HK1242838B - Property enhancing fillers for transparent coatings and transparent conductive films - Google Patents

Description

Property-enhancing fillers for transparent coatings and transparent conductive films

Cross-reference to related applications

This application claims priority to U.S. Patent Application No. 14/577,669, filed December 19, 2014, by Virkar et al., entitled “Property Enhancing Fillers for Transparent Coatings and Transparent Conductive Films,” and U.S. Provisional Application No. 62/059,376, filed October 3, 2014, by Virkar et al., entitled “Property Enhancing Fillers for Coatings and Transparent Conductive Films,” both of which are incorporated herein by reference.

Technical Field

The present invention relates to thin polymer films loaded with property-enhancing nanoparticles (e.g., nanoparticles that contribute to hardness and wear resistance, thermal conductivity, and/or high dielectric constant). The present 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 conductivity and/or a coating associated with a transparent conductive layer). The present invention also relates to transparent polymer-based films comprising nanodiamonds. Additionally, the present invention relates to coating solutions comprising dissolved polymer, dispersed property-enhancing nanoparticles, other optional compositions (e.g., processing aids or stabilizing compositions), and optionally, metal nanowires.

Background Art

Transparent polymer films are used in a wide range of products. While these films can be used for many purposes, they generally provide some protection against various mechanical and/or environmental insults. The protection provided by the film can be directed both to the underlying structure and to the film itself, as, for example, a scratched surface of the film can degrade the film's desired properties by reducing transparency and increasing haze or turbidity. Surface protection can be important both during use of the final product and during processing to form the product and during transport of components for assembly into the product.

Functional films can serve important purposes in a range of environments. For example, conductive films can be important for dissipating static electricity, which can be undesirable or dangerous. Optical films can be used to provide various functions, such as polarization, antireflection, phase shifting, brightness enhancement, or other functions. High-quality displays can include one or more optical coatings.

Transparent conductors are used in several optoelectronic applications, including, for example, touch screens, liquid crystal displays (LCDs), flat-panel displays, organic light-emitting diodes (OLEDs), solar cells, and smart windows. Historically, indium tin oxide (ITO) has been the material of choice due to its relatively high transparency at high conductivity. However, ITO has several drawbacks. For example, ITO is a brittle ceramic that must be deposited using sputtering, a manufacturing process that involves high temperatures and a vacuum and can therefore be relatively slow. Furthermore, ITO is known to crack easily on flexible substrates.

Summary of the Invention

In a first aspect, the present invention relates to an optical structure comprising a transparent substrate and a coating comprising a polymer binder and nanodiamonds.

In another aspect, the present invention relates to a transparent conductive film comprising a transparent substrate, a transparent 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 from 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 mixtures 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.

In an additional aspect, the present invention relates to a transparent conductive film comprising a transparent substrate and a transparent conductive layer comprising a polymer binder, a sparse metal conductive element, and nanoparticles. In some embodiments, the nanoparticles may have an average primary particle size of no more than about 100 nm and may be formed from 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 mixtures 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.

In other aspects, the present invention relates to an optical structure comprising a transparent substrate and a transparent coating. The transparent coating may include a polymer binder and from about 0.05 wt. % to about 30 wt. % of nanoparticles having an average primary particle diameter of no more than about 100 nm, and may have a pencil hardness at least about 1 level greater than the pencil hardness of the clear coating without filler and a total reduction in visible light transmission attributable to the clear hard coating of no more than about 5%.

The present invention also relates to a solution comprising a solvent, a curable polymer binder, and nanoparticles. The nanoparticles may have an average primary particle diameter of no more than about 100 nm and may include: 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/(m·K); 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 mixtures thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

1 is a fragmentary side view of a film having a sparse metallic conductive layer and various additional transparent layers on either side of the sparse metallic conductive layer.

2 is a top view of a representative schematic patterned structure having three conductive paths formed with a sparse metal conductive layer.

FIG3 is a schematic diagram showing a capacitance-based touch sensor.

FIG4 is a schematic diagram showing a resistance-based touch sensor.

5 is a scanning electron micrograph (SEM) of a transparent conductive film with an overcoat layer having 10 wt % nanodiamonds at a first magnification.

FIG. 6 is a SEM image of the transparent conductive film of FIG. 5 at a higher magnification.

FIG. 7 is a SEM image of a transparent conductive film having an overcoat layer having 5 wt % nanodiamonds.

FIG8 is a SEM image of a transparent conductive film having an overcoat layer having 3 wt % nanodiamonds.

DETAILED DESCRIPTION

Transparent coatings have been developed that have a polymer matrix with property-enhancing nanoparticle fillers to provide the coating with desirable properties, such as increased hardness and/or higher thermal conductivity, in a thin coating with good optical clarity. Suitable fillers for the polymer matrix include, for example, nanodiamonds, which can provide the desired hardness, increased dielectric constant, and thermal conductivity to a coating formed with nanodiamonds without unduly reducing optical clarity. Other suitable nanoparticles or combinations thereof can similarly be incorporated into the polymer matrix. The nanoparticles used as fillers can be formed from materials having high bulk hardness values and/or high bulk thermal conductivity and/or high bulk dielectric constant. In some embodiments, the coating formed from the particle-loaded polymer can have a thickness of no more than about 5 microns. The enhanced coating can be formed via a solution coating process in which the matrix polymer is dissolved in a solvent and the nanoparticles are dispersed in the solution. Such coatings may be suitable for protecting transparent conductive layers, but other transparent coating applications can effectively utilize the enhanced coatings described herein. Specifically, the transparent conductive layer can be formed from metal nanowires. In additional or alternative embodiments, the desired filler can be added directly to the conductive ink used to form the sparse metal conductive element, with a corresponding increase in hardness and other property improvements after being coated with a polymer topcoat. The protective coating can be used to reduce damage from scratches, environmental aggressors (e.g., dilute acids and bases); reduce thermal damage; reduce vulnerability to high voltages and/or provide other valuable protection.

As described herein, an enhanced loading coating can be formed in which the total transmittance of visible light is moderately reduced. Various polymer matrices can be introduced into coatings with relatively good mechanical strength to provide a good high transparency foundation for further enhancement. Generally speaking, a coating with a small thickness can be formed, and the enhanced mechanical properties can effectively stabilize the coating mechanically even with a small thickness. In some embodiments, since conductivity can be maintained via a thin outer coating, a small thickness may be required for the use of an adjacent transparent conductive layer. Therefore, in the case of a coating having an average thickness of no more than about 25 microns and in some embodiments no more than 1 micron, and typically at least about 50nm thick, significant mechanical stability can be obtained. In addition, the thermal conductivity properties of the enhanced coating may be required to dissipate heat so that heating damage can be reduced. Improved thermal conductivity can provide other desirable uses for specific applications. Coatings with high dielectric fillers can be used to prevent sparse metal conductive layers from being damaged by high voltage.

Good coating properties generally relate to the good dispersion of the nanoparticle filler in the solution of the matrix polymer so that the gained coating has the particle agglomeration effect of minimization. The nanoparticle filler has an average primary particle diameter of no more than about 100nm usually so that the particle can be incorporated into the relatively smooth thin coating and the particle does not change optical properties excessively. In general, the coating has a nanoparticle loading that is no more than about 70 % by weight. The concentration of polymer binder and filler particles can be adjusted to produce the desired coating properties of solution in the coating solution, such as the thickness of viscosity and final coating. The ratio of the concentration of solid can be adjusted to produce desired coating concentration in case the coating is dry in the coating solution. The polymer component of coating can be cross-linked to further strengthen the coating by UV radiation or other means that are suitable for polymer binder usually.

In general, property-enhancing nanoparticle fillers can be incorporated into a passive protective coating and/or directly into the transparent conductive layer. A passive transparent protective coating may or may not be used to cover the transparent conductive layer. A common feature of these coatings is the compatibility of the components in the coating solution and the resulting composite material. Compatibility refers to the ability to effectively disperse into a relatively uniform material without an unacceptable degree of aggregation (e.g., by agglomeration) of the components. Specifically, compatibility allows for good distribution of the materials within the coating solution to provide for the formation of a reasonably uniform composite material that forms the coating. A more uniform composite material is believed to contribute to the desired optical properties of the coating, such as good transparency and low turbidity.

For passive coatings, the coating solution may include a solvent, a dissolved matrix polymer, nanoparticles having selected properties, possible combinations thereof, and optional additional components. A range of matrix polymers suitable for transparent films may be used, as described below. Wetting agents (e.g., surfactants) and other processing aids may be used. Generally, the solvent may include water, an organic solvent, or a suitable mixture thereof. For active coatings, the coating solution typically further includes components that contribute to active functionality (e.g., metal nanowires for conductivity). Examples of two types of coatings are described in the Examples below. For overcoats used as transparent conductive layers based on metal nanowires, stabilizers introduced into the overcoat have been found to stabilize the conductivity of the transparent conductive layer. The stabilizers are consistent with maintaining good transparency and process compatibility of the coating solution and are further described below.

Regarding the desired filler, nanodiamonds are particularly interesting due to the desirable properties that can be introduced that are consistent with maintaining good optical transparency and relatively low turbidity. Diamond is a crystalline form of carbon with ^sp3 hybrid orbitals that contrasts with graphitic carbon, amorphous carbon, and other forms of carbon. Commercial nanodiamonds typically have a core of crystalline diamond carbon, a shell of amorphous and/or graphitic carbon, and are dielectrics. The surface chemical properties of nanodiamonds can reflect the synthesis method and possible additional processing. Commercial nanodiamonds that can be functionalized or unfunctionalized after purification can be purchased from various suppliers, as listed below. Nanodiamonds share extremely high hardness and thermal conductivity values with macro diamonds, and these properties can be used to provide the desired properties to transparent coatings incorporating nanodiamonds.

