U.S. patent application number 14/577669 was filed with the patent office on 2016-04-07 for property enhancing fillers for transparent coatings and transparent conductive films.
The applicant listed for this patent is C3Nano Inc.. Invention is credited to Hua Gu, Faraz Azadi Manzour, Ajay Virkar, Xiqiang Yang.
Application Number | 20160096967 14/577669 |
Document ID | / |
Family ID | 55631590 |
Filed Date | 2016-04-07 |
United States Patent
Application |
20160096967 |
Kind Code |
A1 |
Virkar; Ajay ; et
al. |
April 7, 2016 |
PROPERTY ENHANCING FILLERS FOR TRANSPARENT COATINGS AND TRANSPARENT
CONDUCTIVE FILMS
Abstract
Optically transparent films can comprise a coating of
nanodiamonds to introduce desirable properties, such as hardness,
good thermal conductivity and an increased dielectric constant. In
general, transparent conductive films can be formed with desirable
property enhancing nanoparticles included in a transparent
conductive layer and/or in a coating layer. Property enhancing
nanoparticles can be formed from materials having a large hardness
parameter, a large thermal conductivity and/or a large dielectric
constant. Suitable polymers are incorporated as a binder in the
layers with the property enhancing nanoparticles. The coatings with
property enhancing nanoparticles can be solution coated and
corresponding solutions are described.
Inventors: |
Virkar; Ajay; (San
Francisco, CA) ; Manzour; Faraz Azadi; (Berkeley,
CA) ; Yang; Xiqiang; (Hayward, CA) ; Gu;
Hua; (Dublin, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
C3Nano Inc. |
Hayward |
CA |
US |
|
|
Family ID: |
55631590 |
Appl. No.: |
14/577669 |
Filed: |
December 19, 2014 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
62059376 |
Oct 3, 2014 |
|
|
|
Current U.S.
Class: |
428/215 ; 252/74;
252/75; 252/76; 428/323; 428/336; 428/337; 428/413; 428/418;
428/457; 428/464; 428/532 |
Current CPC
Class: |
C09D 101/10 20130101;
C08K 3/04 20130101; C09D 133/14 20130101; C09D 101/26 20130101;
C09D 101/18 20130101; G02B 1/14 20150115; C09D 163/00 20130101;
G02B 1/16 20150115; C09D 101/02 20130101; C08K 3/04 20130101; C09D
133/14 20130101; C09D 133/14 20130101; C08K 3/04 20130101 |
International
Class: |
C09D 101/02 20060101
C09D101/02; G02B 1/14 20060101 G02B001/14; C09D 133/14 20060101
C09D133/14; C09D 163/00 20060101 C09D163/00; C09D 133/08 20060101
C09D133/08 |
Claims
1-20. (canceled)
21. A solution comprising a solvent, a curable polymer binder and
nanoparticles having an average primary particle diameter of no
more than about 100 nm and comprising a material having a bulk
Vickers Hardness of at least about 1650 HV, a high thermal
conductivity material having a bulk thermal conductivity of at
least about 30 W/(mK), a high dielectric material selected from the
group consisting of barium titanate, strontium titanate, lead
titanate, lead zirconium titanate, calcium copper titanate and
mixtures thereof, or a mixture thereof, wherein the curable polymer
binder comprises acrylic resin, copolymers thereof or mixtures
thereof.
22. The solution of claim 21 having from about 0.005 wt % to about
5 wt % nanoparticles.
23. The solution of claim 21 wherein the nanoparticles comprise
nanodiamond.
24. The solution of claim 21 wherein the solvent comprises water,
ethanol, isopropyl alcohol, isobutyl alcohol, tertiary butyl
alcohol, methyl ethyl ketone, methyl isobutyl ketone, cyclic
ketones, glycol ethers, toluene, hexane, ethyl acetate, butyl
acetate, ethyl lactate, propylene carbonate, dimethyl carbonate,
PGMEA (2-methoxy-1-methylethylacetate), N,N-dimethylformamide,
N,N-dimethylacetamide, acetonitrile, formic acid, or mixtures
thereof.
25. The solution of claim 21 wherein the curable polymer binder
further comprises polysiloxanes, polyurethanes, epoxy containing
polymers, copolymers thereof, and mixtures thereof.
26. The solution of claim 21 having a polymer binder concentration
from about 0.025 wt % to about 50 wt % and a nanoparticle
concentration from 0.005 wt % to about 5 wt %.
27. The solution of claim 21 wherein the solvent is
non-aqueous.
28. The solution of claim 21 wherein the solution can be coated to
form a transparent coating with an average thickness of a micron
onto a sparse metal conductive element having a pencil hardness of
at least about 1 grade greater than the pencil hardness of the
transparent coating without the filler and a decrease in the value
of total transmission of visible light expressed as a percent due
to the transparent coating of no more than about 5.
29. The solution of claim 21 wherein the nanoparticles comprise
dielectric nanodiamonds at a concentration in the solution from
about 0.005 wt % to about 0.05 wt % and wherein the solution can be
coated to form a transparent coating with an average thickness of a
micron having a decrease in the value of total transmission of
visible light expressed as a percent due to the dielectric
nanodiamonds of no more than about 5.
30. A solution comprising a solvent, a curable polymer binder and
nanoparticles having an average primary particle diameter of no
more than about 100 nm and comprising a material having a bulk
Vickers Hardness of at least about 1650 HV, a high thermal
conductivity material having a bulk thermal conductivity of at
least about 30 W/(mK), a high dielectric material selected from the
group consisting of barium titanate, strontium titanate, lead
titanate, lead zirconium titanate, calcium copper titanate and
mixtures thereof, or a mixture thereof, wherein the solution has a
polymer binder concentration from about 0.025 wt % to about 50 wt %
and a solvent concentration of at least about 45%.
31. The solution of claim 30 having from about 0.005 wt % to about
5 wt % nanoparticles.
32. The solution of claim 30 wherein the nanoparticles comprise
dielectric nanodiamond.
33. The solution of claim 30 wherein the solvent comprises ethanol,
isopropyl alcohol, isobutyl alcohol, tertiary butyl alcohol, methyl
ethyl ketone, methyl isobutyl ketone, cyclic ketones, glycol
ethers, toluene, hexane, ethyl acetate, butyl acetate, ethyl
lactate, propylene carbonate, dimethyl carbonate, PGMEA
(2-methoxy-1-methylethylacetate), N,N-dimethylformamide,
N,N-dimethylacetamide, acetonitrile, formic acid, or mixtures
thereof.
34. The solution of claim 30 wherein the curable polymer binder
comprises polysiloxanes, polysilsesquioxanes, polyurethanes,
acrylic resins, acrylic copolymers, cellulose ethers and esters,
nitrocellulose, other water insoluble structural polysaccharides,
polyethers, polyesters, styrene-acrylate copolymers,
styrene-butadiene copolymers, acrylonitrile butadiene styrene
copolymers, polysulfides, epoxy containing polymers, copolymers
thereof, and mixtures thereof.
35. The solution of claim 30 wherein the curable polymer binder
resin comprises polyurethanes, acrylic resins, acrylic copolymers,
epoxy containing polymers, copolymers thereof, and mixtures
thereof.
36. The solution of claim 30 having from about 0.01 wt % to about 1
wt % nanoparticles.
37. The solution of claim 30 wherein the nanoparticles comprise
dielectric nanodiamond at a concentration in the solution from
about 0.005 wt % to about 0.05 wt % and wherein the solution can be
coated to form a transparent coating with an average thickness of a
micron and a decrease in the value of total transmission of visible
light expressed as a percent due to the nanoparticles of no more
than about 5.
38. A solution comprising a solvent, a curable polymer binder and
dielectric nanodiamonds particles having an average primary
particle diameter of no more than about 100 nm at a concentration
in the solution from about 0.005 wt % to about 0.05 wt %, wherein
the solution can be coated to form a transparent coating with an
average thickness of a micron and a decrease in the value of total
transmission of visible light expressed as a percent due to the
nanodiamond particles of no more than about 5.
39. The solution of claim 38 wherein the curable polymer binder
resin comprises polyurethanes, acrylic resins, acrylic copolymers,
epoxy containing polymers, copolymers thereof, and mixtures
thereof.
40. The solution of claim 38 having from about 0.01 wt % to about 5
wt % nanoparticles.
41. The solution of claim 38 wherein the solvent is
non-aqueous.
42. The solution of claim 38 wherein the coating is formed on a
sparse metal conductive element and has a pencil hardness of at
least about 1 grade greater than the pencil hardness of the
transparent coating without the filler.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application claims priority to copending U.S.
provisional patent application Ser. No. 62/059,376, filed Oct. 3,
2014 to Virkar et al., entitled "Property Enhancing Fillers for
Coatings and Transparent Conductive Films," incorporated herein by
reference.
FIELD OF THE INVENTION
[0002] The invention is related to thin polymer films loaded with
property enhancing nanoparticles, such as nanoparticles
contributing to hardness and abrasion resistance, thermal
conductivity and/or a high dielectric constant. The invention
further relates to transparent conductive films incorporating a
thin polymer layer loaded with property enhancing nanoparticles,
which may or may not be in a layer providing the electrical
conductivity and/or a coating layer associated with the transparent
conductive layer. The invention also relates to transparent
polymer-based films comprising nanodiamonds. In addition, the
invention relates to coating solutions that comprise dissolved
polymers, dispersed property enhancing nanoparticles, other
optional compositions, such as processing aids or stabilization
compositions, and optional metal nanowires.
BACKGROUND OF THE INVENTION
[0003] Transparent polymer films are used in a wide range of
products. While the films can serve many purposes, generally the
films provide some protection from various mechanical and/or
environmental assaults. Protection provided by the film can be
directed both to underlying structure as well as the film itself
since, for example, a scratched surface of the film can degrade the
desired performance of the film by decreasing transparency and
increasing blurring or haze. Protection of surfaces can be
significant both in use of the ultimate product as well as during
processing to form the product and transporting components for
assembly into the product.
[0004] Functional films can provide important roles in a range of
contexts. For example, electrically conductive films can be
important for the dissipation of static electricity when static can
be undesirable or dangerous. Optical films can be used to provide
various functions, such as polarization, anti-reflection, phase
shifting, brightness enhancement or other functions. High quality
displays can comprise one or more optical coatings.
[0005] Transparent conductors can be used for several
optoelectronic applications including, for example, touch-screens,
liquid crystal displays (LCD), flat panel display, organic light
emitting diode (OLED), solar cells and smart windows. Historically,
indium tin oxide (ITO) has been the material of choice due to its
relatively high transparency at high conductivities. There are
however several shortcomings with ITO. For example, ITO is a
brittle ceramic which needs to be deposited using sputtering, a
fabrication process that involves high temperatures and vacuum and
therefore can be relatively slow. Additionally, ITO is known to
crack easily on flexible substrates.
SUMMARY OF THE INVENTION
[0006] In a first aspect, the invention pertains to an optical
structure comprising a transparent substrate and a coating
comprising a polymer binder and nanodiamonds.
[0007] In a further aspect, the invention pertains to a transparent
conductive film comprising a transparent substrate, a transparent
electrically conductive layer and a protective coating comprising a
polymer binder and nanoparticles. In some embodiments, the
nanoparticles have an average primary particle diameter of no more
than about 100 nm and are formed of a material having a bulk
Vickers Hardness of at least about 1650 HV, a high thermal
conductivity material selected from the group consisting of
diamond, graphene, silicon nitride, boron nitride, aluminum
nitride, gallium arsenide, indium phosphide or a mixture thereof
and/or a high dielectric constant material selected from the group
consisting of barium titanate, strontium titanate, lead titanate,
lead zirconium titanate, calcium copper titanate and mixtures
thereof.
[0008] In additional aspects, the invention pertains to a
transparent conductive film comprising a transparent substrate and
a transparent electrically conductive layer comprising a polymer
binder, a sparse metal conductive element and nanoparticles. In
some embodiments, the nanoparticles can have an average primary
particle size of no more than about 100 nm and can be formed of a
material having a bulk Vickers Hardness of at least about 1650 HV,
a high thermal conductivity material selected from the group
consisting of diamond, graphene, silicon nitride, boron nitride,
aluminum nitride, gallium arsenide, indium phosphide or a mixture
thereof and/or a high dielectric constant material selected from
the group consisting of barium titanate, strontium titanate, lead
titanate, lead zirconium titanate, calcium copper titanate and
mixtures thereof.
[0009] In other aspects, the invention pertains to an optical
structure comprising a transparent substrate and a transparent
coating. The transparent coating can comprise a polymer binder and
from about 0.05 weight percent to about 30 weight percent
nanoparticles with an average primary particle diameter of no more
than about 100 nm, and can have a pencil hardness of at least about
1 grade greater than the pencil hardness of the transparent coat
without the filler and a decrease in total transmission of visible
light due to the transparent hard coat of no more than about
5%.
[0010] Moreover, the invention pertains to a solution comprising a
solvent, a curable polymer binder and nanoparticles. The
nanoparticles can have an average primary particle diameter of no
more than about 100 nm and can comprise a material having a bulk
Vickers Hardness of at least about 1650 HV, a high thermal
conductivity material having a bulk thermal conductivity of at
least about 30 W/(mK), a high dielectric constant material selected
from the group consisting of barium titanate, strontium titanate,
lead titanate, lead zirconium titanate, calcium copper titanate and
mixtures thereof, or a mixture thereof.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] FIG. 1 is a fragmentary side view of a film with a sparse
metal conductive layer and various additional transparent layers on
either side of the sparse metal conductive layer.
[0012] FIG. 2 is a top view of a representative schematic patterned
structure with three electrically conductive pathways formed with
sparse metal conductive layers.
