment, an
organic substrate having the aforementioned composite coating
deposited thereon. Suitable organic substrates include plastics
including optical lenses. In one embodiment, the method of the
invention for coating a substrate with the coating comprises
introducing to the surface of a substrate (such as an organic
substrate) a colloidal anti-reflective surface coating solution
comprising a high index colloid and sol-gel low index oxide sols
thereof to introduce a coating onto the substrate. The coating is
preferably cured at a temperature from about 50.degree. C. to
100.degree. C.
[0040] A method of the invention is an inexpensive, generally
simpler process compared to prior techniques as it eliminates the
need for expensive evaporators or plasma equipment. The formulating
and aging of a sol-gel does not require complex or expensive
equipment. Simple film application techniques, including spinning,
dipping, or spraying permit the coating of large complicated
shapes, and even simultaneous coating of the inner and outer
surfaces of a tube. The large scale application of a colloidal
surface coating is possible without a limit on the size of the part
to be coated. As noted above, there are many prior art sol-gel AR
surface coatings applied to vitreous substrates. However, the
process of the invention eliminates the necessity to heat and/or
etch the sol-gel coating to produce a porous, low index,
anti-reflecting glassy film.
[0041] In addition to its AR property, the composite coating of the
invention also possesses an anti-static property. In the embodiment
described above, the composite coating is conductive, having a
sheet resistance on the order of 1.7.times.10.sup.6 to
2.3.times.10.sup.6 ohms per square. The anti-static property arises
from the conductive nature of the coating. The anti-static property
of the composite coating of the invention allows a coated substrate
to resist the collection of dust particles.
[0042] The composite coating of the invention is particularly
useful for coating various substrates such as glass, ceramics,
metals, and organic polymeric materials to increase light
transmission, anti-static and EMI shielding performance, without
need to subject the coating or the substrate to high temperature
curing processes. Substrates coated with the coating of the
invention may be used, for example, in ophthalmic lenses, display
filters and solar photovoltaic applications.
[0043] The invention deposits a composition having a well
controlled thickness and refractive index, thereby exhibiting
minimum reflectance at a predetermined wavelength. FIG. 2 shows the
reflectance of an embodiment of a composite coating according to
the invention (ITO/SiO.sub.2) over a wavelength range of 450 to 750
nanometers. Optical measurements of reflectance or transmittance
are made using a Shimazu UV-1601 spectrophotometer equipped with
5-degree specular reflectance accessory. The composite coating is
cured at relatively low temperatures to form a durable coating
having anti-static characterics. In one case, the coating is formed
on a plastic substrate, such as a plastic lens.
[0044] Having been generally described, the following examples are
given as particular embodiments of the invention, to illustrate
some of the properties and demonstrate the practical advantages
thereof, and to allow one skilled in the art to utilize the
invention. It is understood that these examples are to be construed
as merely illustrative.
EXAMPLE 1
[0045] The preparation of a high refractive index metal oxide, ITO
(Indium Tin Oxide) colloidal coating solution is made as follows:
15 grams of ITO powder, 60 grams of dimethylformamide (DMF), 75
grams of ethanol, and 200 grams of zirconia bead are placed in a
250 cm.sup.3 glass bottle. The ITO powder is ground for
approximately 1 to 24 hours using a ball mill. The pH of the ITO
colloid is controlled in the range of 2 to 8. The ITO colloid is
diluted to 1.0 to 15.0 weight percent with a mixed solvent of
methanol, ethanol, butanol, 2-methoxyethanol, and 1-methoxy
2-propanol.
EXAMPLE 2
[0046] The preparation of a low refractive index metal oxide,
silica colloidal coating solution is made as follows: 200 grams of
tetraethylorthosilicate (TEOS), 100 grams of ethanol, and 200 grams
of water are mixed together for 30 minutes. The pH of the mixed
solution is controlled to 1.0 to 4.0. The pH controlled solution is
aged at 50.degree. C. for 2 to 24 hours. The silica colloid is
diluted to 0.5 to 6.0 weight percent with a mixed solvent of
methanol, ethanol, isopropanol, butanol, 2-methoxyethanol,
diacetone alcohol, and 1-methoxy2-propanol.
EXAMPLE 3
[0047] The example illustrates the introduction of a composite
coating according to the invention on a plastic substrate having a
hardcoat of polysiloxane on a surface thereof. Hardcoat is applied
on plastic, acrylic polymer substrate to a thickness of 1-10 .mu.m.
The 2.3 weight percent ITO colloidal coating solution of Example 1
is spin-coated on the hardcoat of the plastic substrate at the rate
of 300 rpm for one minute to a thickness of 500-1500 .ANG.. Silica
coating solutions of different concentrations are spin-coated on
different substrates over the ITO layer at the rate of 300 rpm for
one minute to a thickness of about 500-1000 .ANG.. The substrates
with the hardcoat/ITO/silica are baked at 100.degree. C. for 30
minutes to form a hard coating. Table 1 shows the reflectance of
the baked substrates at a wavelength of 550 nm.
1TABLE 1 Absolute reflectance Concentration of ITO Concentration of
at the wavelength coating solution silica coating solution of 550
nm 2.3 wt. % 1.4 wt. % 0.35 2.3 wt. % 1.5 wt. % 0.38 2.3 wt. % 1.6
wt. % 0.57
[0048] In the preceding detailed description, the invention is
described with reference to specific embodiments thereof. It will,
however, be evident that various modifications and changes may be
made thereto without departing from the broader spirit and scope of
the invention as set forth in the claims. The specification and
drawings are, accordingly, to be regarded in an illustrative rather
than a restrictive sense.
* * * * *