Nanodiamonds with an average primary particle diameter of typically no more than about 50 nm and in some embodiments no more than about 10 nm are commercially available, but nanodiamonds with an average primary particle diameter of no more than about 100 nm may be useful in some embodiments. As used herein, unless otherwise indicated, the particle diameter is the average value along the principal axis of the particle, which can be roughly estimated from a transmission electron micrograph. Commercial nanodiamonds are synthetically produced to have possible surface modifications, and their overall structure can be confirmed using spectroscopic techniques. Surface modification of nanodiamonds can be useful for processing nanodiamonds and for compatibility with specific solvents and adhesives. As described in the examples below, commercial nanodiamonds can be well dispersed in a range of solvents to produce high-quality optical coatings with good transparency and low turbidity. Other nanoparticle fillers may have an average particle diameter within the same range as nanodiamonds. Nanoparticles may have a roughly spherical shape or other convenient shape. One of ordinary skill in the art will recognize that additional ranges within the above-defined average particle diameter ranges for nanodiamonds or other property-enhancing nanoparticles are encompassed, and such ranges are within the scope of the present invention.

Nanodiamonds can provide a desired degree of hardness and thermal conductivity to composite coatings incorporating nanodiamonds. In addition, diamond is a good dielectric, so nanodiamond composite coatings can facilitate the dissipation of strong electric fields that can damage films in structures. Other nanoparticles can similarly be introduced to provide similar properties to composites incorporating functional nanoparticles consistent with the good optical transparency of the resulting coatings. To form transparent conductive films, other suitable nanoparticles for providing hardness include, but are not limited to, for example, boron nitride, _B4C , cubic _BC2N , silicon carbide, crystalline alpha alumina (sapphire), or the like. Nanoparticles that contribute to hardness can be formed from a bulk material having a Vickers hardness of at least about 1650 kgf/ ^mm2 (16.18 GPa).

Regarding thermal conductivity, in addition to nanodiamonds, graphene, silicon nitride, boron nitride, aluminum nitride, gallium arsenide, indium phosphide, and mixtures thereof may also be suitable for introducing high thermal conductivity. In some embodiments, high thermal conductivity materials may have a thermal conductivity of at least about 30 W/(m·K), with graphene and diamond having thermal conductivities among the highest known. Specifically, high dielectric constant materials that can be introduced as nanoparticles include, but are not limited to, for example, barium titanate, strontium titanate, lead titanate, lead zirconium titanate, calcium copper titanate, and mixtures thereof. Regarding the hardness of the protective polymer-based coating, the hardness can be measured by a pencil hardness test on the film, as further described below. Scratch resistance was also evaluated using steel wool in the following examples.

The coating is typically formed by solution coating. Nanoparticles (e.g., nanodiamonds) can be dispersed, and the dispersion of nanoparticles can then be blended with a coating solution of a polymer binder, although the order of processing can be adjusted depending on the choice of solvent and the dispersion properties of the particles. The nanoparticles in the coating solution can have a concentration ranging 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%. One of ordinary skill in the art will recognize that additional concentration ranges falling within the explicit ranges above are contemplated and are within the scope of the present invention.

Transparent conductive elements (e.g., films) of particular interest herein include sparse metal conductive layers. Conductive layers are typically sparse to provide the desired amount of optical transparency, so the metal coverage above the conductive element layer has significant gaps. For example, a transparent conductive film may include metal nanowires deposited along a layer where sufficient contact for percolation to provide a suitable conductive path is provided. In other embodiments, the transparent conductive film may include a network of fused metal nanostructures, which have been found to exhibit desirable electrical and optical properties. Unless otherwise specifically indicated, conductivity referenced herein refers to electrical conductivity.

The loaded polymer films described herein can provide desirable properties for transparent optical films in general and for protecting sparse metal conductive elements in transparent conductive films in particular. The film thickness can be selected to be thin enough to allow good electrical conductivity through the film. The film's hardness can make the structure resistant to scratches and deformation, and the high thermal conductivity can facilitate heat removal to limit potential damage to the sparse metal conductive elements due to heat. Regardless of the specific structure, sparse metal conductive elements are susceptible to environmental attack.

In general, various sparse metallic conductive layers can be formed from metal nanowires. U.S. Patent No. 8,049,333 to Alden et al., entitled “Transparent Conductors Comprising Metal Nanowires,” which is incorporated herein by reference, describes films formed from metal nanowires that have been treated to flatten the nanowires at junctions to improve conductivity. U.S. Patent No. 8,748,749 to Srinivas et al., entitled “Patterned Transparent Conductors and Related Manufacturing Methods,” which is incorporated herein by reference, describes structures that include surface-embedded metal nanowires to improve metal conductivity. However, it has been discovered that fused metal nanostructured networks have desirable properties in terms of high conductivity and desirable optical properties with respect to transparency and low haze. Fusion of adjacent metal nanowires can be performed based on chemical processes under commercially suitable processing conditions.

Metal nanowires can be formed from a range of metals and are commercially available. Although metal nanowires themselves are conductive, it is believed that the majority of the resistance in metal nanowire-based films is due to the junctions between the nanowires. Depending on the processing conditions and the properties of the nanowires, the sheet resistance of deposited, relatively transparent nanowire films can be extremely high, for example, in the billions of ohms/sq range or even higher. Various methods have been proposed to reduce the resistance of nanowire films without compromising optical transparency. Low-temperature chemical melting to form metal nanostructured networks has been found to be particularly effective in reducing resistance while maintaining optical transparency.

In particular, a significant advance towards achieving conductive films based on metal nanowires has been the discovery of a tractable process for forming molten metal networks in which adjacent segments of metal nanowires are fused. The fusion of metal nanowires using various fusion sources is further described in the following published U.S. patent applications: 2013/0341074 to Wilka et al., entitled “Metal Nanowire Networks and Transparent Conductive Material”; 2013/0342221 to Wilka et al., entitled “Metal Nanostructured Networks and Transparent Conductive Material” (the '221 application); 2014/0238833 to Wilka et al., entitled “Fused Metal Nanostructured Networks, Fusing Solutions With Reducing Agents and Methods for Forming Metal Networks” (the '833 application); and Yang et al., entitled “Transparent Conductive Coatings Based on Metal Nanowires and Polymer Binders, Solution Processing and Patterning Methods Therefor.” and Patterning Approaches” to Li et al., U.S. Patent Application No. 2015/0144380 (the '380 application), and co-pending U.S. Patent Application 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.

Transparent conductive films generally include several components or layers that contribute to the processability and/or mechanical properties of the structure without adversely altering the optical properties. Sparse metallic conductive layers can be designed to have desired optical properties when incorporated into transparent conductive films. The sparse metallic conductive layer may or may not further include a polymer binder. Unless otherwise indicated, references to thickness refer to the average thickness of the referenced layer or film, and adjacent layers may be entangled at their boundaries depending on the specific materials. In some embodiments, the overall film structure may 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 of no more than about 250 ohms/sq after formation, although significantly better performance is described herein.

For incorporation into a clear coating for a transparent conductive film or directly into an ink for forming a sparse metallic conductive layer, a loaded overcoat typically does not significantly increase sheet resistance, and in some embodiments, the sheet resistance is increased by no more than about 20% relative to the sheet resistance of a corresponding unloaded film; in other embodiments, the sheet resistance is increased by no more than about 15%; and in additional embodiments, the sheet resistance is increased by no more than about 10%. For general optical applications, the overcoat may reduce the total visible light transmittance by no more than about 5% relative to the total transmittance of a corresponding unloaded film; in other embodiments, the total visible light transmittance is reduced by no more than about 3%; in additional embodiments, the total visible light transmittance is reduced by no more than about 2%; and in other embodiments, the total visible light transmittance is reduced by no more than about 1%. Furthermore, it may be desirable that haze not be significantly increased by the fillers in the coating. In some embodiments, the haze value, typically reported as a percentage, may be increased by no more than about 0.5 relative to the haze value of a corresponding unloaded film; in other embodiments, the haze value is increased by no more than about 0.4; and in additional embodiments, the haze value is increased by no more than about 0.3. In some embodiments, turbidity may be reduced. One of ordinary skill in the art will recognize that additional ranges of sheet resistance increase, total transmittance change, and turbidity change within the explicit ranges above are contemplated and are within the scope of the present invention. A reference unloaded film was produced from a coating solution having the same concentrations of the other components in the solvent and processed in the same manner, such that the final thickness may be slightly different.

It has been discovered that extremely effective stabilization of the sparse metallic conductive layer can be achieved through appropriate design of the overall structure. Specifically, the stabilizing composition can be placed in a layer (which can be an overcoat or primer) adjacent to the sparse metallic conductive element. Furthermore, an optically clear adhesive (e.g., as a component of the film) can be used to prepare for attaching the transparent conductive film to the device, and the choice of optically clear adhesive has been found to significantly contribute to achieving the desired degree of stability. Specifically, the optically clear adhesive can include a double-sided adhesive layer on a carrier layer. The carrier layer can be a polyester (e.g., PET or a commercial barrier layer material) that provides the desired moisture and gas barrier to protect the sparse metallic conductive layer, although applicants do not wish to be limited by the operating principle of a particular optically clear adhesive.

Transparent conductive films are widely used in applications such as solar cells and touch screens. Transparent conductive films formed from metal nanowire components offer the promise of lower processing costs and more adaptable physical properties compared to traditional materials. In multilayer films with various structural polymer layers, the resulting film structures have been found to be robust in processing while maintaining desired conductivity. Furthermore, the incorporation of the desired components described herein can provide stability without degrading the functional properties of the film, allowing devices incorporating the films to have a suitable lifespan under normal use.

Clear coatings and films

Transparent coatings having nanoparticle-loaded polymers as described herein are typically applied to transparent substrates for incorporation into desired structures. General structures are described, and specific applications of transparent conductive films are found in the following sections. Generally speaking, precursor solutions for transparent filled coatings can be deposited onto transparent substrates using appropriate coating methods to form transparent structures. In some embodiments, the transparent substrate can be a film for incorporation into a final device or, alternatively or additionally, an integral optical component (e.g., a light emitting device or a light receiving device). The discussion focuses on simple passive transparent substrates and then discusses other structures accordingly.