[0013] FIG. 3 is a schematic diagram showing a capacitance based
touch sensor.
[0014] FIG. 4 is a schematic diagram showing a resistance based
touch sensor.
[0015] FIG. 5 is a scanning electron micrograph (SEM) of a
transparent conductive film with an overcoat having 10 wt %
nanodiamonds at a first magnification.
[0016] FIG. 6 is an SEM image of the transparent conductive film of
FIG. 5 at a greater magnification.
[0017] FIG. 7 is an SEM image of a transparent conductive film with
an overcoat having 5 wt % nanodiamonds.
[0018] FIG. 8 is an SEM image of a transparent conductive film with
an overcoat having 3 wt % nanodiamonds.
DETAILED DESCRIPTION OF THE INVENTION
[0019] Transparent coatings have been developed with polymer
matrices with property enhancing nanoparticle fillers to provide
desired properties for the coatings, such as increased hardness
and/or greater thermal conductivity, in a thin coating with good
optical transparency. Suitable fillers for the polymer matrices
include, for example, nanodiamonds that can provide desirable
hardness, increased dielectric constant, and thermal conductivity
to a coating formed with the nanodiamonds without decreasing the
optical transparency by an undesirable amount. Other appropriate
nanoparticles or combinations thereof can be similarly incorporated
into the polymer matrix. The nanoparticles for use as fillers can
be formed from materials that have a high bulk hardness value
and/or a high bulk thermal conductivity and/or a high bulk
dielectric constant. In some embodiments, the coatings formed from
the particle loaded polymer can have a thickness of no more than
about 5 microns. The enhanced coatings can be formed through a
solution coating process in which the matrix polymer is dissolved
in a solvent and the nanoparticles are dispersed in the solution.
The coatings can be suitable for protection of transparent
electrically conductive layers, although other transparent coating
applications can make effective use of the enhanced coatings
described herein. In particular, transparent conductive layers can
be formed from metal nanowires. In additional or alternative
embodiments, the desirable fillers can be added directly to a
conductive ink used to form a sparse metal conductive element with
a corresponding increase in hardness and other property
improvements following the coating with a polymer overcoat. The
protective coatings can be useful to reduce damage from scratching,
environmental assaults, such as dilute acids and bases, reduce
thermal damage, decrease vulnerability from high voltages and/or
provide other valuable protection.
[0020] As described herein, the enhanced loaded coatings can be
formed with a modest drop in total transmission of visible light.
Various polymer matrices can be introduced for the coatings with
relatively good mechanical strength to provide a good high
transparency base for further enhancement. Generally, the coatings
can be formed with small thicknesses, and the enhanced mechanical
properties can be effective to mechanically stabilize the coatings
even with the small thicknesses. In some embodiments, small
thicknesses can be desirable for use adjacent transparent
conductive layers since electrical conductivity can be maintained
through the thin overcoats. Thus, with coatings having average
thicknesses of no more than about 25 microns and in some
embodiments no more than a micron, and generally at least about 50
nm thick, significant mechanical stability can be obtained.
Furthermore, thermal conductivity properties of the enhanced
coatings can be desirable to dissipate heat so that damage from
heating can be reduced. Improved thermal conductivity can provide
other desirable uses for specific applications. A coating with a
high dielectric filler can be useful to protect sparse metal
conductive layers from damage from high voltage.
[0021] Good coating properties generally involve the formation of a
good dispersion of the nanoparticle fillers within a solution of
the matrix polymer so that the resulting coating has reduced
effects of particle clumps. The nanoparticle fillers generally have
an average primary particle diameter of no more than about 100 nm
so that the particles can be incorporated into a relatively smooth
thin coating and so that the particles do not alter the optical
properties more than desired. In general, the coatings have a
nanoparticle loading of no more than about 70 weight percent. The
concentrations of polymer binder and filler particles in a coating
solution can be adjusted to yield desirable coating properties for
the solution, such as viscosity, and thickness of the final
coating. The ratio of concentration of solids in the coating
solution can be adjusted to yield the coating concentrations
desired once the coating is dried. The polymer component of the
coating generally can be crosslinked with UV radiation or other
means appropriate for the polymer binder to further strengthen the
coating.
[0022] In general, the property enhancing nanoparticle fillers can
be introduced into a passive protective coating and/or directly
into a transparent conductive layer. Passive transparent protective
coatings may or may not be used to cover a transparent conductive
layer. A common feature for these coatings is the compatibility of
the components in a coating solution as well as in the resulting
composite material. Compatibility refers to the ability to
effectively disperse into a relatively uniform material without an
unacceptable degree of aggregation of the components, such as with
clumping. In particular, the compatibility can allow for good
distribution of the materials within the coating solutions to
provide for formation of a reasonably uniform composite material
forming the coating. A more uniform composite material is believed
to contribute to desirable optical properties of the coating, such
as good transparency and low haze.
[0023] For the passive coatings, the coating solutions can comprise
a solvent, dissolved matrix polymer, nanoparticles with selected
properties, possible combinations thereof and optional additional
components. A range of matrix polymers can be used that are
suitable for transparent films, as described below. Wetting agents,
such as surfactants, can be used as well as other processing aids.
In general, the solvents can comprise water, organic solvents or
suitable mixtures thereof. For the active coatings, the coating
solutions generally further comprise components that contribute to
the active functionality, such as metal nanowires for contributing
electrical conductivity. Examples of both types of coatings are
described below in the Examples. For use as an overcoat for a metal
nanowire based transparent conductive layers, it has been found
that stabilizers introduced into the overcoat can stabilize the
electrical conductivity of the transparent conductive layer. The
stabilizers are consistent with maintaining good transparency and
process compatibility for the coating solutions, and are described
further below.
[0024] With respect to desirable fillers, nanodiamonds are of
particular interest due to desirable properties that can be
introduced consistent with maintaining good optical transparency
and relatively low haze. Diamond is a crystalline form of carbon
with sp.sup.3 hybridized orbitals, in contrast with graphitic
carbon, amorphous carbon and other forms of carbon. Commercial
nanodiamonds generally can have a core of crystalline diamond
carbon with a shell of amorphous and/or graphitic carbon, and are
dielectrics. The surface chemistry of the nanodiamonds can reflect
the synthesis approach and possibly additional processing.
Commercial nanodiamonds, which can be functionalized or
unfunctionalized following purification, are available from various
suppliers as listed below. Nanodiamond share with macroscopic
diamonds very high values of hardness and thermal conductivity, and
these properties can be used to deliver desirable properties to
transparent coatings incorporating nanodiamonds.
[0025] Nanodiamonds are commercially available with average primary
particle diameters generally no more than about 50 nm and in some
embodiments no more than about 10 nm, although nanodiamonds may be
useful in some embodiments with average primary particle diameters
of no more than about 100 nm. As used herein unless indicated
otherwise, particle diameters are an average of values along the
principle axes of the particle, which can be roughly estimated from
transmission electron micrographs. Commercial nanodiamonds are
produced synthetically with possible surface modification, and
their overall structure can be confirmed using spectroscopic
techniques. Surface modification of the nanodiamonds can be useful
for processing of the nanodiamonds and for compatibility with
particular solvents and binders. As described in the examples
below, the commercial nanodiamonds can be well dispersed in a range
of solvents for the production of high quality optical coatings
with good transparency and low haze. Other nanoparticle fillers can
have average particle diameters over the same ranges as the
nanodiamonds. The nanoparticles can have roughly spherical shapes
or other convenient shapes. A person of ordinary skill in the art
will recognize that additional ranges within the explicit average
particle diameter ranges above for nanodiamonds or other property
enhancing nanoparticles are contemplated and are within the present
disclosure.
[0026] The nanodiamonds can provide a desirable degree of hardness
and thermal conductivity to a composite coating incorporating the
nanodiamonds. Also, diamonds are a good dielectric so that a
nanodiamond composite coatings can facilitate dissipation of strong
electric fields that can damage films in the structure. Other
nanoparticles can be similarly introduced to provide similar
properties to composites incorporating the functional nanoparticles
consistent with good optical transparency of a resulting coating.
For the formation of transparent conductive films, other suitable
nanoparticles for providing hardness include but not limited to,
for example, boron nitride, B.sub.4C, cubic-BC.sub.2N, silicon
carbide, crystalline alpha-aluminum oxide (sapphire), or the like.
Hardness contributing nanoparticles can be formed from a bulk
material having a Vickers hardness of at least about 1650
kgf/mm.sup.2 (16.18 GPa).
[0027] With respect to thermal conductivity, in addition to
nanodiamonds, graphene, silicon nitride, boron nitride, aluminum
nitride, gallium arsenide, indium phosphide and mixtures thereof
can be suitable for introducing high thermal conductivity. In some
embodiments, high thermal conductivity materials can have a thermal
conductivity of at least about 30 W/(mK), and graphene and diamond
have among the highest thermal conductivities known. Particularly
high dielectric constant materials that can be introduced as
nanoparticles include but not limited to, for example, barium
titanate, strontium titanate, lead titanate, lead zirconium
titanate, calcium copper titanate and mixtures thereof. With
respect to the hardness of the protective polymer based coatings,
hardness can be measured with the pencil hardness test for films,
as described further below. Scratch resistance is also evaluated
with the use of steel wool in the Examples below.
[0028] The coatings are generally formed by solution coating. The
nanoparticles, such as the nanodiamonds, can be dispersed and then
the dispersion of nanoparticles can be blended with the coating
solution of the polymer binder, although processing orders may be
suitable depending on the selection of solvent and the dispersion
properties of the particles. The nanoparticles in the coating
solution can have a concentration in the ranges from about 0.005 wt
% to about 5.0 wt %, in further embodiments from about 0.0075 wt %
to about 1.5 wt % and in additional embodiments from about 0.01 wt
% to about 1.0 wt %. A person of ordinary skill in the art will
recognize that additional ranges of concentrations within the
explicit ranges above are contemplated and are within the present
disclosure.
[0029] Transparent electrically conductive elements, e.g., films,
of particular interest herein comprise a sparse metal conductive
layer. The conductive layers are generally sparse to provide
desired amount of optical transparency, so the coverage of the
metal has very significant gaps over the layer of the conductive
element. For example, transparent electrically conductive films can
comprise metal nanowires deposited along a layer where sufficient
contact can be provided for percolation to provide suitable
conduction pathways. In other embodiments, the transparent
electrically conductive film can comprise a fused metal
nanostructured network, which has been found to exhibit desirable
electrical and optical properties. Conductivity referenced herein
refers to electrical conductivity unless specifically indicated
otherwise.
[0030] The loaded polymer films described herein can provide
desirable properties generally for transparent optical films and in
particular for protection of sparse metal conductive elements in
transparent conductive films. The thicknesses of the film can be
selected thin enough that good electrical conductivity can take
place through the films. The hardness of the films can make the
structure resistant to scratching and deformation and high thermal
conductivity can facilitate removal of heat to limit potential
damage of a sparse metal conductive element due to heat. Sparse
metal conductive elements, regardless of the specific structures,
are vulnerable to environmental assaults.
[0031] In general, various sparse metal conductive layers can be
formed from metal nanowires. Films formed with metal nanowires that
are processed to flatten the nanowires at junctions to improve
conductivity is described in U.S. Pat. No. 8,049,333 to Alden et
al., entitled "Transparent Conductors Comprising Metal Nanowires,"
incorporated herein by references. Structures comprising surface
embedded metal nanowires to increase metal conductivity are
described in U.S. Pat. No. 8,748,749 to Srinivas et al., entitled
"Patterned Transparent Conductors and Related Manufacturing
Methods," incorporated herein by reference. However, desirable
properties have been found for fused metal nanostructured networks
with respect to high electrical conductivity and desirable optical
properties with respect to transparency and low haze. Fusing of
adjacent metal nanowires can be performed based on chemical
processes under commercially appropriate processing conditions.
[0032] Metal nanowires can be formed from a range of metals, and
metal nanowires are available commercially. While metal nanowires
are inherently electrically conducting, the vast majority of
resistance in the metal nanowires based films is believed to due to
the junctions between nanowires. Depending on processing conditions
and nanowire properties, the sheet resistance of a relatively
transparent nanowire film, as deposited, can be very large, such as
in the giga-ohms/sq range or even higher. Various approaches have
been proposed to reduce the electrical resistance of the nanowire
films without destroying the optical transparency. Low temperature
chemical fusing to form a metal nanostructured network has been
found to be very effective at lowering the electrical resistance
while maintaining the optical transparency.
[0033] In particular, a significant advance with respect to
achieving electrically conductive films based on metal nanowires
has been the discovery of well controllable processes to form a
fused metal network where adjacent sections of the metal nanowires
fuse. Fusing of metal nanowires with various fusing sources is
described further in published U.S. patent applications
2013/0341074 to Virkar et al., entitled "Metal Nanowire Networks
and Transparent Conductive Material," and 2013/0342221 to Virkar et
al. (the '221 application), entitled "Metal Nanostructured Networks
and Transparent Conductive Material," 2014/0238833 to Virkar et al.
(the '833 application), entitled "Fused Metal Nanostructured
Networks, Fusing Solutions With Reducing Agents and Methods for
Forming Metal Networks," and copending U.S. patent application Ser.
No. 14/087,669 to Yang et al. (the '669 application), entitled
"Transparent Conductive Coatings Based on Metal Nanowires, Solution
Processing Thereof, and Patterning Approaches," copending U.S.
patent application Ser. No. 14/448,504 to Li et al, entitled "Metal
Nanowire Inks for the Formation of Transparent Conductive Films
With Fused Networks," all of which are incorporated herein by
reference.