In general, any reasonable transparent substrate may be suitable. Thus, a suitable substrate may be formed from, for example, inorganic glass (e.g., silicate glass), a transparent polymer film, an inorganic crystal, or the like. In some embodiments, the substrate is a polymer film. Suitable polymers for the substrate include, for example, polyethylene terephthalate (PET), polyethylene naphthalate (PEN), polyacrylates, poly(methyl methacrylate), polyolefins, polyvinyl chloride, fluoropolymers, polyamides, polyimides, polysulfones, silanes, polyetheretherketones, polynorbornene, polyesters, polystyrene, polyurethanes, polyvinyl alcohol, polyvinyl acetate, acrylonitrile butadiene styrene copolymers, cycloolefin polymers, cycloolefin copolymers, polycarbonates, copolymers thereof, blends thereof, or the like. Fluoropolymers include, for example, polyvinyl fluoride, polyvinylidene fluoride, polytetrafluoroethylene, hexafluoropropylene, perfluoropropyl vinyl ether, perfluoromethyl vinyl ether, polychlorotrifluoroethylene, and the like. The polymer film used in some embodiments may have a thickness of from about 5 microns to about 5 mm; in other embodiments, from about 10 microns to about 2 mm; and in additional embodiments, from about 15 microns to about 1 mm. One of ordinary skill in the art will recognize that additional thickness ranges are encompassed within the explicit ranges above and are within the scope of the present invention. The substrate may include multiple layers distinguished by composition and/or other properties. More specific ranges of materials suitable for the substrate of the transparent conductive film are presented below, and the general substrate range will include these specific materials and properties. Suitable polymers for the coating may include, for example, radiation-curable polymers and/or thermally curable polymers, such as polyurethanes, acrylic resins, acrylic copolymers, cellulose ethers and cellulose esters, other structural polysaccharides, polyethers, polyesters, polymers containing epoxy groups, copolymers thereof, and mixtures thereof.

The transparent coating layer with the property-enhancing nanoparticle filler may typically 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. A transparent coating layer formed from a nanoparticle-loaded polymer may include from about 0.01 weight percent (wt%) to about 70 wt% of the property-enhancing nanoparticles; in further embodiments, from about 0.05 wt% to about 60 wt% of the property-enhancing nanoparticles; in other embodiments, from about 0.1 wt% to about 50 wt% of the property-enhancing nanoparticles; and in additional embodiments, from about 0.2 wt% to about 40 wt% of the property-enhancing nanoparticles. The transparent coating layer may further include a polymer binder for the transparent conductive film and, optionally, the sparse metal conductive layer, optional property modifiers (e.g., crosslinkers, wetting agents, viscosity modifiers), and/or stabilizers (e.g., antioxidants and/or UV stabilizers). One of ordinary skill in the art will recognize that additional ranges of loaded polymer thickness and nanoparticle concentrations within the explicit ranges above are contemplated and are within the scope of the present invention.

Regarding property-enhancing nanoparticles, nanodiamonds exhibit desirable properties, particularly with respect to hardness and thermal conductivity, and to some extent, with respect to dielectric constant. Both the hardness and thermal conductivity of bulk diamond are among the highest among known materials. However, additional materials provide the desired values for these properties. For convenience, although nanoparticle properties generally directly reflect bulk properties, since the values for nanoparticles can be more difficult to obtain, reference is made to the material properties of the corresponding bulk material. The materials for property-enhancing nanoparticles are typically inorganic materials or carbon materials in which the majority of the material is elemental carbon, such as fullerenes, 3-dimensional crystals (diamonds), 2-dimensional crystals (graphitic carbon), amorphous forms (e.g., carbon black), and the like. Nanoparticles can have surface modifications, including organic surface modifications, without changing the identification of nanoparticles based on the majority of the core material.

For relevant materials, the hardness of the bulk material can be measured with reference to Vickers hardness. Vickers hardness is a measurement of the indentation of a material. Vickers hardness can be measured by recognized standards (including ASTM E384 and ISO 6507-1-2005, both of which are incorporated herein by reference). A table shows the Vickers hardness of many materials of interest. Vickers hardness is usually reported in HV (Vickers Pyramid Value, kilogram-force/square millimeter (kg-force/mm ² )), but it can be reported in Pascals, even though it is not actually pressure. In some embodiments, the bulk material corresponding to the nanoparticles may have a Vickers hardness of at least about 1650 HV; in some embodiments, a Vickers hardness of at least about 1750 HV; and in additional embodiments, a Vickers hardness of at least about 1800 HV. In addition to nanodiamonds, additional hard materials for property-enhancing nanoparticles include, for example, boron nitride, _B4C , cubic _BC2N , silicon carbide, tungsten carbide, aluminum boride, crystalline alpha alumina (sapphire), or the like.

With respect to high thermal conductivity materials, suitable materials may have a bulk thermal conductivity of at least about 30 W/(m·K); in further embodiments, a bulk thermal conductivity of at least about 35 W/(m·K); and in some embodiments, a bulk thermal conductivity of at least about 50 W/(m·K). One of ordinary skill in the art will recognize that additional ranges of thermal conductivity within the above-specified ranges are encompassed and within the scope of the present invention. Suitable high thermal conductivity materials include, in addition to nanodiamonds, for example, many elemental metals (non-ionized 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 constants, various titanates have high dielectric constants, such as barium titanate, strontium titanate, lead titanate, lead zirconium titanate, calcium copper titanate, and mixtures thereof.

Relevant nanoparticles are generally commercially available. Sources of nanoparticles include, for example, 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 Nanomaterials, Inc. (Texas, USA); and Nanoscale Technologies, Inc. (Romeoville, Illinois, USA). In addition, laser pyrolysis technology has been developed for the synthesis of a wide range of dispersed nanoparticles, as described in U.S. Patent 7,384,680 to Bi et al., entitled “Nanoparticle-Based Powder Coatings and Corresponding Structures,” which is incorporated herein by reference.

Nanodiamonds or diamond nanoparticles may typically be natural nanodiamonds or synthetic nanodiamonds, and the nanodiamond particles may include a crystalline nanodiamond core surrounded by a shell of graphite and/or amorphous carbon. The surface of the nanodiamond may be formed due to a specific synthesis method and optional post-synthesis processing (e.g., surface functionalization). For commercial applications, suitable diamond nanoparticles are typically synthetic nanodiamonds, which are commercially available. The surface of the nanodiamond may be functionalized to affect the chemical properties of the nanodiamond, such as dispersibility and/or compatibility with a specific polymer binder. The average diameter of the nanodiamond particles may typically 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. One of ordinary skill in the art will recognize that additional ranges of nanodiamond average diameters within the above-specified ranges are encompassed and are within the scope of the present invention.

Synthetic nanodiamonds can be produced by several methods. For example, gas phase formation (e.g., chemical vapor deposition), ion irradiation of graphite, chlorination of carbides, and techniques using shock wave energy are some of the several possible methods for producing such diamond particles or thin nanodiamond films. In addition to diamond nanoparticles with roughly spherical morphology, other 1- and 2-dimensional nanodiamond structures have been produced, such as nanodiamond rods, flakes, sheets, and the like, which can also be used in UV protection compositions (for methods of synthesizing these structures, see O. Shenderova and G. McGuire, "Types of Nanodiamonds," chapter "Ultrananocrystalline diamond: Synthesis, Properties and Applications," ed. O. Shenderova and D. Gruen, William Andrews Publishers, 2006, the contents of which are incorporated herein by reference). Commercial nanodiamond particles are typically formed by controlled detonation techniques, such as those described in U.S. Patent No. 5,916,955, entitled “Diamond-Carbon Material and Method for Producing Thereof,” by Vereschagin et al., which is incorporated herein by reference. Improved purification methods for detonation nanodiamonds are described, for example, in published PCT application WO 2013/135305, entitled “Detonation Nanodiamond Material Purification Method and Product Thereof,” by Dolmatov et al., which is incorporated herein by reference. Commercial nanodiamonds with various surface chemistries or dispersed in various solvents are commercially available from NanoCarbon Research Institute Co., Ltd. (Japan); PlasmaChem (Germany); Carbodeon Ltd. (Finland); NEOMOND (South Korea); Sigma-Aldrich (USA); and Ray Technology Ltd. (Israel).

Nanodiamond particles generally each include a mechanically stable, chemically inert crystalline core and a surface that is generally considered to be relatively chemically active. By functionalizing the surface of the nanodiamond particles with a target substance, the nanodiamond can have modified chemical and/or physical properties. Functionalization can be accomplished by various chemical, photochemical, and electrochemical methods to graft different organic functional groups onto the nanodiamond. Depending on the desired physical properties and application of the nanodiamond, the functionalized nanodiamond material can be fluorinated, chlorinated, carboxylated, aminated, hydroxylated, hydrogenated, sulfonated, or a mixture thereof. See, for example, Yao's published U.S. patent application 2011/0232199, entitled "Process for Production of Dispersion of Fluorinated Nanodiamond," and (for carboxylated nanodiamonds) Mariachi et al.'s published PCT application WO 2014/174150, entitled "A Method for Producing Zeta-Negative Nanodiamond Dispersion and Zeta-Negative Nanodiamond Dispersion," which are incorporated herein by reference. Functionalization and/or purification can be used to help remove and/or break up nanoparticle agglomerates. Generally, commercial nanodiamonds are sufficiently unagglomerated to be processed into relatively uniform thin films as described herein. Solution pH, concentration, solvent, and other dispersion properties can be adjusted to further aid in 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.

The hardness of loaded polymer films can be measured using the Pencil Hardness Test for Films based on ASTM D3363. Following the pencil sharpening method, a constant downward force is applied while the pencil is held at a 45° angle. A 500-gram or 750-gram pencil hardness kit is used for the measurement. Hardness is determined by analyzing the effect of pencils of varying graphite grade scales on the base conductive layer. The film is considered acceptable if it does not damage the base layer. The film is examined under a Leica microscope at 20x magnification. The hardness scale ranges from 9B to 9H, with higher B values corresponding to lower hardness values and higher H values corresponding to increased hardness. The F value connects the B and H ranges, with the lowest "B" value being HB, followed by B, 2B, ..., 9B. In some embodiments, a coating with property-enhancing nanoparticles can have a pencil hardness that is at least one hardness grade higher; in some embodiments, at least about two grades higher; and in other embodiments, at least about three grades higher, relative to an otherwise equivalent coating without the property-enhancing nanoparticles. Other scales and tests for hardness are available and will follow qualitatively similar trends. Scratch resistance was also evaluated by rubbing steel wool against the surface with a 100 g weight, as further described in the Examples below. Ultrafine steel wool was used to scratch the film by rubbing the surface after the clear top coat was applied.

Transparent loaded coatings can be formed by coating the precursor solution using an appropriate coating method. The property-enhancing nanoparticles and/or stabilizing composition can be incorporated into a suitable solvent selected to deposit a coating having appropriate compatibility. Suitable solvents generally include, for example, water, alcohols, ketones, esters, ethers (e.g., glycol ethers), aromatic compounds, alkanes, and the like, and mixtures thereof. Specific solvents include, for example, water, ethanol, isopropanol, isobutanol, tert-butanol, methyl ethyl ketone, methyl isobutyl ketone, cyclic ketones (e.g., cyclopentanone and cyclohexanone), glycol ethers, toluene, hexane, ethyl acetate, butyl acetate, ethyl lactate, propylene carbonate, dimethyl carbonate, PGMEA (2-methoxy-1-methylethyl acetate), N,N-dimethylformamide, N,N-dimethylacetamide, acetonitrile, formic acid, or mixtures thereof.

In general, the polymers used for the coating, which are typically crosslinkable polymers, can be supplied as commercial coating compositions or formulated from selected polymer compositions. Suitable classes of radiation-curable and/or thermally curable polymers include, for example, silanes, polysilsesquioxanes, polyurethanes, acrylic resins, acrylic copolymers, cellulose ethers and cellulose esters, nitrocellulose, other water-insoluble structural polysaccharides, polyethers, polyesters, polystyrenes, polyimides, fluoropolymers, 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); coatings from Hybrid Plastics Co., Ltd. (Mississippi, USA); silica-filled siloxane coatings from California Hard Coatings, Inc. (California, USA); and crystal-coated UV-curable coatings from SDC Technologies, Inc. (California, USA). The polymer concentration and, correspondingly, the concentration of other non-volatile agents can be selected to achieve the desired rheological properties of the coating solution, such as an appropriate viscosity for the selected coating process. Solvents can be added or removed to adjust the total solids concentration. The relative amounts of solids can be selected to adjust the composition of the finished coating composition, and the total amount of solids can be adjusted to achieve the desired thickness of the dried coating. Generally, the coating solution can have a polymer concentration of 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%. One of ordinary skill in the art will recognize that additional ranges of polymer concentrations within the above-specified ranges are encompassed and are within the scope of the present invention.

The property-enhancing nanoparticles can be incorporated into the coating solution used to form the coating. The coating precursor solution can include from about 0.005 wt% to about 5 wt% of the nanoparticles; in further embodiments, from about 0.01 wt% to about 3 wt% of the nanoparticles; and in additional embodiments, from about 0.025 wt% to about 2 wt% of the property-enhancing nanoparticles. One of ordinary skill in the art will recognize that additional ranges of property-enhancing nanoparticles in the coating solution within the ranges explicitly stated above are contemplated and fall within the scope of the present invention. Additional additives (e.g., wetting agents, viscosity modifiers, dispersing aids, and the like) can be added as desired.

In some embodiments, a transparent coating with property-enhancing nanoparticles may result in a decrease in total visible light transmittance of no more than about 5 percentage points, in further embodiments, no more than about 3 percentage points, and in further embodiments, no more than about 1.5 percentage points, relative to a corresponding coating without the property-enhancing nanoparticles. Furthermore, in some embodiments, a transparent coating with property-enhancing nanoparticles may result in an increase in haze of no more than about 1.5 percentage points, in further embodiments, no more than about 1 percentage point, and in further embodiments, no more than about 0.6 percentage points, relative to a corresponding unloaded coating. One of ordinary skill in the art will recognize that additional ranges of modifications in the optical properties of the loaded polymer coatings are contemplated within the explicit ranges above and are within the scope of the present invention. A corresponding unloaded coating, with the same concentration in the solvent and processed in the same manner, except for the absence of the nanoparticles, may have a slightly different final thickness than the corresponding coating.

For deposition of the coating precursor solution, any suitable deposition method can be used, such as dip coating, spray coating, knife-edge coating, rod coating, Meyer-rod coating, slot die coating, gravure printing, spin coating, or the like. The deposition method controls the amount of liquid deposited, and the concentration of the solution can be adjusted to provide a desired thickness of product coating on the surface. After forming the coating from the dispersion, the coating can be dried to remove the liquid and, if appropriate, crosslinked.

Transparent conductive film

Transparent conductive structures or films generally include: a sparse metal conductive layer that provides electrical conductivity without significantly adversely altering optical properties; and various additional layers that provide mechanical support and protection for the conductive elements. Generally, a polymer overcoat is placed above the sparse metal conductive layer. The property-enhancing nanoparticles described herein can be placed in the overcoat, the optional undercoat, and/or directly in the sparse metal conductive layer. The sparse metal conductive layer is extremely thin and correspondingly susceptible to damage from mechanical and other abuse. The property-enhancing nanoparticles can provide some types of protection, and as described in the previous section, stabilizing compounds and other elements of the film can provide additional protection. Regarding sensitivity to environmental damage, it has been found that the undercoat and/or overcoat can include a stable composition that can provide the desired protection, and certain types of optically clear adhesives and/or barrier layers can also provide valuable protection against light, heat, chemicals, and other environmental damage. While the focus herein is on environmental insults from humid air, heat, and light, the polymer sheet used to protect the conductive layer from these environmental insults can also provide protection from contacts and the like.

Thus, a sparse metal conductive layer can be formed on a substrate that may have one or more layers in its structure. The substrate can generally be identified as a self-supporting film or sheet structure. A thin solution-processed layer, referred to as a primer, can be placed along the top surface of the substrate film and immediately below the sparse metal conductive layer, as needed. In addition, the sparse metal conductive layer can be coated with an additional layer that provides some protection on the side of the sparse metal conductive layer opposite to the substrate. Generally speaking, the conductive structure can be placed in any orientation in the final product, i.e., from the substrate facing outward to the substrate against the surface of the supporting conductive structure of the product. In some embodiments, multiple coatings (i.e., primer and topcoat) can be applied, and each layer can have selected additives for enhancing the corresponding properties.

Referring to FIG1 , a representative transparent conductive film 100 includes a substrate 102, a basecoat 104, a sparse metal conductive layer 106, an overcoat 108, an optically clear adhesive layer 110, and a protective surface layer 112, although not all embodiments include all layers. Specifically, a roll of transparent conductive film can be dispensed with the overcoat as a top layer for subsequent processing (which may or may not involve the subsequent addition of additional overlying layers). In these embodiments, having a mechanically hard overcoat may be desirable to reduce the risk of damage to the conductive film. The transparent conductive film generally includes a sparse metal conductive layer and at least one layer on each side of the sparse metal conductive layer. In some embodiments, the total thickness of the transparent conductive film may have a thickness of from 5 microns to about 3 millimeters (mm); in other embodiments, it may have a thickness of from about 10 microns to about 2.5 mm; and in other embodiments, it may have a thickness of from about 15 microns to about 1.5 mm. One of ordinary skill in the art will recognize that additional ranges of thicknesses within the above-specified ranges are contemplated and fall within the scope of the present invention. In some embodiments, the length and width of the film as produced can be selected to suit a specific application so that the film can be directly introduced for further processing into a product. In additional or alternative embodiments, the width of the film can be selected for a specific application, while the length of the film can be long (where it is desired that the film can be cut into the desired length for use). For example, the film can be in the form of a long sheet or roll. Similarly, in some embodiments, the film can be in a roll or another large standard format, and the elements of the film can be cut according to the desired length and width for use.

Substrate 102 generally comprises a durable support layer formed from a suitable polymer or polymers. In some embodiments, the substrate may have a thickness from about 10 microns to about 1.5 mm; in other embodiments, from about 15 microns to about 1.25 mm; and in additional embodiments, from about 20 microns to about 1 mm. Those skilled in the art will recognize that additional ranges of substrate thicknesses within the explicit ranges above are contemplated and fall within the scope of the present invention. Suitable optically clear polymers with excellent transparency, low haze, and good protective properties can be used for the substrate. Suitable polymers include, for example, polyethylene terephthalate (PET), polyethylene naphthalate (PEN), polyacrylates, poly(methyl methacrylate), polyolefins, polyvinyl chloride, fluoropolymers, polyamides, polyimides, polysulfones, siloxanes, polyetheretherketones, polynorbornenes, polyesters, polystyrene, polyurethanes, polyvinyl alcohol, polyvinyl acetate, acrylonitrile butadiene styrene copolymers, cyclic olefin polymers, cyclic olefin copolymers, polycarbonates, copolymers thereof, or blends thereof, or the like. Suitable commercial polycarbonate substrates include, for example: MAKROFOL SR243 1-1CG, available from Bayer Materials Science; plastic, available from TAP Plastics; and LEXAN ^™ 8010CDE, available from SABIC Innovative Plastics. The protective surface layer 112 can independently have a thickness and composition covering the same thickness ranges and composition ranges as the substrates described above in this paragraph.

An optional undercoat layer 104 and/or an optional overcoat layer 108, which may be independently selected for inclusion, may be disposed below or above the sparse metallic conductive layer 106, respectively. The optional coating layers 104, 108 may comprise a curable polymer, such as a thermally curable or radiation curable polymer. Suitable polymers for the coating layers 104, 108 are described below as binders for inclusion in the metal nanowire ink; and the list of polymers, corresponding crosslinkers, and additives also applies to the optional coating layers 104, 108 without explicitly repeating the discussion here. The coating layers 104, 108 may have a thickness of from about 25 nm to about 2 microns; in further embodiments, from about 40 nm to about 1.5 microns; and in yet another embodiment, from about 50 nm to about 1 micron. One of ordinary skill in the art will recognize that additional ranges of overcoat layer thicknesses within the explicit ranges above are contemplated and fall within the scope of the present invention. Generally, the small thickness of the overcoat 108 allows for electrical conduction through the overcoat 108 so that electrical connection to the sparse metallic conductive layer 106 is possible, but in some embodiments the overcoat may include sublayers where electrical conductivity is provided through some but not necessarily all sublayers.