[0034] The transparent conductive films generally comprise several
components or layers that contribute to the processability and/or
the mechanical properties of the structure without detrimentally
altering the optical properties. The sparse metal conductive layers
can be designed to have desirable optical properties when
incorporated into the transparent conductive films. The sparse
metal conductive layer may or may not further comprise a polymer
binder. Unless otherwise indicated, references to thicknesses refer
to average thicknesses over the referenced layer or film, and
adjacent layers may intertwine at their boundaries depending on the
particular materials. In some embodiments, the total film structure
can have a total transmission of visible light of at least about
85%, a haze of no more than about 2 percent and a sheet resistance
after formation of no more than about 250 ohms/sq, although
significantly better performance is described herein.
[0035] For incorporation into transparent coatings for transparent
conductive films or directly into the ink for the formation of a
sparse metal conductive layer, the loaded overcoats generally do
not increase the sheet resistance significantly, and in some
embodiments the sheet resistance increases relative to the sheet
resistance of corresponding unloaded films by no more than about
20%, in further embodiments, no more than about 15% and in
additional embodiments, no more than about 10%. For general optical
applications, the overcoat can decrease the total transmittance of
visible light relative to the value of total transmission in
percent of a corresponding unloaded film by no more than about 5,
in further embodiments no more than about 3, in additional
embodiments no more than about 2 and in other embodiments no more
than about 1. Also, it can be desirable for the haze to not
increase by a large amount with the filler in the coating. In some
embodiments, the haze value can increase relative to the haze value
of a corresponding unloaded film by no more than about 0.5, in
further embodiments by no more than about 0.4 and in additional
embodiments no more than about 0.3 in units of haze generally
reported as a percent. In some embodiments, haze may decrease. A
person of ordinary skill in the art will recognize that additional
ranges of sheet resistance increase, total transmittance change and
haze change within the explicit ranges above are contemplated and
are within the present disclosure. A reference unloaded film is
produced with the coating solution that has the same concentrations
of other components in the solvent and is processed the same way so
that the final thickness may be slightly different.
[0036] It has been found that very effective stabilization of the
sparse metal conductive layer can be achieved through the
appropriate design of the overall structure. In particular, a
stabilization composition can be placed in a layer adjacent the
sparse metal conductive element, which can be an overcoat layer or
an undercoat layer. Furthermore, an optically clear adhesive, e.g.
as a component of the film, can be used to provide for attaching
the transparent conductive film to a device, and the selection of
the optically clear adhesive has been found to significantly
facilitate obtaining a desired degree of stabilization. In
particular, optically clear adhesives can comprise a double sided
adhesive layers on a carrier layer. The carrier layer can be a
polyester, such as PET or a commercial barrier layer material,
which may provide a desirable moisture and gas barrier to protect
the sparse metal conductive layers, although Applicant does not
want to be limited by a theory of operation of particular optically
clear adhesives.
[0037] Transparent, electrically conductive films find important
applications, for example in solar cells and touch screens.
Transparent conductive films formed from metal nanowire components
offer the promise of lower processing cost and more adaptable
physical properties relative to traditional materials. In a
multilayered film with various structural polymer layer(s), the
resulting film structure has been found to be robust with respect
to processing while maintaining desirable electrical conductivity,
and the incorporation of desirable components as described herein
can additionally provide stabilization without degrading the
functional properties of the film so that devices incorporating the
films can have suitable lifetimes in normal use.
Transparent Coatings and Films
[0038] The transparent coatings with nanoparticle loaded polymers
described herein are generally coated onto a transparent substrate
for incorporation into a desired structure. General structures are
described, and specific applications for transparent conductive
films are found in the following section. In general, a precursor
solution for the transparent filled coatings can be deposited using
appropriate coating methods onto a transparent substrate to form a
transparent structure. In some embodiments, the transparent
substrate can be a film for incorporation into an ultimate device
or alternatively or additionally an integral optical component,
such as a light emitting device or a light receiving device. The
discussion focuses on a simple passive transparent substrate and
other structures follow accordingly.
[0039] In general, any reasonable transparent substrate can be
suitable. Thus, suitable substrates can be formed, for example,
from inorganic glasses, such as silicate glasses, transparent
polymer films, inorganic crystals or the like. In some embodiments,
the substrate is a polymer film. Suitable polymers for a substrate
include, for example, polyethylene terephthalate (PET),
polyethylene naphthalate (PEN), polyacrylate, poly(methyl
methacrylate), polyolefin, polyvinyl chloride, fluoropolymers,
polyamide, polyimide, polysulfone, polysiloxane,
polyetheretherketone, polynorbornene, polyester, polystyrene,
polyurethane, polyvinyl alcohol, polyvinyl acetate,
acrylonitrile-butadiene-styrene copolymer, cyclic olefin polymer,
cyclic olefin copolymer, polycarbonate, copolymers thereof or blend
thereof or the like. Fluoropolymers include, for example,
polyvinylfluoride, polyvinylidenefluoride, polytetrafluoroethylene,
hexafluoropropylene, perfluoropropylvinylether,
perfluoromethylvinylether, polychlorotrifluoroethylene, and the
like. Polymer films for some embodiments can have a thickness from
about 5 microns to about 5 mm, in further embodiments, from about
10 microns to about 2 mm and in additional embodiment from about 15
microns to about 1 mm. A person of ordinary skill in the art will
recognize that additional ranges of thicknesses within the explicit
ranges above are contemplated and are within the present
disclosure. Substrates can comprise a plurality of layers
distinguished by composition and/or other properties. More specific
ranges of materials suitable for substrates for transparent
conductive films are presented below, and the general substrate
ranges would include these specific materials and properties.
Suitable polymers for the coatings can include, for example,
radiation curable polymers and/or heat curable polymers, such as
polyurethanes, acrylic resins, acrylic copolymers, cellulose ethers
and esters, other structural polysaccharides, polyethers,
polyesters, epoxy containing polymers, copolymers thereof, and
mixtures thereof.
[0040] The transparent coating with property enhancing nanoparticle
filler generally can have a thickness of no more than about 25
microns, in further embodiments from about 20 nanometers (nm) to
about 10 microns, in other embodiments from about 35 nm to about 5
microns, and in additional embodiments from about 50 nm to about 2
microns. The transparent coatings formed from the nanoparticle
loaded polymers can comprise from about 0.01 weight percent (wt %)
to about 70 wt % property enhancing nanoparticles, in further
embodiments from about 0.05 wt % to about 60 wt %, in other
embodiments from about 0.1 wt % to about 50 wt %, and in additional
embodiments from about 0.2 wt % to about 40 wt % property enhancing
nanoparticles. The transparent coatings can further comprise
polymer binder, optional property modifiers, such as crosslinking
agents, wetting agents, viscosity modifiers, and/or stabilizers,
such as antioxidants and/or UV stabilizers, for transparent
conductive films, and optionally a sparse metal conductive layer. A
person of ordinary skill in the art will recognize that additional
ranges of thickness and nanoparticle concentrations in the loaded
polymers within the explicit ranges above are contemplated and are
within the present disclosure.
[0041] With respect to property enhancing nanoparticles,
nanodiamonds present desirable properties in particular with
respect to hardness and thermal conductivity as well as to some
degree dielectric constant. Bulk diamonds have among the greatest
values of known materials with respect to both hardness and thermal
conductivity. However, additional materials provide desirable
values for these properties. For convenience, the material
properties are referenced to the corresponding bulk materials since
the values for the nanoparticles may be less available, although
the nanoparticle properties generally directly reflect roughly the
bulk properties. The material of the property enhancing
nanoparticles is generally either inorganic materials or carbon
materials with a majority of the material being elemental carbon,
which are known, for example, in fullerenes, 3-dimensional crystals
(diamond), 2-dimensional crystals (graphitic carbon), amorphous
forms (e.g., carbon blacks), and the like. The nanoparticle can
have surface modifications, including organic surface
modifications, without altering identification of the nanoparticles
according to the majority core material.
[0042] For the materials of relevance, the hardness of the bulk
materials can be referenced to a Vickers hardness measurement.
Vickers hardness is a measure of indenting the material. Vickers
hardness can be measured with accepted standards, which include
ASTM E384 and ISO 6507-1-2005, both of which are incorporated
herein by reference. Vickers hardness is tabulated for many
materials of interest. Vickers hardness is generally reported in
units of HV (Vickers pyramid number, kg-force/mm.sup.2), although
it may be reported in units of Pascal even though it is not
actually a pressure. In some embodiments, the bulk material
corresponding to the nanoparticles can have a Vickers Hardness of
at least about 1650 HV, in some embodiments at least about 1750 HV
and in additional embodiments at least about 1800 HV. In addition
to nanodiamonds, additional hard materials for the property
enhancing nanoparticles include, for example, boron nitride,
B.sub.4C, cubic-BC.sub.2N, silicon carbide, tungsten carbide,
aluminum boride, crystalline alpha-aluminum oxide (sapphire), or
the like.
[0043] With respect to high thermal conductivity materials,
suitable materials can have bulk thermal conductivities of at least
about 30 W/(mK), in further embodiments at least about 35 W/(mK),
and in some embodiments at least about 50 W/(mK). A person of
ordinary skill in the art will recognize that additional ranges of
thermal conductivity within the explicit ranges above are
contemplated and are within the present disclosure. Suitable high
thermal conductivity materials, apart from nanodiamonds, include,
for example, many elemental metals (unionized elemental form) and
metal alloys, graphene, silicon nitride, boron nitride, aluminum
nitride, gallium arsenide, indium phosphide, aluminum oxide, and
mixtures thereof. With respect to high dielectric constant, various
titanates have high dielectric constants, such as barium titanate,
strontium titanate, lead titanate, lead zirconium titanate, calcium
copper titanate and mixtures thereof.
[0044] Relevant nanoparticles are generally available commercially.
Nanoparticles sources include, for example, US Research
Nanomaterials, Inc. (Texas, USA), which sells many of the materials
of interest, BYK-Chemie GMbH. (Germany), Sigma-Aldrich (Missouri,
USA), Nanostructured and Amorphous Materials (Texas, USA), Sky
Spring Nano Materials Inc. (Texas, USA) and Nanophase Technologies
Corp. (Romeoville, Ill., USA). Also, laser pyrolysis techniques
have been developed for the synthesis of a wide range of
dispersible nanoparticles, as described in U.S. Pat. No. 7,384,680
to Bi et al., entitled "Nanoparticle-Based Powder Coatings and
Corresponding Structures," incorporated herein by reference.
[0045] Nanodiamonds, or diamond nanoparticles, can be generally
natural nanodiamonds or synthetic nanodiamonds, and a nanodiamond
particle can comprise a crystalline nanodiamond core surrounded by
a shell of graphitic and/or amorphous carbon. The surface of the
nanodiamond may be formed due to the particular synthesis approach
as well as optional post synthesis processing, such as surface
functionalization. For commercial applications, suitable diamond
nanoparticles are generally synthetic nanodiamonds, which are
available commercially. The surface of a nanodiamond may be
functionalized to influence the chemical properties of the
nanodiamonds, such as the dispersability and/or compatibility with
a particular polymer binder. The average diameter of nanodiamond
particles generally can be no more than about 100 nm, in further
embodiments from about 2 nm to about 75 nm and in additional
embodiments from about 2.5 nm to about 50 nm. A person of ordinary
skill in the art will recognize that additional ranges of
nanodiamond average diameters within the explicit ranges above are
contemplated and are within the present disclosure.
[0046] Synthetic nanodiamonds can be produced by several means. For
example, vapor phase formation such as chemical vapor deposition,
ion irradiation of graphite, chlorination of carbides, and
techniques using shock wave energies are some of the several
possible methods to produce such diamond particles or thin
nanodiamond films. In addition to diamond nanoparticles of rough
spherical form, other 1- and 2-dimensional nanodiamond structures
had been fabricated such as nanodiamond rods, sheets, flakes, and
the like, which can also be used in UV protecting compositions (on
methods of synthesis of these structures see O. Shenderova and G.
McGuire, "Types of Nanodiamonds,", book chapter in
"Ultrananocrystalline diamond: Synthesis, Properties and
Applications", Editors: O. Shenderova, D. Gruen, William-Andrews
Publisher, 2006), incorporated herein by reference). Commercial
nanodiamond particles are generally formed by controlled explosive
techniques, such as described in U.S. Pat. No. 5,916,955 to
Vereschagin et al., entitled "Diamond-Carbon Material and Method
for Producing Thereof," incorporated herein by reference. Improved
purification methods for detonation nanodiamonds are described, for
example, in published PCT application, WO 2013/135305 to Dolmatov
et al., entitled "Detonation Nanodiamond Material Purification
Method and Product Thereof," incorporated herein by reference.
Commercial nanodiamonds with various surface chemistries or
dispersed in ranges of solvents are available from NanoCarbon
Research Institute Co., Ltd. (Japan), PlasmaChem (Germany),
Carbodeon Limited OY (Finland), NEOMOND (Korea), Sigma-Aldrich
(USA), and Ray Techniques Ltd. (Israel).
[0047] The nanodiamond particles each generally comprise a
mechanically stable, chemically inert crystalline core and a
surface generally considered relatively chemically active. By
functionalizing the nanodiamond particle surface with targeted
species, the nanodiamond can be provided with modified chemical
and/or physical properties. Functionalization can be done by
various chemical, photochemical, and electrochemical methods to
graft different organic functionalities onto the nanodiamond.
Depending on the desired physical property and application of the
nanodiamond, functionalized nanodiamond materials can be
fluorinated, chlorinated, carboxylated, aminated, hydroxylated,
hydrogenated, sulfonated or a mixture thereof. See, for example,
published U.S. patent application 2011/0232199 to Yao, entitled
"Process for Production of Dispersion of Fluorinated Nano Diamond,"
and (carboxylated nanodiamonds) published PCT application WO
2014/174150 to Myllyaki et al., entitled "A Method for Producing
Zeta Negative Nanodiamond Dispersion and Zeta Negative Nanodiamond
Dispersion," incorporated herein by reference. The
functionalization and/or purification can be used to help to remove
and/or break up nanoparticle agglomerates. In general, commercial
nanodiamonds are sufficiently unagglomerated for processing into
relatively uniform thin films as described herein. The pH of the
solutions, concentration, solvent and other dispersion properties
can be adjusted to further assist with dispersing the nanodiamonds.
For example, carboxylated nanodiamonds are generally stably
dispersed in higher pH solutions, and hydrogenated and aminated
nanodiamonds are generally stably dispersed in lower pH
solutions.
[0048] Hardness of the loaded polymer films can be measured with
the pencil hardness test for films based on ASTM D3363. Following
pencil sharpening methodology, a constant downward applied force is
used while holding the pencil at a 45.degree. angle. A Pencil
hardness Kit was used for the measurements with 500 grams or 750
grams. Hardness was determined by analyzing the effect of different
pencils in the graphite grading scale on the base conductive layer.
If no damage was done to the base layer, the film was considered to
have passed. The film was checked under a Leica microscope at a
20.times. magnification. The hardness scales range with grade
values from 9B to 9H, with higher values of B corresponding to
lower values of hardness and larger values of H corresponding to
increased hardness, and a value of F connects the B and H ranges
and the lowest "B" value is HB followed by B, 2B, . . . , 9B. In
some embodiments, the coating with the property enhancing
nanoparticles can have a pencil hardness of at least one grade
greater hardness, in some embodiments at least bout 2 grades
greater, and in further embodiments at least about 3 grades greater
pencil hardness relative to an equivalent coating in all other
respects except without the property enhancing nanoparticles. Other
scales and tests for hardness are available, and qualitatively
similar trends should follow. Scratch resistance is also evaluated
with the use of steel wool rubbed against the surface with a 100 g
weight, as described further in the Examples below. Superfine steel
wool was used to scratch the film by rubbing the surface after the
transparent overcoat is applied.
[0049] The transparent loaded coatings can be formed by coating a
precursor solution using appropriate coating methods. Property
enhancing nanoparticles and/or stabilization compositions can be
incorporated into a suitable solvent selected to deposit the
coating with appropriate compatibility. Suitable solvents generally
include, for example, water, alcohols, ketones, esters, ethers,
such as glycol ethers, aromatic compounds, alkanes, and the like
and mixtures thereof. Specific solvents include, for example,
water, ethanol, isopropyl alcohol, isobutyl alcohol, tertiary butyl
alcohol, methyl ethyl ketone, methyl isobutyl ketone, cyclic
ketones such as cylcopentanone and cyclohexanone, glycol ethers,
toluene, hexane, ethyl acetate, butyl acetate, ethyl lactate,
propylene carbonate, dimethyl carbonate, PGMEA
(2-methoxy-1-methylethylacetate), N,N-dimethylformamide,
N,N-dimethylacetamide, acetonitrile, formic acid, or mixtures
thereof.
[0050] In general, the polymer for the coating, generally a
crosslinkable polymer, can be supplied as a commercial coating
composition or formulated with selected polymer compositions.
Suitable classes of radiation curable polymers and/or heat curable
polymers include, for example, polysiloxanes, polysilsesquioxanes,
polyurethanes, acrylic resins, acrylic copolymers, cellulose ethers
and esters, nitrocellulose, other water insoluble structural
polysaccharides, polyethers, polyesters, polystyrene, polyimide,
fluoropolymer, styrene-acrylate copolymers, styrene-butadiene
copolymers, acrylonitrile butadiene styrene copolymers,
polysulfides, epoxy containing polymers, copolymers thereof, and
mixtures thereof. Suitable commercial coating compositions include,
for example, coating solutions from Dexerials Corporation (Japan),
POSS.RTM. Coatings from Hybrid Plastics, Inc. (Mississippi, USA),
silica filled siloxane coatings from California Hardcoating Company
(California, USA), CrystalCoat UV-curable coatings from SDC
Technologies, Inc. (California, USA). The polymer concentrations
and correspondingly the concentrations of other non-volatile agents
can be selected to achieve desired rheology of the coating
solution, such as an appropriate viscosity for the selected coating
process. Solvent can be added or removed to adjust total solid
concentrations. Relative amounts of solids can be selected to
adjust the composition of the finished coating composition, and the
total amounts of solids can be adjusted to achieve a desired
thickness of the dried coating. Generally, the coating solution can
have a polymer concentration from about 0.025 wt % to about 50 wt
%, in further embodiments from about 0.05 wt % to about 25 wt % and
in additional embodiments from about 0.075 wt % to about 20 wt %. A
person of ordinary skill in the art will recognize that additional
ranges of polymer concentrations within the specific ranges above
are contemplated and are within the present disclosure.
[0051] Property enhancing nanoparticles can be incorporated into
the coating solution for forming a coating layer. A coating
precursor solution can comprise from about 0.005 wt % to about 5 wt
% nanoparticles, in further embodiments from about 0.01 wt % to
about 3 wt % and in additional embodiments from about 0.025 wt % to
about 2 wt % property enhancing nanoparticles. A person of ordinary
skill in the art will recognize that additional ranges of property
enhancing nanoparticles in a coating solution within the explicit
ranges above are contemplated and are within the present
disclosure. Additional additives, such as wetting agents, viscosity
modifiers, dispersing aids, and the like can be added as
desired.
[0052] The transparent coating with property enhancing
nanoparticles in some embodiments can cause a decrease of the total
transmittance of visible light relative to a corresponding coating
without the property enhancing nanoparticles by no more than about
5 percentage points, in further embodiments no more than about 3
and in additional embodiments no more than about 1.5 percentage
points. Also, the transparent coating with property enhancing
nanoparticles can cause an increase of the haze in some embodiments
relative to corresponding unloaded coatings by no more than about
1.5 percentage points, in further embodiments by no more than about
1, and in additional embodiments by no more than about 0.6
percentage points. A person of ordinary skill in the art will
recognize that additional ranges of modifications of optical
properties due to loaded polymer coatings within the explicit
ranges above are contemplated and are within the present
disclosure. The corresponding unloaded coatings have the same
concentrations in the solvent of components other than the absent
nanoparticles and is processed the same way so that the final
thickness of the coating may be slightly different for the
corresponding coating.
[0053] For the deposition of the coating precursor solutions, any
reasonable deposition approach can be used, such as dip coating,
spray coating, knife edge coating, bar coating, Meyer-rod coating,
slot-die coating, gravure printing, spin coating or the like. The
deposition approach directs the amount of liquid deposited, and the
concentration of the solution can be adjusted to provide the
desired thickness of product coating on the surface. After forming
the coating with the dispersion, the coating can be dried to remove
the liquid and crosslinked appropriately.
Transparent Conductive Films
[0054] The transparent electrically conductive structures or films
generally comprise a sparse metal conductive layer that provides
the electrical conductivity without significantly adversely
altering the optical properties and various additional layers that
provide mechanical support as well as protection of the conductive
element. Generally, a polymer overcoat is placed over the sparse
metal conductive layer. The property enhancing nanoparticles as
described herein can be placed in an overcoat layer, an optional
undercoat layer and/or directly into the sparse metal conductive
layer. The sparse metal conductive layer is very thin and
correspondingly susceptible to damage by mechanical and other
abuses. The property enhancing nanoparticles can provide some types
of protection, and stabilization compounds, as described in the
previous section, as well as other elements of the films can
provide additional protections. With respect to sensitivities to
environmental damage, it has been found that an undercoat and/or
overcoat can comprise a stabilization composition that can provide
desirable protection, and certain classes of optically clear
adhesives and/or barrier layers can also provide valuable
protection from light, heat, chemicals and other environmental
damage. While the focus herein is on environmental assaults from
humid air, heat and light, polymer sheets used to protect the
conductive layers from these environmental assaults can also
provide protection from contact and the like.
[0055] Thus, the sparse metal conductive layer can be formed on a
substrate that can have one or more layers in the structure of the
substrate. The substrate generally can be identified as a self
supporting film or sheet structure. A thin solution processed
layer, referred to as an undercoat, can be optionally placed along
the top surface of the substrate film and immediately under the
sparse metal conductive layer. Also, the sparse metal conductive
can be coated with additional layers that provide some protection
on the side of the sparse metal conductive layer opposite the
substrate. In general, the electrically conductive structure can be
placed in either orientation in the final product, i.e., with the
substrate facing outward to the substrate against the surface of
the product supporting the electrically conductive structure. In
some embodiments, a plurality of coatings, i.e., undercoats and
overcoats, can be applied, and each layer can have selected
additives for corresponding property enhancement.
[0056] Referring to FIG. 1, representative transparent conductive
film 100 comprises a substrate 102, undercoat layer 104, sparse
metal conductive layer 106, overcoat layer 108, optically clear
adhesive layer 110 and protective surface layer 112, although not
all embodiments include all layers. In particular, rolls of
transparent conductive film can be distributed with the overcoat as
the top layer for later processing that may or may not involve
subsequent addition of additional over-layers. In these
embodiments, having a mechanically hard overcoat can be desirable
in terms of reducing risk of damage to the electrically conductive
film. A transparent conductive film generally comprises a sparse
metal conductive layer and at least one layer on each side of the
sparse metal conductive layer. The total thickness of the
transparent conductive film can have in some embodiments a
thickness from 5 microns to about 3 millimeters (mm), in further
embodiments from about 10 microns to about 2.5 mm and in other
embodiments from about 15 microns to about 1.5 mm. A person of
ordinary skill in the art will recognize that additional ranges of
thicknesses within the explicit ranges above are contemplated and
are within the present disclosure. In some embodiments, the length
and width of the film as produced can be selected to be appropriate
for a specific application so that the film can be directly
introduced for further processing into a product. In additional or
alternative embodiments, a width of the film can be selected for a
specific application, while the length of the film can be long with
the expectation that the film can be cut to a desired length for
use. For example, the film can be in long sheets or a roll.
Similarly, in some embodiments, the film can be on a roll or in
another large standard format and elements of the film can be cut
according to a desired length and width for use.
[0057] Substrate 102 generally comprises a durable support layer
formed from an appropriate polymer or polymers. In some
embodiments, the substrate can have a thickness from about 10
microns to about 1.5 mm, in further embodiments from about 15
microns to about 1.25 mm and in additional embodiments from about
20 microns to about 1 mm. A person of ordinary skill in the art
will recognize that additional ranges of thicknesses of the
substrate within the explicit ranges above are contemplated and are
within the present disclosure. Suitable optically clear polymers
with very good transparency, low haze and good protective abilities
can be used for the substrate. Suitable polymers include, for
example, polyethylene terephthalate (PET), polyethylene naphthalate
(PEN), polyacrylate, poly(methyl methacrylate), polyolefin,
polyvinyl chloride, fluoropolymers, polyamide, polyimide,
polysulfone, polysiloxane, polyetheretherketone, polynorbornene,
polyester, polystyrene, polyurethane, polyvinyl alcohol, polyvinyl
acetate, acrylonitrile-butadiene-styrene copolymer, cyclic olefin
polymer, cyclic olefin copolymer, polycarbonate, copolymers thereof
or blend thereof or the like. Suitable commercial polycarbonate
substrates include, for example, MAKROFOL SR243 1-1 CG,
commercially available from Bayer Material Science; TAP.RTM.
Plastic, commercially available from TAP Plastics; and LEXAN.TM.
8010 CDE, commercially available from SABIC Innovative Plastics.
Protective surface layer 112 can independently have a thickness and
composition covering the same thickness ranges and composition
ranges as the substrate as described in this paragraph above.
[0058] Optional undercoat 104 and/or optional overcoat 108,
independently selectable for inclusion, can be placed respectively
under or over sparse metal conductive layer 106. Optional coatings
104, 108 can comprise a curable polymer, e.g., heat curable or
radiation curable polymers. Suitable polymers for coatings 104, 108
are described below as binders for inclusion in the metal nanowire
inks, and the list of polymers, corresponding cross linking agents
and additives apply equally to optional coatings 104, 108 without
repeating the discussion explicitly here. Coatings 104, 108 can
have a thickness from about 25 nm to about 2 microns, in further
embodiments from about 40 nm to about 1.5 microns and in additional
embodiments from about 50 nm to about 1 micron. A person of
ordinary skill in the art will recognize that additional ranges of
overcoat thickness within the explicit ranges above are
contemplated and are within the present disclosure. In general, the
thinness of overcoat 108 allows for electrical conduction through
overcoat 108 so that electrical connection can be made to sparse
metal conductive layer 106, although in some embodiments, an
overcoat can comprise sublayers in which electrical conductivity is
provided through some but not necessarily all of the sublayers.
[0059] Optional optically clear adhesive layer 110 can have a
thickness from about 10 microns to about 300 microns, in further
embodiments from about 15 microns to about 250 microns and in other
embodiments from about 20 microns to about 200 microns. A person of
ordinary skill in the art will recognize that additional ranges of
thicknesses of optically clear adhesive layers within the explicit
ranges above are contemplated and are within the present
disclosure. Suitable optically clear adhesives can be contact
adhesives. Optically clear adhesives include, for example, coatable
compositions and adhesive tapes. UV curable liquid optically clear
adhesives are available based on acrylic or polysiloxane
chemistries. Suitable adhesive tapes are available commercially,
for example, from Lintec Corporation (MO series); Saint Gobain
Performance Plastics (DF713 series); Nitto Americas (Nitto Denko)
(LUCIACS CS9621T and LUCIAS CS9622T); DIC Corporation (DAITAC LT
series OCA, DAITAC WS series OCA and DAITAC ZB series); PANAC
Plastic Film Company (PANACLEAN series); Minnesota Mining and
Manufacturing (3M, Minnesota U.S.A.--product numbers 8146, 8171,
8172, 8173 and similar products) and Adhesive Research (for example
product 8932).