The optional optically clear adhesive layer 110 may have a thickness of 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. One of ordinary skill in the art will recognize that additional ranges of optically clear adhesive layer thicknesses within the explicit ranges above are encompassed and within the scope of the present invention. Suitable optically clear adhesives may be contact adhesives. Optically clear adhesives include, for example, coatable compositions and tapes. UV-curable liquid optically clear adhesives are available based on acrylic or silicone chemistries. Suitable tapes are commercially available, for example, from Lintec Corporation of Japan (MO series); Saint Gobain Performance Plastics (DF713 series); Nitto Americas (NittoDenko) (LUCIACS CS9621T and LUCIAS CS9622T); DIC Corporation (DAITAC LT series OCA, DAITAC WS series OCA, and DAITAC ZB series); PANAC Plastic Films (PANACLEAN series); Minnesota Mining and Manufacturing Company (3M, Minnesota, USA - Product Nos. 8146, 8171, 8172, 8173, and similar products); and Adhesive Research (e.g., product 8932).

The amount of nanowires delivered to the substrate for the sparse metallic conductive layer 106 can involve a balance of factors to achieve the desired amount of transparency and conductivity. While the thickness of the nanowire network can, in principle, be assessed using scanning electron microscopy, the network can be relatively sparse to provide optical transparency, which can complicate measurement. Generally, a sparse metallic conductive element (e.g., a fused metal nanowire network) will 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, sparse metallic conductive elements are typically relatively open structures with a distinct surface texture at the submicron scale. The loading of the nanowires can provide a useful network parameter that can be easily assessed, and the loading value provides a surrogate parameter related to thickness. Therefore, as used herein, the loading of nanowires on a substrate is generally expressed as milligrams of nanowires per square meter of substrate. Generally, the metal conductive network (whether fused or not) can have a loading of from about 0.1 milligrams (mg)/ ^m2 to about 300 mg/ ^m2 ; in further embodiments, from about 0.5 mg/ ^m2 to about 200 mg/ ^m2 ; and in other embodiments, from about 1 mg/ ^m2 to about 150 mg/ ^m2 . Within the conductive network, the transparent conductive layer can include from about 0.5 wt% to about 70 wt% metal; in other embodiments, from about 0.75 wt% to about 60 wt%; and in yet other embodiments, from about 1 wt% to about 50 wt% metal. One of ordinary skill in the art will recognize that additional ranges of thickness and metal loading within the explicit ranges above are contemplated and within the scope of the present invention. If the sparse metal conductive layer is patterned, the thickness and loading discussion applies only to areas where the metal is not excluded or significantly reduced by the patterning process. In addition to polymer binders and other processing aids, and the like, the sparse metal conductive layer can also include property-enhancing nanoparticles. The ranges of concentration of property-enhancing nanoparticles described above for loading in transparent polymer layers are generally also applicable to sparse metallic conductive layers.

In general, within the overall thickness ranges described above for the specific components of film 100, layers 102, 104, 106, 108, 110, 112 can be further divided into sublayers, for example, having compositions different from those of other sublayers. For example, the overcoat layer can include sublayers having different property-enhancing components. In some embodiments, the top overcoat sublayer can include high-dielectric nanoparticles, which can inhibit electrical conduction through the layer. Electrical connection can then be established via a window, metal tab, or the like (which may include, for example, nanodiamonds and/or a stabilizing composition) that penetrates the top sublayer of overcoat 108, but not necessarily the outer coating sublayers. In addition, multiple layers of optically clear adhesive are discussed above. Thus, more complex layer stacks can be formed. A sublayer can be processed similarly to or differently from other sublayers within a particular layer, for example, one sublayer can be laminated while another sublayer can be coated and cured.

Stabilizing compositions can be placed in appropriate layers to stabilize the sparse metal conductive layer. For embodiments in which the sparse metal conductive layer comprises a molten nanostructured metal network, the sparse metal conductive layer formed may not include stabilizing compounds itself, as the presence of these compounds may inhibit the chemical melting process. In alternative embodiments, it may be acceptable to include a stabilizer in the coating solution used to form the sparse metal conductive layer. Similarly, stabilizing compounds may be included in optically clear adhesive compositions. However, it has been found that stabilizing compounds can be effectively included in coatings, which can correspondingly make the coatings relatively thin while still providing effective stabilization. Specific descriptions of coatings with stabilizing compositions are described in the previous sections. Since the layer with the stabilizing composition can be thin, desired stabilization can be achieved by a low total amount of stabilizers, which can be desirable from a processing perspective and has a low impact on optical properties.

For some applications, it is necessary to pattern the conductive portions of the film to introduce desired functionality, such as different areas of a touch sensor. Patterning can be performed by varying the metal loading on the substrate surface (by printing metal nanowires at selected locations with virtually no metal elsewhere, or by etching or otherwise stripping metal from selected locations before and/or after melting the nanowires). However, it has been discovered that a high contrast in conductivity can be achieved between melted and unmelted portions of the layer with substantially equivalent metal loading, allowing patterning to be performed by selectively melting the metal nanowires. This ability to pattern based on melting provides an important additional patterning option based on selective melting of nanowires (e.g., via selective delivery of a molten solution or vapor). Patterning based on selective melting of metal nanowires is described in the '833 application and the '380 application, supra.

As an illustrative example, a molten metal nanostructure network can form a conductive pattern along a substrate surface 120, wherein a plurality of conductive paths 122, 124, and 126 are surrounded by resistive regions 128, 130, 132, and 134, as shown in FIG2 . As shown in FIG2 , the molten zone corresponds to three different conductive regions corresponding to conductive paths 122, 124, and 126. Although three independently connected conductive regions have been described in FIG2 , it should be understood that a pattern having two, four, or more than four electrically conductive independent conductive paths or regions can be formed as desired. For many commercial applications, a relatively intricate pattern having a large number of elements can be formed. Specifically, by using available patterning techniques suitable for patterning the films described herein, extremely fine patterns with high-resolution features can be formed. Similarly, the shape of a specific conductive region can be selected as desired.

Transparent conductive films are typically built around sparse metallic conductive elements that are deposited to form the functional features of the film. Various layers are coated, laminated, or otherwise added to the structure using appropriate film processing methods. As described herein, the properties of the layers can significantly alter the long-term performance of the transparent conductive film. Deposition of sparse metallic conductive layers is further described below in the context of fused metal nanostructure layers, but unfused metal nanowire coatings can be similarly deposited (albeit without the fused component).

The sparse metallic conductive layer is typically solution coated onto a substrate, which may or may not have a coating on top that then forms an undercoat adjacent to the sparse metallic conductive layer. In some embodiments, an overcoat may be solution coated onto the sparse metallic conductive layer. Crosslinking by application of UV light, heat, or other radiation may be performed to crosslink the polymer binder in the coating and/or sparse metallic conductive layer, which may be performed in one step or in multiple steps.

Sparse metal conductive layer

Sparse metallic conductive layers are typically formed from metal nanowires. With adequate loading and selected nanowire properties, reasonable conductivity can be achieved with nanowires having correspondingly appropriate optical properties. The stable film structures described herein are expected to yield desirable properties for films with a variety of sparse metallic conductive structures. However, particularly desirable properties have been achieved with fused metal nanostructured networks.

As outlined above, several practical methods have been developed to achieve metal nanowire melting. Metal loading can be balanced to achieve desired levels of conductivity and good optical properties. Generally, metal nanowire processing can be achieved by depositing two inks, wherein the first ink comprises the metal nanowires and the second ink comprises the melting composition, or by depositing an ink that combines melting elements into a metal nanowire dispersion. The inks may or may not further include additional processing aids, binders, or the like. Appropriate patterning methods can be selected to suit a particular ink system.

In general, one or more solutions or inks used to form a metal nanostructure network may collectively include well-dispersed metal nanowires, a flux, and optional additional components such as 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 used in the metal nanowire ink and/or the molten solution (if different from the nanowire ink) may include an aqueous solvent, an organic solvent, or a mixture thereof. Specifically, suitable solvents include, for example, water, alcohols, ketones, esters, ethers (e.g., glycol ethers), aromatic compounds, alkanes, and the like, and mixtures thereof. Specific solvents include, for example, water, ethanol, isopropanol, isobutanol, tert-butanol, methyl ethyl ketone, glycol ethers, methyl isobutyl ketone, toluene, hexane, ethyl acetate, butyl acetate, ethyl lactate, PGMEA (2-methoxy-1-methylethyl acetate), or mixtures thereof. While the solvent should be selected based on its ability to form a good dispersion of metal nanowires, the solvent should also be compatible with the other selected additives so that the additives are soluble in the solvent. For embodiments where the flux and metal nanowires are contained in a single solution, the solvent or a component thereof may or may not be a significant component of the molten solution (e.g., an alcohol) and can be selected accordingly if desired.

Metal nanowire inks in single-ink or dual-ink configurations may 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. One of ordinary skill in the art will recognize that additional ranges of metal nanowire concentrations within the explicit ranges above are contemplated and fall within the scope of the present invention. The concentration of metal nanowires affects the metal loading on the substrate surface and the physical properties of the ink.

In general, 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 may be desirable due to their high electrical conductivity. Commercial metal nanowires are available from Sigma-Aldrich (Missouri, USA), Cangzhou Nanochannel Materials Co., Ltd. (China), Blue Nano (North Carolina, USA), EMFUTUR (Spain), Seashell Technologies (California, USA), Aiden (South Korea), nanoComposix (USA), Nanopyxis (South Korea), K&B (South 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 be synthesized using various known synthetic routes or variations thereof. In order to have good transparency and low turbidity, the nanowires need to have a range of small diameters. Specifically, the metal nanowires are desirably of 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 longer lengths are expected to provide better conductivity within the network. Generally, the metal nanowires may have an average length of at least 1 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 enable even longer nanowires. The aspect ratio may be specified as the ratio of the average length divided by the average diameter, and in some embodiments, the nanowires may 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. One of ordinary skill in the art will recognize that additional ranges of nanowire sizes within the explicit ranges above are contemplated and fall within the scope of the present invention.