[0060] The amount of nanowires delivered onto the substrate for
sparse metal conductive layer 106 can involve a balance of factors
to achieve desired amounts of transparency and electrical
conductivity. While thickness of the nanowire network can in
principle be evaluated using scanning electron microscopy, the
network can be relatively sparse to provide for optical
transparency, which can complicate the measurement. In general, the
sparse metal conductive element, e.g., fused metal nanowire
network, would have an average thickness of no more than about 5
microns, in further embodiments no more than about 2 microns and in
other embodiments from about 10 nm to about 500 nm. However, the
sparse metal conductive elements are generally relatively open
structures with significant surface texture on a submicron scale.
The loading levels of the nanowires can provide a useful parameter
of the network that can be readily evaluated, and the loading value
provides an alternative parameter related to thickness. Thus, as
used herein, loading levels of nanowires onto the substrate is
generally presented as milligrams of nanowires for a square meter
of substrate. In general, the metal conductive networks, whether or
not fused, can have a loading from about 0.1 milligrams
(mg)/m.sup.2 to about 300 mg/m.sup.2, in further embodiments from
about 0.5 mg/m.sup.2 to about 200 mg/m.sup.2, and in other
embodiments from about 1 mg/m.sup.2 to about 150 mg/m.sup.2. The
transparent conductive layer can comprise from about 0.5 wt % to
about 70 wt % metal, in other embodiments from about 0.75 wt % to
about 60 wt % and in further embodiments from about 1 wt % to about
50 wt % metal in a conductive network. A person of ordinary skill
in the art will recognize that additional ranges of thickness and
metal loading within the explicit ranges above are contemplated and
are within the present disclosure. If the sparse metal conductive
layer is patterned, the thickness and loading discussion applies
only to the regions where metal is not excluded or significantly
diminished by the patterning process. The sparse metal conductive
layer can comprise property enhancing nanoparticles in addition to
a polymer binder and other processing aids and the like. Ranges of
concentration of property enhancing nanoparticles described above
for loadings in transparent polymer layers generally also apply to
sparse metal conductive layers.
[0061] Generally, within the total thicknesses above for particular
components of film 100, layers 102, 104, 106, 108, 110, 112 can be
subdivided into sublayers, for example, with different compositions
from other sublayers. For example, an overcoat layer can comprise
sublayers with different property enhancing components. In some
embodiments, a top overcoat sublayer may comprise high dielectric
nanoparticles, that may inhibit electrical conduction through the
layer. Then an electrical connection can be established through a
window, metal tab or the like penetrating top sublayer of overcoat
108 without necessarily penetrating an overcoat sublayer, which may
comprise for example, nanodiamonds and/or a stabilization
composition. Also, multiple layer optically clear adhesives are
discussed above. Thus, more complex layer stacks can be formed.
Sublayers may or may not be processed similarly to other sublayers
within a particular layer, for example, one sublayer can be
laminated while another sublayer can be coated and cured.
[0062] Stabilization compositions can be placed in appropriate
layers to stabilize the sparse metal conductive layers. For
embodiments in which the sparse metal conductive layers comprise
fused nanostructured metal networks, the sparse metal conductive
layer itself as formed may not comprise a stabilization compound
since the presence of such compounds may inhibit the chemical
fusing process. In alternative embodiments, it may be acceptable to
include the stabilization agents in coating solutions for forming
the sparse metal conductive layer. Similarly, stabilization
compounds can be included in an optically clear adhesive
composition. However, it has been found that the stabilization
compounds can be included effectively in a coating layer, which can
correspondingly be made relatively thin while still providing
effective stabilization. Specific descriptions of coatings with
stabilization compositions are described in the previous section.
Since the layers with the stabilization compositions can be thin,
desirable stabilization can be obtained with low totals of
stabilization agents, which can be desirable from a processing
perspective as well as having a low effect on the optical
properties.
[0063] For some applications, it is desirable to pattern the
electrically conductive portions of the film to introduce desired
functionality, such as distinct regions of a touch sensor.
Patterning can be performed by changing the metal loading on the
substrate surface either by printing metal nanowires at selected
locations with other locations being effectively barren of metal or
to etch or otherwise ablate metal from selected locations either
before and/or after fusing the nanowires. However, it has been
discovered that high contrast in electrical conductivity can be
achieved between fused and unfused portions of a layer with
essentially equivalent metal loading so that patterning can be
performed by selectively fusing the metal nanowires. This ability
to pattern based on fusing provides significant additional
patterning options based on selective fusing of the nanowires, for
example, through the selective delivery of a fusing solution or
vapor. Patterning based on selective fusing of metal nanowires is
described in the '833 application and the '669 application
above.
[0064] As a schematic example, a fused metal nanostructured network
can form conductive patterns along a substrate surface 120 with a
plurality of electrically conductive pathways 122, 124, and 126
surrounded by electrically resistive regions 128, 130, 132, 134, as
shown in FIG. 2. As shown in FIG. 2, the fused area corresponds
with three distinct electrically conductive regions corresponding
with electrically conductive pathways 122, 124, and 126. Although
three independently connected conductive regions have been
illustrated in FIG. 2, it is understood that patterns with two,
four or more than 4 conductive independent conductive pathways or
regions can be formed as desired. For many commercial applications,
fairly intricate patterns can be formed with a large number of
elements. In particular, with available patterning technology
adapted for the patterning of the films described herein, very fine
patterns can be formed with highly resolved features. Similarly,
the shapes of the particular conductive regions can be selected as
desired.
[0065] The transparent conductive film is generally built up around
the sparse metal conductive element which is deposited to form the
functional feature of the film. Various layers are coated,
laminated or otherwise added to the structure using appropriate
film processing approaches. As described herein, the nature of the
layers can significantly alter the long term performance of the
transparent conductive film. The deposit of the sparse metal
conductive layer is described further below in the context of a
fused metal nanostructured layers, but un-fused metal nanowire
coatings can be similarly deposited except that the fusing
components are absent.
[0066] The sparse metal conductive layer generally is solution
coated onto a substrate, which may or may not have a coating layer
on top of the substrate that then forms an undercoat adjacent the
sparse metal conductive layer. An overcoat can be solution coated
onto the sparse metal conductive layer in some embodiments.
Crosslinking, with application of UV light, heat or other
radiation, can be performed to crosslink polymer binders in the
coating layers and/or the sparse metal conductive layer, which can
be performed in one step or multiple steps.
Sparse Metal Conductive Layers
[0067] Sparse metal conductive layers are generally formed from
metal nanowires. With sufficient loading and selected nanowire
properties, reasonable electrical conductivity can be achieved with
the nanowires with corresponding appropriate optical properties. It
is expected that the stabilized film structures described herein
can yield desirable performance for films with various sparse metal
conductive structures. However, particularly desirable properties
have been achieved with fused metal nanostructured networks.
[0068] As summarized above, several practical approaches have been
developed to accomplish the metal nanowire fusing. The metal
loading can be balanced to achieve desirable levels of electrical
conductivity with good optical properties. In general, the metal
nanowire processing can be accomplished through deposition of two
inks with the first ink comprising the metal nanowires and the
second ink comprising a fusing composition, or through the
deposition of an ink that combines the fusing elements into the
metal nanowire dispersion. The inks may or may not further comprise
additional processing aids, binders or the like. Suitable
patterning approaches can be selected to be suitable for the
particular ink system.
[0069] In general, one or more solutions or inks for the formation
of the metal nanostructured network can collectively comprise well
dispersed metal nanowires, a fusing agent, and optional additional
components, for example, a polymer binder, a crosslinking agent, a
wetting agent, e.g., a surfactant, a thickener, a dispersant, other
optional additives or combinations thereof. The solvent for the
metal nanowire ink and/or the fusing solution if distinct from the
nanowire ink can comprise an aqueous solvent, an organic solvent or
mixtures thereof. In particular, suitable solvents include, for
example, water, alcohols, ketones, esters, ethers, such as glycol
ethers, aromatic compounds, alkanes, and the like and mixtures
thereof. Specific solvents include, for example, water, ethanol,
isopropyl alcohol, isobutyl alcohol, tertiary butyl alcohol, methyl
ethyl ketone, glycol ethers, methyl isobutyl ketone, toluene,
hexane, ethyl acetate, butyl acetate, ethyl lactate, PGMEA
(2-methoxy-1-methylethylacetate), or mixtures thereof. While the
solvent should be selected based on the ability to form a good
dispersion of metal nanowires, the solvents should also be
compatible with the other selected additives so that the additives
are soluble in the solvent. For embodiments in which the fusing
agent is included in a single solution with the metal nanowires,
the solvent or a component thereof may or may not be a significant
component of the fusing solution, such as alcohols and can be
selected accordingly if desired.
[0070] The metal nanowire ink, in either a one ink or two ink
configuration, can include from about 0.01 to about 1 weight
percent metal nanowires, in further embodiments from about 0.02 to
about 0.75 weight percent metal nanowires and in additional
embodiments from about 0.04 to about 0.5 weight percent metal
nanowires. A person of ordinary skill in the art will recognize
that additional ranges of metal nanowire concentrations within the
explicit ranges above are contemplated and are within the present
disclosure. The concentration of metal nanowires influences the
loading of metal on the substrate surface as well as the physical
properties of the ink.
[0071] In general, the nanowires can be formed from a range of
metals, such as silver, gold, indium, tin, iron, cobalt, platinum,
palladium, nickel, cobalt, titanium, copper and alloys thereof,
which can be desirable due to high electrical conductivity.
Commercial metal nanowires are available from Sigma-Aldrich
(Missouri, USA), Cangzhou Nano-Channel Material Co., Ltd. (China),
Blue Nano (North Carolina, U.S.A.), EMFUTUR (Spain), Seashell
Technologies (California, U.S.A.), Aiden (Korea), nanoComposix
(U.S.A.), Nanopyxis (Korea), K&B (Korea), ACS Materials
(China), KeChuang Advanced Materials (China), and Nanotrons (USA).
Silver in particular provides excellent electrical conductivity,
and commercial silver nanowires are available. Alternatively,
silver nanowires can also be synthesized using a variety of known
synthesis routes or variations thereof. To have good transparency
and low haze, it is desirable for the nanowires to have a range of
small diameters. In particular, it is desirable for the metal
nanowires to have an average diameter of no more than about 250 nm,
in further embodiments no more than about 150 nm, and in other
embodiments from about 10 nm to about 120 nm. With respect to
average length, nanowires with a longer length are expected to
provide better electrical conductivity within a network. In
general, the metal nanowires can have an average length of at least
a micron, in further embodiments, at least 2.5 microns and in other
embodiments from about 5 microns to about 100 microns, although
improved synthesis techniques developed in the future may make
longer nanowires possible. An aspect ratio can be specified as the
ratio of the average length divided by the average diameter, and in
some embodiments, the nanowires can have an aspect ratio of at
least about 25, in further embodiments from about 50 to about
10,000 and in additional embodiments from about 100 to about 2000.
A person of ordinary skill in the art will recognize that
additional ranges of nanowire dimensions within the explicit ranges
above are contemplated and are within the present disclosure.
[0072] Polymer binders and the solvents are generally selected
consistently such that the polymer binder is soluble or dispersible
in the solvent. In appropriate embodiments, the metal nanowire ink
generally comprises from about 0.02 to about 5 weight percent
binder, in further embodiments from about 0.05 to about 4 weight
percent binder and in additional embodiments from about 0.1 to
about 2.5 weight percent polymer binder. In some embodiments, the
polymer binder comprises a crosslinkable organic polymer, such as a
radiation crosslinkable organic polymer and/or a heat curable
organic binder. To facilitate the crosslinking of the binder, the
metal nanowire ink can comprise in some embodiments from about
0.0005 wt % to about 1 wt % of a crosslinking agent, in further
embodiments from about 0.002 wt % to about 0.5 wt % and in
additional embodiments from about 0.005 wt % to about 0.25 wt %.
The nanowire ink can optionally comprise a rheology modifying agent
or combinations thereof. In some embodiments, the ink can comprise
a wetting agent or surfactant to lower the surface tension, and a
wetting agent can be useful to improve coating properties. The
wetting agent generally is soluble in the solvent. In some
embodiments, the nanowire ink can comprise from about 0.01 weight
percent to about 1 weight percent wetting agent, in further
embodiments from about 0.02 to about 0.75 weight percent and in
other embodiments from about 0.03 to about 0.6 weight percent
wetting agent. A thickener can be used optionally as a rheology
modifying agent to stabilize the dispersion and reduce or eliminate
settling. In some embodiments, the nanowire ink can comprise
optionally from about 0.05 to about 5 weight percent thickener, in
further embodiments from about 0.075 to about 4 weight percent and
in other embodiments from about 0.1 to about 3 weight percent
thickener. A person of ordinary skill in the art will recognize
that additional ranges of binder, wetting agent and thickening
agent concentrations within the explicit ranges above are
contemplated and are within the present disclosure.
[0073] A range of polymer binders can be suitable for
dissolving/dispersing in a solvent for the metal nanowires, and
suitable binders include polymers that have been developed for
coating applications. Hard coat polymers, e.g., radiation curable
coatings, are commercially available, for example as hard coat
materials for a range of application, that can be selected for
dissolving in aqueous or non-aqueous solvents. Suitable classes of
radiation curable polymers and/or heat curable polymers include,
for example, polysiloxanes, polysilsesquioxanes, polyurethanes,
acrylic resins, acrylic copolymers, cellulose ethers and esters,
nitrocellulose, other water insoluble structural polysaccharides,
polyethers, polyesters, polystyrene, polyimide, fluoropolymer,
styrene-acrylate copolymers, styrene-butadiene copolymers,
acrylonitrile butadiene styrene copolymers, polysulfides, epoxy
containing polymers, copolymers thereof, and mixtures thereof.
Examples of commercial polymer binders include, for example,
NEOCRYL.RTM. brand acrylic resin (DMS NeoResins), JONCRYL.RTM.
brand acrylic copolymers (BASF Resins), ELVACITE.RTM. brand acrylic
resin (Lucite International), SANCURE.RTM. brand urethanes
(Lubrizol Advanced Materials), cellulose acetate butyrate polymers
(CAB brands from Eastman.TM. Chemical), BAYHYDROL.TM. brand
polyurethane dispersions (Bayer Material Science), UCECOAT.RTM.
brand polyurethane dispersions (Cytec Industries, Inc.),
MOWITOL.RTM. brand polyvinyl butyral (Kuraray America, Inc.),
cellulose ethers, e.g., ethyl cellulose or hydroxypropyl methyl
cellulose, other polysaccharide based polymers such as Chitosan and
pectin, synthetic polymers like polyvinyl acetate, and the like.
The polymer binders can be self-crosslinking upon exposure to
radiation, and/or they can be crosslinked with a photoinitiator or
other crosslinking agent. In some embodiments, photocrosslinkers
may form radicals upon exposure to radiation, and the radicals then
induce crosslinking reactions based on radical polymerization
mechanisms. Suitable photoinitiators include, for example,
commercially available products, such as IRGACURE.RTM. brand
(BASF), GENOCURE.TM. brand (Rahn USA Corp.), and DOUBLECURE.RTM.
brand (Double Bond Chemical Ind., Co, Ltd.), combinations thereof
or the like.
[0074] Wetting agents can be used to improve the coatability of the
metal nanowire inks as well as the quality of the metal nanowire
dispersion. In particular, the wetting agents can lower the surface
energy of the ink so that the ink spreads well onto a surface
following coating. Wetting agents can be surfactants and/or
dispersants. Surfactants are a class of materials that function to
lower surface energy, and surfactants can improve solubility of
materials. Surfactants generally have a hydrophilic portion of the
molecule and a hydrophobic portion of the molecule that contributes
to its properties. A wide range of surfactants, such as nonionic
surfactants, cationic surfactant, anionic surfactants, zwitterionic
surfactants, are commercially available. In some embodiments, if
properties associated with surfactants are not an issue,
non-surfactant wetting agents, e.g., dispersants, are also known in
the art and can be effective to improve the wetting ability of the
inks. Suitable commercial wetting agents include, for example,
COATOSIL.TM. brand epoxy functionalized silane oligomers (Momentum
Performance Materials), SILWET.TM. brand organosilicone surfactant
(Momentum Performance Materials), THETAWET.TM. brand short chain
non-ionic fluorosurfactants (ICT Industries, Inc.), ZETASPERSE.RTM.
brand polymeric dispersants (Air Products Inc.), SOLSPERSE.RTM.
brand polymeric dispersants (Lubrizol), XOANONS WE-D545 surfactant
(Anhui Xoanons Chemical Co., Ltd), EFKA.TM. PU 4009 polymeric
dispersant (BASF), MASURF FP-815 CP, MASURF FS-910 (Mason
Chemicals), NOVEC.TM. FC-4430 and FC-4432 fluorinated surfactants
(3M), mixtures thereof, and the like.
[0075] Thickeners can be used to improve the stability of the
dispersion by reducing or eliminating settling of the solids from
the metal nanowire inks. Thickeners may or may not significantly
change the viscosity or other fluid properties of the ink. Suitable
thickeners are commercially available and include, for example,
CRAYVALLAC.TM. brand of modified urea such as LA-100 (Cray Valley
Acrylics, USA), polyacrylamide, THIXOL.TM. 53 L brand acrylic
thickener, COAPUR.TM. 2025, COAPUR.TM. 830 W, COAPUR.TM. 6050,
COAPUR.TM. XS71 (Coatex, Inc.), BYK.RTM. brand of modified urea
(BYK Additives), Acrysol DR 73, Acrysol RM-995, Acrysol RM-8W (Dow
Coating Materials), Aquaflow NHS-300, Aquaflow XLS-530
hydrophobically modified polyether thickeners (Ashland Inc.),
Borchi Gel L 75 N, Borchi Gel PW25 (OMG Borchers), and the
like.
[0076] As noted above, the inks for depositing the sparse metal
conductive layers can further comprise property enhancing
nanoparticles. Suitable property enhancing nanoparticles include
nanodiamonds as well as other property enhancing nanoparticle
materials presented above which are specifically incorporated into
the present discussion. Also, the ranges of nanoparticle sizes are
summarized above in the context of coatings and are similarly
incorporated here. The solution to form the sparse metal conductive
layer can comprise from about 0.001 wt % to about 10 wt %
nanoparticles, in further embodiments from about 0.002 wt % to
about 7 wt % and in additional embodiments from about 0.005 to
about 5 wt % property enhancing nanoparticles. A person of ordinary
skill in the art will recognize that additional ranges of
nanoparticle concentrations within the explicit ranges above are
contemplated and are within the present disclosure.
[0077] Additional additives can be added to the metal nanowire ink,
generally each in an amount of no more than about 5 weight percent,
in further embodiments no more than about 2 weight percent and in
further embodiments no more than about 1 weight percent. Other
additives can include, for example, anti-oxidants, UV stabilizers,
defoamers or anti-foaming agents, anti-settling agents, viscosity
modifying agents, or the like.
[0078] As noted above, fusing of the metal nanowires can be
accomplished through various agents. Without wanting to be limited
by theory, the fusing agents are believed to mobilize metal ions,
and the free energy seems to be lowered in the fusing process.
Excessive metal migration or growth may lead in some embodiments to
a degeneration of the optical properties, so desirable results can
be achieved through a shift in equilibrium in a reasonably
controlled way, generally for a short period of time, to generate
sufficient fusing to obtain desired electrical conductivity while
maintaining desired optical properties. In some embodiments,
initiation of the fusing process can be controlled through a
partial drying of the solutions to increase concentrations of the
components, and quenching of the fusing process can be
accomplished, for example, through rinsing or more completing
drying of the metal layer. The fusing agent can be incorporated
into a single ink along with the metal nanowires. The one ink
solution can provide appropriate control of the fusing process.
[0079] In some embodiments, a process is used in which a sparse
nanowire film is initially deposited and subsequent processing with
or without depositing another ink provides for the fusing of the
metal nanowires into a metal nanostructured network, which is
electrically conducting. The fusing process can be performed with
controlled exposure to a fusing vapor and/or through the deposition
of a fusing agent in solution. Sparse metal conductive layers are
generally formed on a selected substrate surface. The as-deposited
nanowire film generally is dried to remove the solvent. Processing
can be adapted for patterning of the film as described further
below.
[0080] For the deposition of the metal nanowire ink, any reasonable
deposition approach can be used, such as dip coating, spray
coating, knife edge coating, bar coating, Meyer-rod coating,
slot-die coating, gravure printing, spin coating or the like. The
ink can have properties, such as viscosity, adjusted appropriately
with additives for the desired deposition approach. Similarly, the
deposition approach directs the amount of liquid deposited, and the
concentration of the ink can be adjusted to provide the desired
loading of metal nanowires on the surface. After forming the
coating with the dispersion, the sparse metal conductive layer can
be dried to remove the liquid.
[0081] The films can be dried, for example, with a heat gun, an
oven, a thermal lamp or the like, although the films that can be
air dried can be desired in some embodiments. In some embodiments,
the films can be heated to temperatures from about 50.degree. C. to
about 150.degree. C. during drying. After drying, the films can be
washed one or more times, for example, with an alcohol or other
solvent or solvent blend, such as ethanol or isopropyl alcohol, to
remove excess solids to lower haze. Patterning can be achieved in
several convenient ways. For example, printing of the metal
nanowires can directly result in patterning. Additionally or
alternatively, lithographic techniques can be used to remove
portions of the metal nanowires, prior to or after fusing, to form
a pattern.
[0082] Clear protective films covering the sparse metal conductive
layer can be formed with holes or the like in appropriate locations
to provide for electrical connections to the conductive layer. In
general, various polymer film processing techniques and equipment
can be used to the processing of these polymer sheets, and such
equipment and techniques are well developed in the art, and future
developed processing techniques and equipment can be
correspondingly adapted for the materials herein.
Transparent Film Electrical and Optical Properties
[0083] The fused metal nanostructured networks can provide low
electrical resistance while providing good optical properties.
Thus, the films can be useful as transparent conductive electrodes
or the like. The transparent conductive electrodes can be suitable
for a range of applications such as electrodes along light
receiving surfaces of solar cells. For displays and in particular
for touch screens, the films can be patterned to provide
electrically conductive patterns formed by the film. The substrate
with the patterned film, generally has good optical properties at
the respective portions of the pattern.
[0084] Electrical resistance of thin films can be expressed as a
sheet resistance, which is reported in units of ohms per square
(.OMEGA./.quadrature. or ohms/sq) to distinguish the values from
bulk electrical resistance values according to parameters related
to the measurement process. Sheet resistance of films is generally
measured using a four point probe measurement or another suitable
process. In some embodiments, the fused metal nanowire networks can
have a sheet resistance of no more than about 300 ohms/sq, in
further embodiments no more than about 200 ohms/sq, in additional
embodiments no more than about 100 ohms/sq and in other embodiments
no more than about 60 ohms/sq. A person of ordinary skill in the
art will recognize that additional ranges of sheet resistance
within the explicit ranges above are contemplated and are within
the present disclosure. Depending on the particular application,
commercial specifications for sheet resistances for use in a device
may not be necessarily directed to lower values of sheet resistance
such as when additional cost may be involved, and current
commercially relevant values may be for example, 270 ohms/sq,
versus 150 ohms/sq, versus 100 ohms/sq, versus 50 ohms/sq, versus
40 ohms/sq, versus 30 ohms/sq or less as target values for
different quality and/or size touch screens, and each of these
values defines a range between the specific values as end points of
the range, such as 270 ohms/sq to 150 ohms/sq, 270 ohms/sq to 100
ohms/sq, 150 ohms/sq to 100 ohms/sq and the like with 15 particular
ranges being defined. Thus, lower cost films may be suitable for
certain applications in exchange for modestly higher sheet
resistance values. In general, sheet resistance can be reduced by
increasing the loading of nanowires, but an increased loading may
not be desirable from other perspectives, and metal loading is only
one factor among many for achieving low values of sheet
resistance.
[0085] For applications as transparent conductive films, it is
desirable for the fused metal nanowire networks to maintain good
optical transparency. In principle, optical transparency is
inversely related to the loading with higher loadings leading to a
reduction in transparency, although processing of the network can
also significantly affect the transparency. Also, polymer binders
and other additives can be selected to maintain good optical
transparency. The optical transparency can be evaluated relative to
the transmitted light through the substrate. For example, the
transparency of the conductive film described herein can be
measured by using a UV-Visible spectrophotometer and measuring the
total transmission through the conductive film and support
substrate. Transmittance is the ratio of the transmitted light
intensity (I) to the incident light intensity (I.sub.o). The
transmittance through the film (T.sub.film) can be estimated by
dividing the total transmittance (T) measured by the transmittance
through the support substrate (T.sub.sub). (T=I/I.sub.o and
T/T.sub.sub=(I/I.sub.o)/(I.sub.sub/I.sub.o)=I/I.sub.sub=T.sub.film)
Thus, the reported total transmissions can be corrected to remove
the transmission through the substrate to obtain transmissions of
the film alone. While it is generally desirable to have good
optical transparency across the visible spectrum, for convenience,
optical transmission can be reported at 550 nm wavelength of light.
Alternatively or additionally, transmission can be reported as
total transmittance from 400 nm to 700 nm wavelength of light, and
such results are reported in the Examples below. In general, for
the fused metal nanowire films, the measurements of 550 nm
transmittance and total transmittance from 400 nm to 700 nm (or
just "total transmittance" for convenience) are not qualitatively
different. In some embodiments, the film formed by the fused
network has a total transmittance (TT %) of at least 80%, in
further embodiments at least about 85%, in additional embodiments,
at least about 90%, in other embodiments at least about 94% and in
some embodiments from about 95% to about 99%. Transparency of the
films on a transparent polymer substrate can be evaluated using the
standard ASTM D1003 ("Standard Test Method for Haze and Luminous
Transmittance of Transparent Plastics"), incorporated herein by
reference. A person or ordinary skill in the art will recognize
that additional ranges of transmittance within the explicit ranges
above are contemplated and are within the present disclosure. When
adjusting the measured optical properties for the films in the
Examples below for the substrate, the films have very good
transmission and haze values, which are achieved along with the low
sheet resistances observed.