The polymer binder and solvent are generally selected in concert such that the polymer binder is soluble or dispersible in the solvent. In suitable embodiments, the metal nanowire ink generally comprises from about 0.02 wt% to about 5 wt% binder; in further embodiments, from about 0.05 wt% to about 4 wt% binder; and in additional embodiments, from about 0.1 wt% to about 2.5 wt% polymer binder. In some embodiments, the polymer binder comprises a crosslinkable organic polymer, such as a radiation-crosslinkable organic polymer and/or a thermally curable organic binder. To promote crosslinking of the binder, in some embodiments, the metal nanowire ink may comprise from about 0.0005 wt% to about 1 wt% 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 may optionally include a rheology modifier or a combination thereof. In some embodiments, the ink may include a humectant or surfactant to reduce surface tension, and the humectant may be suitable for improving coating properties. Wetting agents are typically soluble in solvents. In some embodiments, the nanowire ink may include from about 0.01% to about 1% by weight of wetting agent; in other embodiments, from about 0.02% to about 0.75% by weight; and in other embodiments, from about 0.03% to about 0.6% by weight of wetting agent. A thickener may optionally be used as a rheology modifier to stabilize the dispersion and reduce or eliminate sedimentation. In some embodiments, the nanowire ink may optionally include from about 0.05% to about 5% by weight of thickener; in other embodiments, from about 0.075% to about 4% by weight; and in other embodiments, from about 0.1% to about 3% by weight of thickener. One of ordinary skill in the art will recognize that additional ranges of binder, wetting agent, and thickener concentrations within the explicit ranges above are contemplated and within the scope of the present invention.

A range of polymer binders may be suitable for dissolving/dispersing in the solvent of the metal nanowires, and suitable binders include polymers developed for coating applications. Hardcoat polymers (e.g., radiation curable coatings) are commercially available, for example as hardcoat materials that can be selected for dissolution in aqueous or non-aqueous solvents for a range of applications. Suitable classes of radiation curable polymers and/or thermally curable polymers include, for example, silanes, polysilsesquioxanes, polyurethanes, acrylic resins, acrylic copolymers, cellulose ethers and cellulose esters, nitrocellulose, other water-insoluble structural polysaccharides, polyethers, polyesters, polystyrenes, polyimides, fluoropolymers, 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, GLUTAMATE® brand acrylic resins (DMS Resins), GLUTAMATE® brand acrylic copolymers (BASF Resins), GLUTAMATE® brand acrylic resins (Lucite International), GLUTAMATE® brand polyurethanes (Lubrizol Advanced Materials), cellulose acetate butyrate polymers (CAB brand from Eastman ^™ Chemical), BAYHYDROL ^™ brand polyurethane dispersions (Bayer MaterialScience), GLUTAMATE® brand polyurethane dispersions (Cytec Industries, Inc.), GLUTAMATE® brand polyvinyl butyral (Kuraray USA, Inc.), cellulose ethers (e.g., ethyl cellulose or hydroxypropyl methylcellulose), other polysaccharide-based polymers (e.g., chitosan and pectin), synthetic polymers like polyvinyl acetate, and the like. The polymer binder may self-crosslink after exposure to radiation and/or may be crosslinked with a photoinitiator or other crosslinking agent. In some embodiments, the photocrosslinker may form free radicals after exposure to radiation, and the free radicals may then induce a crosslinking reaction based on a free radical polymerization mechanism. Suitable photoinitiators include, for example, commercially available products such as the GLUTAMATE brand (BASF), GENOCURE ^™ brand (Ryanon USA Inc.), and GLUTAMATE brand (Double Bond Chemical Co., Ltd.), combinations thereof, or the like.

Wetting agents can be used to improve the coatability of metal nanowire inks and the quality of metal nanowire dispersions. Specifically, wetting agents can reduce the surface energy of the ink so that the ink spreads well on the surface after application. Wetting agents can be surfactants and/or dispersants. Surfactants are a class of materials that act to reduce surface energy and improve the solubility of materials. Surfactants typically have a hydrophilic portion and a hydrophobic portion that contribute to their properties. A wide range of surfactants are commercially available, such as nonionic surfactants, cationic surfactants, anionic surfactants, and zwitterionic surfactants. In some embodiments, if the properties associated with the surfactant are not an issue, non-surfactant wetting agents (e.g., dispersants) are also known in the art and can effectively improve the wetting ability of the ink. Suitable commercial wetting agents include, for example, COATOSIL ^™ brand epoxy-functional silane oligomers (Momentum Performance Materials), SILWET ^™ brand organosilicone surfactants (Momentum Performance Materials), THETAWET ^™ brand short-chain nonionic fluorosurfactants (ICT Industries, Ltd.), FLEX brand polymeric dispersants (Air Products, Inc.), FLEX brand polymeric dispersants (Lubrizol), XOANONS WE-D545 surfactant (Anhui Jiazhi Xinnuo Chemical Co., Ltd.), EFKA ^™ PU 4009 polymeric dispersant (BASF), MASURF FP-815 CP, MASURF FS-910 (Mason Chemical), NOVEC ^™ FC-4430 and FC-4432 fluorinated surfactants (3M), mixtures thereof, and the like.

Thickeners can be used to improve the stability of the dispersion by reducing or eliminating the settling of solids from the metal nanowire ink. 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 ^™ brand modified urea (e.g., LA-100) (Crayvalley Acrylics, USA), polyacrylamide, THIXOL ^™ 53L brand acrylic thickener, COAPUR ^™ 2025, COAPUR ^™ 830W, COAPUR ^™ 6050, COAPUR ^™ XS71 (Gotis Co., Ltd.), brand modified urea (BYK Additives), Acrysol DR 73, Acrysol RM-995, Acrysol RM-8W (Dow Coatings), 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.

As mentioned above, the ink used to deposit the sparse metallic conductive layer may further include property-enhancing nanoparticles. Suitable property-enhancing nanoparticles include nanodiamonds as well as the other property-enhancing nanoparticle materials presented above (which are specifically incorporated into this discussion). In addition, the ranges of nanoparticle sizes are outlined above in the context of the coating and are similarly incorporated herein. The solution forming the sparse metallic conductive layer may include 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 wt% to about 5 wt% property-enhancing nanoparticles. One 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 scope of the present invention.

Additional additives may be added to the metal nanowire ink, typically each in an amount of no more than about 5% by weight; in further embodiments, no more than about 2% by weight; and in further embodiments, no more than about 1% by weight. Other additives may include, for example, antioxidants, UV stabilizers, defoamers or anti-foaming agents, anti-settling agents, viscosity modifiers, or the like.

As indicated above, melting of the metal nanowires can be achieved via various agents. Without wishing to be bound by theory, the melting agent is believed to mobilize the metal ions, and the free energy appears to be reduced during the melting process. In some embodiments, excessive metal migration or growth can lead to degradation of optical properties, so the desired result can be achieved by shifting the equilibrium in a reasonably controlled manner (typically for a short period of time) to produce sufficient melting to obtain the desired conductivity while maintaining the desired optical properties. In some embodiments, the initiation of the melting process can be controlled by partially drying the solution to increase the concentration of the components, and quenching the melting process can be achieved, for example, by rinsing or more complete drying of the metal layer. The melting agent can be incorporated into a single ink along with the metal nanowires. A single ink solution can provide adequate control over the melting process.

In some embodiments, a process is used to initially deposit a sparse nanowire film, with or without subsequent processing to deposit another ink, to provide for fusing the metal nanowires into a conductive metal nanostructure network. The fusing process can be performed by controlled exposure to molten vapor and/or by deposition of a flux from solution. The sparse metallic conductive layer is typically formed on a selected substrate surface. The deposited nanowire film is typically dried to remove the solvent. As further described below, the process can be adapted for patterning the film.

For deposition of the metal nanowire ink, any suitable deposition method can be used, such as dip coating, spray coating, knife-edge coating, rod coating, meyer rod coating, slot die coating, gravure printing, spin coating, or the like. The ink can have properties, such as viscosity, appropriately adjusted by additives for the desired deposition method. Similarly, the deposition method controls 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 from the dispersion, the sparse metal conductive layer can be dried to remove the liquid.

The film can be dried, for example, by a heat gun, oven, heat lamp, or the like, although air-dried films may be desirable in some embodiments. In some embodiments, the film can be heated to a temperature of from about 50° C. to about 150° C. during drying. After drying, the film can be washed one or more times with alcohol or other solvents or solvent blends (e.g., ethanol or isopropanol) to remove excess solids and reduce turbidity. Patterning can be achieved in several convenient ways. For example, printing of metal nanowires can directly result in patterning. Additionally or alternatively, lithographic techniques can be used to remove portions of the metal nanowires before or after melting to form the pattern.

A clear protective film covering the sparse metal conductive layer can be formed with holes or the like in appropriate locations to provide electrical connections to the conductive layer. Generally speaking, various polymer film processing techniques and equipment can be used for processing these polymer sheets, and such equipment and techniques are well-established in the art, and future developed processing techniques and equipment can be adapted accordingly for the materials herein.

Electrical and optical properties of transparent films

The fused metal nanostructured network can provide low electrical resistance while also providing good optical properties. Therefore, the film is suitable for use as a transparent conductive electrode or the like. Transparent conductive electrodes are suitable for a range of applications, such as electrodes along the light-receiving surface of a solar cell. For displays, and particularly touch screens, the film can be patterned to provide a conductive pattern formed by the film. Substrates with patterned films typically have good optical properties in the corresponding portions of the pattern.

The resistance of a thin film can be expressed as a sheet resistance, which is reported in ohms per square (Ω/□ or ohm/sq) to distinguish it from bulk resistance values that are based on parameters related to the measurement process. The sheet resistance of a film is typically measured using a four-point probe measurement or another suitable process. In some embodiments, the fused metal nanowire network may have a sheet resistance of no more than about 300 ohm/sq; in other embodiments, no more than about 200 ohm/sq; in additional embodiments, no more than about 100 ohm/sq; and in other embodiments, no more than about 60 ohm/sq. One of ordinary skill in the art will recognize that additional ranges of sheet resistance within the explicit ranges above are contemplated and fall within the scope of the present invention. Depending on the specific application, commercial specifications for sheet resistance for use in devices may not necessarily target lower sheet resistance values (e.g., when additional costs may be involved), and currently commercially relevant values may be, for example, 270 ohm/sq, versus 150 ohm/sq, versus 100 ohm/sq, versus 50 ohm/sq, versus 40 ohm/sq, versus 30 ohm/sq, or less than 30 ohm/sq as target values for touch screens of varying qualities and/or sizes, with each of these values defining a range between specific values as endpoints of the range, such as 270 ohm/sq to 150 ohm/sq, 270 ohm/sq to 100 ohm/sq, 150 ohm/sq to 100 ohm/sq, and the like, with 15 specific ranges defined. Thus, lower-cost films may be suitable for certain applications at the expense of slightly higher sheet resistance values. Generally speaking, sheet resistance can be reduced by increasing the loading of the nanowires, but increased loading may not be desirable from other perspectives, and metal loading is only one factor among many in achieving low sheet resistance values.