[0086] The fused metal networks can also have low haze along with
high transmission of visible light while having desirably low sheet
resistance. Haze can be measured using a hazemeter based on ASTM
D1003 referenced above, and the haze contribution of the substrate
can be removed to provide haze values of the transparent conductive
film. In some embodiments, the sintered network film can have a
haze value of no more than about 1.2%, in further embodiments no
more than about 1.1%, in additional embodiments no more than about
1.0% and in other embodiments from about 0.9% to about 0.2%. As
described in the Examples, with appropriately selected silver
nanowires very low values of haze and sheet resistance have been
simultaneously achieved. The loading can be adjusted to balance the
sheet resistance and the haze values with very low haze values
possible with still good sheet resistance values. Specifically,
haze values of no more than 0.8%, and in further embodiments from
about 0.4% to about 0.7%, can be achieved with values of sheet
resistance of at least about 45 ohms/sq. Also, haze values of 0.7%
to about 1.2%, and in some embodiments from about 0.75% to about
1.05%, can be achieved with sheet resistance values of from about
30 ohms/sq to about 45 ohms/sq. All of these films maintained good
optical transparency. A person of ordinary skill in the art will
recognize that additional ranges of haze within the explicit ranges
above are contemplated and are within the present disclosure.
[0087] With respect to the corresponding properties of the
multilayered films, the additional components are generally
selected to have a small effect on the optical properties, and
various coatings and substrates are commercially available for use
in transparent elements. Suitable optical coatings, substrates and
associated materials are summarized above. Some of the structural
material can be electrically insulating, and if thicker insulating
layers are used, the film can be patterned to provide locations
where gaps or voids through the insulating layers can provide
access and electrical contact to the otherwise embedded
electrically conductive element.
Touch Sensors
[0088] The transparent conductive films described herein can be
effectively incorporated into touch sensors that can be adapted for
touch screens used for many electronic devices. Some representative
embodiments are generally described here, but the transparent
conductive films can be adapted for other desired designs. A common
feature of the touch sensors generally is the presence of two
transparent conductive electrode structures in a spaced apart
configuration in a natural state, i.e., when not being touched or
otherwise externally contacted. For sensors operating based on
capacitance, a dielectric layer is generally between the two
electrode structures. Referring to FIG. 3, a representative
capacitance based touch sensor 202 comprises a display component
204, an optional bottom substrate 206, a first transparent
conductive electrode structure 208, a dielectric layer 210, such as
a polymer or glass sheet, a second transparent conductive electrode
structure 212, optional top cover 214, and measurement circuit 216
that measures capacitance changes associated with touching of the
sensor. Referring to FIG. 4, a representative resistance based
touch sensor 240 comprises a display component 242, an optional
lower substrate 244, a first transparent conductive electrode
structure 246, a second transparent conductive electrode structure
248, support structures 250, 252 that support the spaced apart
configuration of the electrode structures in their natural
configuration, upper cover layer 254 and resistance measuring
circuit 256.
[0089] Display components 204, 242 can be, for example, LED based
displays, LCD displays or other desired display components.
Substrates 206, 244 and cover layers 214, 254 can be independently
transparent polymer sheets or other transparent sheets. Support
structures can be formed from a dielectric material, and the sensor
structures can comprise additional supports to provide a desired
stable device. Measurement circuits 216, 256 are known in the
art.
[0090] Transparent conductive electrodes 208, 212, 246 and 248 can
be effectively formed using fused metal networks, which can be
patterned appropriately to form distinct sensors, although in some
embodiments the fused metal networks form some transparent
electrode structures while other transparent electrode structures
in the device can comprise materials such as electrically
conductive metal oxides, for example indium tin oxide, aluminum
doped zinc oxide, indium doped cadmium oxide, fluorine doped tin
oxide, antimony doped tin oxide, or the like as thin films or
particulates, carbon nanotubes, graphene, conductive organic
compositions or the like. Fused metal networks can be effectively
patterned as described herein, and it can be desirable for
patterned films in one or more of the electrode structures to form
the sensors such that the plurality of electrodes in a transparent
conductive structure can be used to provide position information
related to the touching process. The use of patterned transparent
conductive electrodes for the formation of patterned touch sensors
is described, for example, in U.S. Pat. No. 8,031,180 to Miyamoto
et al., entitled "Touch Sensor, Display With Touch Sensor, and
Method for Generating Position Data," and published U.S. patent
application 2012/0073947 to Sakata et al., entitled "Narrow Frame
Touch Input Sheet, Manufacturing Method of Same, and Conductive
Sheet Used in Narrow Frame Touch Input Sheet," both of which are
incorporated herein by reference.
EXAMPLES
[0091] The following examples involve the coating of loaded polymer
precursor solutions onto appropriate substrate. Examples are
presented with nanodiamond fillers, aluminum oxide nanoparticle
fillers or zirconium oxide nanoparticle fillers. Some examples
involve formation of passive coated polymer films. Other examples
involve coatings associated with fused metal conductive networks
that result in the formation in a transparent conductive film. For
embodiments of transparent conductive films, examples are presented
with the property enhancing nanoparticles in the layer with the
fused metal conductive network or in a coating placed over the
layer with the fused metal conductive network. The fused metal
conductive network is formed using silver nanowires.
[0092] Commercial silver nanowires were used in the following
examples with an average diameter of between 25 and 50 nm and an
average length of 10-30 microns. The silver nanowire ink was
essentially as described in Example 5 of copending U.S. patent
application Ser. No. 14/448,504 to Li et al., entitled "Metal
Nanowire Inks for the Formation of Transparent Conductive Films
With Fused Networks," incorporated herein by reference. The metal
nanowire ink comprised silver nanowires at a level between 0.01 to
0.5 wt %, between 0.01 mg/mL and 2.0 mg/mL silver ions, and a
cellulose based binder at concentrations from about 0.02 to 1.0 wt
%. The silver nanowire inks were aqueous solutions with a small
amount of alcohol. The ink was slot coated onto a PET polyester
film. After coating the nanowire inks, the films were then heated
in an oven at 100.degree. C. for 10 min to dry the films. Formation
procedures for the overcoats are described below in the specific
examples.
[0093] The total transmission (TT) and haze of the film samples
were measured using a Haze Meter. To adjust the haze measurements
for the samples below, a value of substrate haze can be subtracted
from the measurements to get approximate haze measurements for the
transparent conductive films alone. The instrument is designed to
evaluate optical properties based on ASTM D 1003 standard
("Standard Test Method for Haze and Luminous Transmittance of
Transparent Plastics"), incorporated herein by reference. The total
transmission and haze of these films include PET substrate which
has base total transmission and haze of .about.92.9% and 0.1%-0.4%,
respectively. In the following examples, several different
formulations of fusing metal nanowire inks are presented along with
optical and sheet resistance measurements.
[0094] Sheet resistance was measured with a 4-point probe method, a
contactless resistance meter or by measuring the resistance of the
film by using a square defined by two solid (non-transparent) lines
of silver formed from silver paste. In some embodiments, to make
sheet resistance measurements, a pair of parallel stripes of silver
paste was sometime used by painting the paste onto the surface of
the samples to define a square, or a rectangular shape, which were
then annealed at roughly 120.degree. C. for 20 minutes in order to
cure and dry the silver paste. Alligator clips were connected to
the silver paste stripes, and the leads were connected to a
commercial resistance measurement device. Electrical connections
are made to exposed end sections of the film. Some samples had
sheet resistance measured by a third party vendor.
[0095] The pencil hardness of the AgNWs film samples were measured
using a Pencil Test Kit. Following the pencil sharpening
methodology, abrasive paper was used for pencil tip modification,
and a constant downward force was applied while holding the pencil
at a 45.degree. angle and the pencil was moved across the surface
of the film sample. This test used a 500 g or 750 g commercial
pencil hardness kit. Hardness was determined by analyzing the
effect of different pencils in the graphite grading scale on the
base conductive layer. If no damage was done to the base layer, the
film was considered to have passed that specific graphite level.
The film was checked under a Leica microscope at a 20.times.
magnification. The film was placed on a very flat surface, which is
significant to avoid scratching by the pencil as the films are very
thin. This testing differed from the corresponding standardized
tests, which rely on visual examination without magnification.
[0096] The steel wool final hardness of the film samples were
measured using superfine 0000 steel wool under specific weights.
For some samples, steel wool, with a constant downward force
provided by 20 g, 50 g, or 100 g weights, was passed over the
coated films once and the film was examined under light for
detection of micro scratches. The number of scratches determine the
scratch resistance of the film. No scratches made by the steel wool
would mean a "pass" for that specific weight on the steel wool. In
the case of not passing, the number of scratches made are indicated
in the results section. For some samples, haze and/or sheet
resistance was also evaluated following testing with the steel
wool.
[0097] In the haze and/or sheet resistance analysis, super fine
steel wool was used to rub the surface after the overcoat was
applied and crosslinked. Steel wool rub was performed very gently,
while keeping a constant downward force. A section of the film
under test was rubbed 10 times back and forth with the steel wool.
Micro scratches tend to contribute much less to the haze increase
compared to deeper scratches. BYK Haze-Gard Plus was used for total
transparency and haze measurements. The change in the sheet
resistance was also measured by a third party service for in-house
OC formulations, as described in Example 4. Haze was measured
before and after the test.
Example 1
Effect of Nanodiamonds on Commercial Overcoat on a Transparent
Substrate
[0098] This example tests the effect on the hardness of a
commercial overcoat loaded with nanodiamonds on PET substrate with
an initial polymer binder overcoat.
[0099] The substrate was prepared by coating a base ink with a
cellulose based polymer binder but without any silver nanowires was
coated onto a transparent PET substrate and dried. The coated
substrate had a haze of 0.72%. A commercial coating polymer from
Dexerials was dissolved in N,N-dimethylformamide (DMF). Six samples
were prepared with two samples at each of 2 wt %, 3 wt % and 4 wt %
polymer concentrations. In one sample at each polymer
concentration, hydrogen terminated nanodiamonds were added,
respectively, at 0.2 wt %, 0.3 wt %, or 0.4 wt % concentrations, so
that in each diamond containing sample, the diamond concentrations
were about one tenth the polymer concentration. The coating
solutions were deposited onto the substrate by slot coating at 1
mil (25.4 microns) wet thickness. The films were then were dried
with an infrared lamp and cured with UV light under nitrogen at 1
J/cm.sup.2 using a Heraeus Fusion UV System (H-bulb). The solid
content of the coating solution correlates with the thickness of
the dried film, and the films formed with coating solutions having
0.3 wt % polymer would have an average thickness of about 75 nm.
Hardness and optical properties were compared between the films
formed with the nanoparticle fillers and films formed without the
nanoparticle fillers. The results are shown in Table 1. In general,
for the thicker dried coatings, inclusion of the nanodiamonds
significantly improved the hardness with a small increase in
haze.
TABLE-US-00001 TABLE 1 Polymer wt % Nanodiamonds Pencil Sample In
Solution wt % in Solution TT % Haze % Hardness 1 2 0 91.6 0.64
<9B 2 2 0.2 92.5 0.62 <9B 3 3 0 92.5 0.61 <9B 4 3 0.3 92.5
0.69 9B-8B 5 4 0 92.4 0.59 8B 6 4 0.4 91.5 1.00 5H
Example 2
Effect of Nanodiamond in Conductive Inks
[0100] This example tests the hardness of films having a fused
metal nanostructured layers with nanodiamonds incorporated into the
conductive layer with a hard coating applied over the conductive
layer.
[0101] A silver nanowire ink was prepared as described above except
for the addition of 0.036 wt % nanodiamonds with hydrogen
terminated surface in the ink. The nanodiamonds were initially
dispersed in a gamma-butyrolactone solvent prior to mixing into the
silver nanowire inks. The nanowire inks were slot coated onto a PET
film substrate and dried to fuse the nanowires into a fused metal
nanostructured network forming a conductive layer. An overcoating
composition was prepared as described in Example 1 except at a
polymer concentration of 0.5 wt % and without nanodiamonds. The
overcoat was processed similarly as described in Example 1 with
slot coating onto the dried fused metal conductive layer, drying of
the coating and UV curing the coating.
[0102] Hardness and optical properties were compared between the
films formed with the nanoparticle fillers in the conductive layer
and films formed without the nanoparticle fillers, as shown in
Table 2. Optical properties were also determined with and without
the overcoat. Inclusion of the nanodiamonds in the nanowire ink
significantly improved the hardness of the film with the overcoat.
With the addition of the nanodiamonds, the sheet resistance
increased somewhat, the total transparency decreased slightly, and
the haze increased somewhat. Note that the overcoat generally
though lowered the haze relative to corresponding samples without
the overcoat.
TABLE-US-00002 TABLE 2 Sheet Resistance Pencil Sample Haze % TT %
(ohms/sq) Hardness AgNW Ink 1.11 92.2 58 AgNW Ink + Overcoat 0.91
91.9 2H AgNW Ink with Nanodiamonds 1.33 91.2 87 AgNW Ink with
Nanodiamonds + 1.19 91.4 ~8H Overcoat
Example 3
Effect of Nanodiamonds in Commercial Overcoats Over a Transparent
Conductive Layer
[0103] This example tests the hardness of transparent conductive
films incorporating commercial overcoats incorporating
nanodiamonds.
[0104] The silver nanowire was deposited and processed as described
above. Following drying, the layer comprised fused metal
nanostructured network within the sparse metal conductive layer.
The sheet resistances for the conductive layers were between 50 and
60 ohms/sq., and the thin overcoat layers did not significantly
change the sheet resistance of the film after applying the and
curing the overcoat. Two different metal nanowire ink systems were
tested in combination with 3 different commercial overcoats, three
different corresponding solvent systems and three different initial
nanodiamond dispersions. The substrates with the fused metal
nanostructured network had an initial haze prior to application of
the overcoat of 1.12% with the first ink system and 1.28% with the
second ink system. Hardness and optical properties were compared
between the films formed with the nanoparticle fillers and films
formed without the nanoparticle fillers.