For applications as transparent conductive films, it is necessary for the fused metal nanowire network to maintain good optical transparency. In principle, optical transparency is inversely related to loading, where higher loading leads to reduced transparency, but processing of the network can also significantly affect transparency. In addition, polymer binders and other additives can be selected to maintain good optical transparency. Optical transparency can be assessed with respect to light transmitted through the substrate. For example, the transparency of the conductive films described herein can be measured by using a UV-visible spectrophotometer and measuring the total transmission through the conductive film and the supporting substrate. Transmittance is the ratio of the transmitted light intensity (I) to the incident light intensity ( _Io ). The transmittance through the film ( _Tfilm ) can be estimated by dividing the measured total transmittance (T) by the transmittance through the supporting substrate ( _Tsub ). (T=I/ _Io and T/ _Tsub =(I/ _Io )/( _Isub / _Io )=I/ _Isub = _Tfilm ). Therefore, the reported total transmission can be corrected to remove the transmission through the substrate, thereby obtaining only the transmission of the film. While good optical transparency across the visible spectrum is generally desirable, for convenience, optical transmission at a wavelength of 550 nm may be reported. Alternatively or additionally, transmission may be reported as total transmittance for wavelengths from 400 nm to 700 nm, and such results are reported in the following examples. Generally, for fused metal nanowire films, there is no qualitative difference between the measurements of 550 nm transmittance and total transmittance from 400 nm to 700 nm (or simply "total transmittance" for convenience). In some embodiments, films formed from the fused network have 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%. The transparency of films on transparent polymer substrates can be assessed using ASTM D1003 ("Standard Test Method for Turbidity and Luminous Transmittance of Transparent Plastics"), which is incorporated herein by reference. One of ordinary skill in the art will recognize that additional ranges of transmittance within the explicit ranges above are contemplated and within the scope of the present invention. When the measured optical properties of the films were adjusted in the following examples of substrates, the films had excellent transmittance and haze values, which were achieved along with the observed low sheet resistance.

The molten metal network can also have low turbidity and high transmittance of visible light, while having a desirably low sheet resistance. Turbidity can be measured using a turbidimeter based on ASTM D1003, as referenced above, and the substrate's turbidity contribution can be removed to provide the turbidity value of the transparent conductive film. In some embodiments, the sintered network film can have a turbidity value of no more than approximately 1.2%; in further embodiments, no more than approximately 1.1%; in additional embodiments, no more than approximately 1.0%; and in other embodiments, from about 0.9% to about 0.2%. As described in the Examples, extremely low turbidity and sheet resistance values have been simultaneously achieved with appropriately selected silver nanowires. The loading can be adjusted to balance sheet resistance and turbidity values, where extremely low turbidity values can still be accompanied by good sheet resistance values. Specifically, turbidity values of no more than 0.8%, and in further embodiments, from about 0.4% to about 0.7%, can be achieved with a sheet resistance value of at least approximately 45 ohm/sq. Furthermore, 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 about 30 ohm/sq to about 45 ohm/sq. All of these films maintain good optical clarity. One of ordinary skill in the art will recognize that additional ranges of haze within the explicit ranges above are contemplated and fall within the scope of the present invention.

With respect to the corresponding properties of multilayer films, the additional components are generally selected to have a small effect on the optical properties, and a variety of 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 materials may be electrically insulating, and if thicker insulating layers are used, the film may be patterned to provide multiple locations where gaps or voids through the insulating layer can provide access and electrical contact to the otherwise embedded conductive elements.

touch sensor

The transparent conductive films described herein can be effectively incorporated into touch sensors suitable for touch screens used in 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 touch sensors is the presence of two transparent conductive electrode structures that are in a spaced-apart configuration in their natural state (i.e., when not touched or otherwise contacted by an external source). For sensors that operate based on capacitance, a dielectric layer is typically between the two electrode structures. Referring to FIG3 , a representative capacitance-based touch sensor 202 includes a display component 204, an optional bottom substrate 206, a first transparent conductive electrode structure 208, a dielectric layer 210 (e.g., a polymer or glass sheet), a second transparent conductive electrode structure 212, an optional top cover 214, and measurement circuitry 216 that measures the change in capacitance associated with a touch on the sensor. 4 , a representative resistive-based touch sensor 240 includes a display component 242 , an optional lower substrate 244 , a first transparent conductive electrode structure 246 , a second transparent conductive electrode structure 248 , spaced-apart support structures 250 , 252 supporting the electrode structures in a natural configuration, an upper cover layer 254 , and resistance measurement circuitry 256 .

Display components 204, 242 may be, for example, LED-based displays, LCD displays, or other desired display components. Substrates 206, 244 and cover layers 214, 254 may be independent transparent polymer sheets or other transparent sheets. Support structures may be formed from dielectric materials, and the sensor structure may include additional supports to provide desired stability. Measurement circuits 216, 256 are known in the art.

Transparent conductive electrodes 208, 212, 246, and 248 can be effectively formed using a molten metal network that can be appropriately patterned to form different sensors. However, in some embodiments, the molten metal network forms some transparent electrode structures, while other transparent electrode structures in the device may include materials such as conductive metal oxides (e.g., 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 particles, carbon nanotubes, graphene, conductive organic compositions, or the like. As described herein, the molten metal network can be effectively patterned, and it may be desirable to form a sensor from a patterned film in one or more electrode structures, such that multiple electrodes in the transparent conductive structure can be used to provide position information related to a touch process. The use of patterned transparent conductive electrodes to form patterned touch sensors is described, for example, in U.S. Patent 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

The following examples involve coating a loaded polymer precursor solution onto a suitable substrate. Examples are presented using nanodiamond fillers, aluminum oxide nanoparticle fillers, or zirconium oxide nanoparticle fillers. Some examples involve forming a passive coated polymer film. Other examples involve coatings associated with a molten metal conductive network that results in a structure in a transparent conductive film. For embodiments of the transparent conductive film, examples are presented using property-enhancing nanoparticles in a layer having a molten metal conductive network or in a coating disposed above a layer having a molten metal conductive network. The molten metal conductive network is formed using silver nanowires.

Commercial silver nanowires with an average diameter between 25 nm and 50 nm and an average length of 10 to 30 microns were used in the following examples. The silver nanowire ink was substantially as described in Example 5 of co-pending 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,” which is incorporated herein by reference. The metal nanowire ink included silver nanowires in an amount between 0.01 wt% and 0.5 wt%; silver ions between 0.01 mg/ml and 2.0 mg/ml; and a cellulose-based binder at a concentration of approximately 0.02 wt% to 1.0 wt%. The silver nanowire ink was an aqueous solution containing a small amount of alcohol. The ink was slot-coated onto a PET polyester film. After coating the nanowire ink, the film was then heated in an oven at 100°C for 10 minutes to dry the film. The procedure for forming the outer coating layer is described in the following specific examples.

The total transmittance (TT) and haze of the film samples were measured using a turbidimeter. To adjust the turbidimetric measurements for the following samples, the substrate haze value was subtracted from the measurements, resulting in an approximate haze measurement for the transparent conductive film. The instrument was designed to evaluate optical properties based on the ASTM D 1003 standard ("Standard Test Method for Turbidity and Luminous Transmittance of Transparent Plastics"), which is incorporated herein by reference. The total transmittance and haze of these films included a PET substrate, which had a base total transmittance and haze of approximately 92.9% and 0.1% to 0.4%, respectively. In the following examples, several different formulations of molten metal nanowire inks are presented, along with optical and sheet resistance measurements.

Sheet resistance was measured using a 4-point probe method, a non-contact resistance meter, or by measuring the resistance of the film using a square defined by two solid (opaque) silver wires formed from silver paste. In some embodiments, sheet resistance measurements were performed using a pair of parallel silver paste strips formed by applying the paste to the surface of the sample to define a square or rectangular shape, followed by annealing the sample at approximately 120°C for 20 minutes to cure and dry the silver paste. Alligator clips were connected to the silver paste strips, and wires were connected to a commercial resistance measurement device. Electrical connections were made to the exposed end portions of the film. Sheet resistance for some samples was measured by a third-party supplier.

The pencil hardness of the AgNWs film samples was measured using a pencil test kit. Following the pencil sharpening method, sandpaper was used to modify the pencil tip and a constant downward force was applied while holding the pencil at a 45° angle and moving the pencil across the surface of the film sample. This test used a 500g or 750g commercial pencil hardness kit. The hardness was determined by analyzing the effect of pencils of different graphite grade scales on the base conductive layer. If no damage was caused to the base layer, the film was considered to have passed the test for that particular graphite grade. The film was examined under a Leica microscope at 20× magnification. The film was placed on a very flat surface, which is very important to avoid being scratched by the pencil because the film is very thin. This test is different from the corresponding standardized test, which relies on visual inspection without magnification.

The final steel wool hardness of the film samples was measured using ultrafine 0000 steel wool subjected to a specific weight. For some samples, steel wool subjected to a constant downward force provided by a 20g, 50g, or 100g weight was passed once over the coated film, and the film was inspected under light to detect microscratches. The number of scratches determined the scratch resistance of the film. The absence of scratches caused by the steel wool would mean a "pass" for the specific weight to which the steel wool was subjected. In the case of a failure, the number of scratches caused was indicated in the results section. For some samples, turbidity and/or sheet resistance were also evaluated after the steel wool test.

When analyzing haze and/or sheet resistance, ultrafine steel wool was used to rub the surface after applying and crosslinking the topcoat. The steel wool rubbing was performed very gently while maintaining a constant downward force. A section of the test film was rubbed back and forth with the steel wool 10 times. Microscratches tend to have a much smaller effect on haze increase than deeper scratches. A BYK Haze-Gard Plus was used for overall transparency and haze measurements. Changes in sheet resistance of the in-house OC formulation were also measured by a third-party service, as described in Example 4. Haze was measured before and after the test.

Example 1 - Effect of Nanodiamonds on Commercial Overcoatings on Transparent Substrates

This example tests the effect on hardness of a commercial nanodiamond loaded overcoat on a PET substrate with an initial polymer binder overcoat.