[0105] A first set of samples were prepared with the first silver
nanowire ink system and an overcoat formed with a coating material
from Hybrid Plastics. The coating solutions for the overcoat were
formed in a formic acid solution. Four solutions were formed with
two solutions having a polymer concentration of 0.5 wt % and two
solutions having polymer concentrations of 0.75 wt %. Of the two
solutions at each polymer concentration, one had added commercial
nanodiamonds in aqueous solvent. The solutions with nanodiamond
fillers had 0.05 wt % nanodiamonds for the 0.5 wt % polymer
solutions and 0.075 wt % nanodiamonds for the 0.75 wt % polymer
solutions. The overcoats were applies, dried and cured. Optical
measurements and hardness measurements were obtained on the cured
films, and the results are presented in Table 3. The haze values in
Table 3 were averages across the film, while the initial haze
values for the steel wool evaluation were specific values measured
at the location where the steel wool was applied. As shown in Table
3, inclusion of the nanodiamonds in these films significantly
improved the hardness, and corresponding experiments also
demonstrated significant improvement in scratch resistance from
steel wool. Representative scanning electron micrograph is shown
for a 10 wt % nanodiamond film at two magnifications in FIGS. 5 and
6. For comparison, FIGS. 7 and 8 show SEM images for 5 wt % and 3
wt % nanodiamond films, respectively.
TABLE-US-00003 TABLE 3 Steel Steel Pencil Wool Wool Hard- Initial
Final Sample TT % Haze % ness Haze % Haze % 0.5 wt % Polymer 91.8
0.83 <9B 0.74 2.22 0.5 wt % Polymer with 91.3 0.84 3H 0.82 0.83
0.05 wt % Nanodiamonds 0.75 wt % Polymer 91.2 0.83 9B 0.78 1.56
0.75 wt % Polymer with 90.8 0.86 5H 0.82 0.82 0.075 wt %
Nanodiamonds
[0106] Two additional samples were prepared with formic acid. These
solutions were prepared with a California Hardcoating Company (CHC)
polymer in the coating solution. The coating solution had polymer
at 0.5 wt %. One solution comprised 0.05 wt % commercial
nanodiamonds in aqueous solution and the second solution did not
include any nanodiamonds. The solutions were coated over a fused
metal nanostructured network formed with the second silver nanowire
ink system. Optical and hardness results were obtained after drying
and curing, and the results are presented in Table 4. The inclusion
of the nanodiamonds significantly increased hardness of the
coatings and decreased the haze increase resulting from the steel
wool test. The initial haze only increased slightly with the
nanodiamonds and the total transmittance only decreased
slightly.
TABLE-US-00004 TABLE 4 Steel Steel Wool Wool Pencil Initial Final
Sample TT % Haze % Hardness Haze % Haze % 0.5 wt % Polymer 91.9
1.07 6B 1.1 2.01 0.5 wt % Polymer with 91.7 1.12 3H 1.1 1.12 0.05
wt % Nanodiamonds
[0107] An additional set of 9 samples were prepared with
N,N-dimethylformamide in the coating solution. The solutions
covered three different polymer concentrations with a coating
polymer from Dexerials, and some samples included nanodiamonds
initially dispersed in ethylene glycol at corresponding
concentrations in the coating solution while other solutions did
not include nanodiamonds. The coatings were applied over a fused
metal nanostructured network formed with the first nanowire ink
solution. Optical and hardness measurements were obtained after
dying and curing the overcoat, and the results are summarized in
Table 5.
TABLE-US-00005 TABLE 5 Steel Steel Pencil Wool Wool TT Haze Hard-
Initial Final Sample % % ness Haze % Haze % 0.3 wt % Polymer 91.7
0.82 HB 0.83 1.04 0.3 wt % Polymer with 91.5 0.89 3H 0.91 1 0.018
wt % Nanodiamonds 0.3 wt % Polymer with 91.3 0.9 5H 0.9 0.9 0.03 wt
% Nanodiamonds 0.5 wt % Polymer 91.5 0.85 H 0.87 1.02 0.5 wt %
Polymer with 91 0.95 5H 1.01 1.03 0.05 wt % Nanodiamonds 0.5 wt %
Polymer with 91.2 0.9 8H 1.04 1.05 0.05 wt % Nanodiamonds 0.75 wt %
Polymer 91.2 0.81 5H 0.9 0.91 0.75 wt % Polymer with 90.4 1.08 8H
0.95 0.96 0.075 wt % Nanodiamonds 0.75 wt % Polymer with 90.1 1.12
8H 1.1 1.1 0.075 wt % Nanodiamonds
[0108] Ten further samples were prepared in non-aqueous solvent for
forming the overcoats. Again, the polymer from Dexerials was used
in a solvent of propylene glycol monomethyl ether (PGME) with 4.5
volume percent N,N-dimethylacetamide (DMA). All of the solutions
included 0.5 wt % polymer. Three different commercial nanodiamonds
were used and for each nanodiamond three different nanodiamond
concentrations were used. The nanodiamonds were commercial
nanodiamonds obtained as dispersions in ethylene glycol (ND-A), in
dispersions of ethylene glycol with particles having
hydrogen-glycol terminated surfaces (ND-H-EG) or in dispersions
gamma-butyrolactone with particles having a hydrogen terminated
surface (ND-H-G). The film samples were prepared as described
above. Optical and hardness measurements were obtained. For these
samples, a micro-scratch analysis was also performed following
rubbing with the steel wool. The results are shown in Table 6. The
nanoparticles significantly improved the scratch resistance of the
films with modest increases in haze and decrease in total
transmittance.
TABLE-US-00006 TABLE 6 Steel Steel Steel Wool Wool Wool at at at
Sample TT % Haze % 20 g 50 g 100 g Overcoat, no 91.5 0.95 1 3-4 ~10
Nanodiamonds Overcoat with 0.0025 wt % 91.4 1.02 pass pass 2 ND-A
Overcoat with 0.005 wt % 91.5 1.03 pass pass 1 ND-A Overcoat with
0.015 wt % 91.5 0.97 pass pass 1 ND-A Overcoat with 0.0025 wt %
91.6 0.91 pass 1 1 ND-H-EG Overcoat with 0.005 wt % 91.3 1.03 pass
pass 1 ND-H-EG Overcoat with 0.015 wt % 91.5 0.96 pass pass 1
ND-H-EG Overcoat with 0.0025 wt % 91.6 0.92 pass pass 1 ND-H-G
Overcoat with 0.005 wt % 91.3 0.94 pass pass pass ND-H-G Overcoat
with 0.015 wt % 91.4 0.95 pass pass pass ND-H-G
Example 4
Effect of Nanodiamonds in Formulated Coating Solutions
[0109] In this example, the effectiveness of nanodiamonds to
improve hardness is examined in samples of transparent conductive
films with in-house formulated overcoats.
[0110] For these experiments, the substrates were prepared with a
fused metal conductive layer formed with the second metal nanowire
ink described in Example 3. Two different in-house coating
solutions (HOC1 and HOC2) were tested. The in-house formulated
coating materials included a blend of a commercial UV crosslinkable
acrylate hard coating composition with a cyclic-siloxane epoxy
resin. HOC1 further comprised a urethane acrylate oligomer, and
HOC2 further comprised an epoxy acrylate oligomer. Epoxy acrylate
hybrid hard coatings are described further, for example, in U.S.
Pat. No. 4,348,462 to Chung, entitled "Abrasion Resistant
Ultraviolet Light Curable Hard Coating Compositions," U.S. Pat. No.
4,623,676 to Kistner, entitled "Protective Coating for Phototools,"
and Sangermano et al., Macromolecular Materials and Engineering,
Volume 293, pp 515-520, (2008), entitled "UV-Cured Interpenetrating
Acrylic-Epoxy Polymer Networks: Preparation and Characterization,"
all three of which are incorporated herein by reference.
[0111] Twelve samples were prepared with two different solvent
systems. Specifically, 8 samples were prepared in a 1:1 by volume
mixture of N,N-dimethylformamide (DMF) and methylethylketone (MEK),
and three samples were prepared in acetonitrile. Samples 1-4 were
prepared with HOC1, and samples 5-12 were prepared with HOC2.
Samples were prepared with two different polymer concentrations in
the coating solution and three different nanodiamond
concentrations. Samples 1-8 had a 0.5 wt % polymer, and samples
9-12 has 0.8 wt % polymer. For four samples, in addition to optical
measurements and hardness measurements, the change in sheet
resistance after applying the steel wool was also measured. The
results are presented in Tables 7 (samples 1-8) and 8 (samples
9-12). The results demonstrate that the solvent had a significant
effect on the coating properties. The nanodiamonds significantly
improved the hardness. The inclusion of the nanodiamonds increased
the haze somewhat.
TABLE-US-00007 TABLE 7 Steel Wool Sample - Nanodiamonds Haze %
Pencil Polymer wt % Solvent TT % Haze % Increase Hardness 1 - HOC1
0.03 DMF + MEK 90.7 1.92 1.17 F 2 - HOC1 0.05 DMF + MEK 90.8 1.84
1.18 B 3 - HOC1 0.1 DMF + MEK 89.8 2.75 1.14 2H 4 - HOC1 0.03
acetonitrile 91.5 1.59 Coating 2B gone 5 - HOC2 0.03 DMF + MEK 90.5
1.35 1.07 H 6 - HOC2 0.05 DMF + MEK 90.7 1.40 1.12 H 7 - HOC2 0.1
DMF + MEK 89.7 1.52 1.10 3H 8 - HOC2 0.03 acetonitrile 91.7 1.28
Coating 3B gone
TABLE-US-00008 TABLE 8 Steel Steel Wool Wool Nanodiamonds Haze %
Resistance Pencil Sample wt % Solvent TT % Haze % Increase Change
Hardness 9 0.03 DMF + 90.4 1.60 1.17 1.31 HB MEK 10 0.05 DMF + 89.8
1.83 1.01 1.06 H MEK 11 0.1 DMF + 88 2.71 1.02 0.97 3H MEK 12 0.03
acetonitrile 91.2 1.56 1.18 1.33 F
[0112] Six samples were prepared with HOC2 based overcoat. Overall,
two different solvent systems were tested and two different types
of nanodiamonds. The samples were prepared as described above. The
results are presented in Table 9. As with the results presented in
Table 7 and 8, the hardness results were significantly dependent on
the solvent system.
TABLE-US-00009 TABLE 9 Nanodiamonds Pencil Sample wt %, type
Solvent (v:v) Hardness 1 0.03, ND-H-EG acetonitrile + HB DMA (95:5)
2 0.05, ND-H-EG acetonitrile + 3H DMA (95:5) 3 0.03, ND-H-G
acetonitrile + 3H DMA (95:5) 4 0.05, ND-H-G acetonitrile + 2H DMA
(95:5) 5 0.03, ND-H-G Acetonitrile + 4H PGME + DMA (48:48:4) 6
0.05, ND-H-G acetonitrile + 6H PGME + DMA (48:48:4)
Example 5
Metal Oxide Fillers
[0113] This example tests the effect on transparent conductive
films with metal oxide nanoparticles in an overcoat over the sparse
metal conductive layer.
[0114] The conductive layer was formed with the second silver
nanowire ink as described in Example 3 above. Six well mixed
coating solution samples were prepared with one of two different
overcoat polymers and one of three different metal oxide
nanoparticles. A first overcoat polymer was obtained from
California Hardcoating Company (CHC), and the second overcoat
polymer was formulated in house (HOC3) similar to the polymers
described in Example 4. The metal oxide nanoparticles were aluminum
oxide nanoparticles (Al.sub.2O.sub.3) from both BYK and US-Nano or
zirconium oxide nanoparticles (ZrO.sub.2) from BYK. All overcoat
solutions were coated, dried and cured as described above. The
average size of the nanoparticles was about 20 nm to about 40 nm.
The coating solutions had about 0.75 wt % polymer and about 0.09 wt
% nanoparticles.
[0115] Sheet resistance (SR) and optical properties were obtained
for films formed with the metal oxide nanoparticles and films
formed without the metal oxide nanoparticles, and the results are
presented in Table 9. In general, the inclusion of aluminum oxide
nanoparticles or zirconium oxide nanoparticles did not
significantly increase sheet resistance or decrease total
transmittance. With zirconium oxide nanoparticles, the haze did not
increase and may have slightly decreased. However, with the
aluminum oxide nanoparticles the haze had a significant
increase.
TABLE-US-00010 TABLE 10 Before Overcoat After Overcoat SR SR Sample
Fusion (Ohms/sq) TT % Haze % (Ohms/sq) TT % Haze % HOC3 with
Al.sub.2O.sub.3 2 passes 59 91.5 1.09 65 91.6 13.7 (BYK) HOC3 with
2 passes 58 91.5 1.08 60 90.8 0.98 ZrO.sub.2 (BYK) CHC with 2
passes 61 91.5 1.07 61 91.4 15.4 Al.sub.2O.sub.3 (BYK) CHC with 2
passes 58 91.5 1.09 65 90.2 1.07 ZrO.sub.2 (BYK) HOC3 with 1 pass
62 90.9 1.34 59 90.5 9.4 Al.sub.2O.sub.3 (US-Nano) CHC with
Al.sub.2O.sub.3 2 passes 43 91.3 1.32 45 90.0 2.7 (US-Nano)
[0116] The embodiments above are intended to be illustrative and
not limiting. Additional embodiments are within the claims. In
addition, although the present invention has been described with
reference to particular embodiments, those skilled in the art will
recognize that changes can be made in form and detail without
departing from the spirit and scope of the invention. Any
incorporation by reference of documents above is limited such that
no subject matter is incorporated that is contrary to the explicit
disclosure herein.
* * * * *