The substrate was prepared by coating a base ink with a cellulose-based polymer binder onto a transparent PET substrate without coating any silver nanowires and drying. The coated substrate had a haze of 0.72%. A commercial coating polymer from Dexerials Corporation was dissolved in N,N-dimethylformamide (DMF). Six samples were prepared, with two samples each having a polymer concentration of 2 wt%, 3 wt%, and 4 wt%. To one sample at each polymer concentration, hydrogen-terminated nanodiamonds were added at a concentration of 0.2 wt%, 0.3 wt%, or 0.4 wt%, respectively, such that in each diamond-containing sample, the diamond concentration was approximately one-tenth of the polymer concentration. The coating solution was deposited onto the substrate by slot coating at a wet thickness of 1 mil (25.4 microns). The film was then dried by infrared lamp and cured with UV light in nitrogen at 1 J/ ^cm² using a Holter-type radiation depth system (H-type bulb). The solids content of the coating solution correlates with the thickness of the dried film, and films formed with a coating solution containing 0.3 wt% polymer have an average thickness of approximately 75 nm. Hardness and optical properties were compared between films formed with and without nanoparticle fillers. The results are shown in Table 1. Generally, for thicker dried coatings, the inclusion of nanodiamonds significantly improved hardness while not increasing turbidity much more.

Table 1

sample Polymer wt% in solution Nanodiamond wt% in solution TT% Turbidity% Pencil 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 Nanodiamonds in Conductive Ink

This example tests the hardness of a film having a fused metal nanostructure layer with nanodiamonds incorporated into a conductive layer over which a hard coating is applied.

A silver nanowire ink was prepared as described above, except that 0.036 wt% of nanodiamonds with hydrogen-terminated surfaces were added to the ink. The nanodiamonds were initially dispersed in a γ-butyrolactone solvent before being mixed into the silver nanowire ink. The nanowire ink was slot-coated onto a PET film substrate and dried to fuse the nanowires into the molten metal nanostructure network that formed the conductive layer. An overcoat composition was prepared as described in Example 1, except that the polymer concentration was 0.5 wt% and no nanodiamonds were present. The overcoat was processed similarly to that described in Example 1 by slot-coating onto the dried molten metal conductive layer, drying the coating, and UV curing the coating.

The hardness and optical properties of films formed with and without nanoparticle fillers in the conductive layer were compared, as shown in Table 2. The optical properties were also determined with and without an overcoat. The inclusion of nanodiamonds in the nanowire ink significantly improved the hardness of the films with the overcoat. The addition of nanodiamonds slightly increased sheet resistance, slightly decreased overall transparency, and slightly increased haze. It should be noted, however, that the overcoat generally reduced haze relative to the corresponding sample without an overcoat.

Table 2

sample Turbidity% TT% Sheet resistance (ohm/sq) Pencil hardness AgNW ink 1.11 92.2 58 AgNW ink + outer coating 0.91 91.9 2H AgNW ink with nanodiamonds 1.33 91.2 87 AgNW ink with nanodiamonds + outer coating 1.19 91.4 ~8H

Example 3 - Effect of Nanodiamonds in a Commercial Overcoat Over a Transparent Conductive Layer

This example tests the hardness of a transparent conductive film incorporating a commercial overcoat incorporating nanodiamonds.

Silver nanowires were deposited and processed as described above. After drying, the layer comprised a molten metal nanostructured network within a sparse metallic conductive layer. The sheet resistance of the conductive layer was between 50 and 60 ohm/sq, and the thin overcoat did not significantly change the sheet resistance of the film after application and curing of the overcoat. Two different metal nanowire ink systems were tested in combination with three different commercial overcoats, three different corresponding solvent systems, and three different initial nanodiamond dispersions. The substrates with the molten metal nanostructured network had an initial haze of 1.12% (first ink system) and 1.28% (second ink system) before application of the overcoat. Hardness and optical properties were compared between films formed with and without nanoparticle fillers.

A first set of samples was prepared using a first silver nanowire ink system and an overcoat formed with a coating material from Hybrid Plastics Ltd. The coating solution for the overcoat was formed in a formic acid solution. Four solutions were formed using two solutions with a polymer concentration of 0.5 wt% and two solutions with a polymer concentration of 0.75 wt%. One of the two solutions at each polymer concentration had commercial nanodiamonds added to the aqueous solvent. The solutions with nanodiamond filler had 0.05 wt% nanodiamonds (for the 0.5 wt% polymer solution) and 0.075 wt% nanodiamonds (for the 0.75 wt% polymer solution). The overcoat was applied, dried, and cured. Optical and hardness measurements were obtained on the cured films, and the results are presented in Table 3. The turbidity values in Table 3 are average values across the films, while the initial turbidity values for the steel wool evaluation are specific values measured at the location where the steel wool was applied. As shown in Table 3, the inclusion of nanodiamonds in these films significantly improved hardness, and corresponding experiments also showed significantly improved resistance to scratching by steel wool. Representative scanning electron micrographs at two magnifications of a 10 wt% nanodiamond film are shown in Figures 5 and 6. For comparison, Figures 7 and 8 show SEM images of 5 wt% and 3 wt% nanodiamond films, respectively.

Table 3

Two additional samples were prepared using formic acid. These solutions were prepared using California Hard Coat Company (CHC) polymer in the coating solution. The coating solution had 0.5 wt% polymer. One solution included 0.05 wt% commercial nanodiamonds in an aqueous solution and the second solution did not contain any nanodiamonds. The solutions were coated over the molten metal nanostructure network formed using the second silver nanowire ink system. Optical and hardness results were obtained after drying and curing and are presented in Table 4. The inclusion of nanodiamonds significantly increased the hardness of the coating and reduced the turbidity increase produced by the steel wool test. The initial turbidity was only slightly increased due to the nanodiamonds and the total transmittance was only slightly reduced.

Table 4

Another set of nine samples was prepared using N,N-dimethylformamide in the coating solution. The solutions covered three different polymer concentrations of a coating polymer from Dexerials Corporation, with some samples containing corresponding concentrations of nanodiamonds initially dispersed in ethylene glycol in the coating solution, while other solutions contained no nanodiamonds. The coating was applied over the molten metal nanostructure network formed with the first nanowire ink solution. Optical and hardness measurements were obtained after drying and curing the overcoat, and the results are summarized in Table 5.

Table 5

Another 10 samples were prepared in a non-aqueous solvent for forming an outer coating. Once again, a polymer from Dexerials Corporation was used in a propylene glycol monomethyl ether (PGME) solvent with 4.5 volume percent of N,N-dimethylacetamide (DMA). All solutions contained 0.5 wt% of polymer. Three different commercial nanodiamonds were used, and for each nanodiamond, three different nanodiamond concentrations were used. The nanodiamonds were commercial nanodiamonds obtained in the form of a dispersion in ethylene glycol (ND-A), an ethylene glycol dispersion with particles having a hydrogen glycol-terminated surface (ND-H-EG), or a γ-butyrolactone dispersion 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, microscratch analysis was also performed after rubbing with steel wool. The results are shown in Table 6. The nanoparticles significantly improved the scratch resistance of the film, and the increase in turbidity and the decrease in total transmittance were not much.

Table 6

Example 4 - Effect of Nanodiamonds in Formulated Coating Solutions

In this example, the effectiveness of nanodiamonds in improving hardness was examined in samples of transparent conductive films with an internally formulated overcoat.

For these experiments, substrates were prepared with a molten metal conductive layer formed using the second metal nanowire ink described in Example 3. Two different internal coating solutions (HOC1 and HOC2) were tested. The internally formulated coating material comprised a blend of a commercial UV-crosslinkable acrylate hardcoat composition and a cyclic siloxane epoxy resin. HOC1 further included a urethane acrylate oligomer, and HOC2 further included an epoxy acrylate oligomer. Epoxy acrylate hybrid hard coatings are further described in, for example, 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., entitled “UV-Cured Interpenetrating Acrylic-Epoxy Polymer Networks: Preparation and Characterization” (Macromolecular Materials and Engineering, Vol. 293, pp. 515-520 (2008)), all of which are incorporated herein by reference.

Twelve samples were prepared using two different solvent systems. Specifically, eight samples were prepared using a 1:1 volume mixture of N,N-dimethylformamide (DMF) and methyl ethyl ketone (MEK), and three samples were prepared using acetonitrile. Samples 1 to 4 were prepared using HOC1, and samples 5 to 12 were prepared using HOC2. The samples were prepared using two different polymer concentrations and three different nanodiamond concentrations in the coating solution. Samples 1 to 8 had 0.5 wt % polymer, and samples 9 to 12 had 0.8 wt % polymer. For four samples, in addition to optical and hardness measurements, the change in sheet resistance after application of steel wool was also measured. The results are presented in Table 7 (Samples 1 to 8) and Table 8 (Samples 9 to 12). The results show that the solvent has a significant effect on the coating properties. Nanodiamonds significantly improve hardness. The inclusion of nanodiamonds slightly increases turbidity.

Table 7

Table 8

Six samples were prepared using an HOC2-based overcoat. Overall, two different solvent systems and two different types of nanodiamonds were tested. The samples were prepared as described above. The results are presented in Table 9. As with the results presented in Tables 7 and 8, the hardness results were significantly dependent on the solvent system.

Table 9

sample Nanodiamond wt%, type Solvent (v:v) Pencil hardness 1 0.03,ND-H-EG Acetonitrile + DMA (95:5) HB 2 0.05,ND-H-EG Acetonitrile + DMA (95:5) 3H 3 0.03,ND-H-G Acetonitrile + DMA (95:5) 3H 4 0.05,ND-H-G Acetonitrile + DMA (95:5) 2H 5 0.03,ND-H-G Acetonitrile + PGME + DMA (48:48:4) 4H 6 0.05,ND-H-G Acetonitrile + PGME + DMA (48:48:4) 6H

Example 5 - Metal Oxide Filler

This example tests the effect of metal oxide nanoparticles in an overcoat layer over a sparse metal conductive layer on a transparent conductive film.

The conductive layer was formed using the second silver nanowire ink described above in Example 3. Six well-mixed coating solution samples were prepared using one of two different overcoat polymers and one of three different metal oxide nanoparticles. The first overcoat polymer was obtained from California Hard Coat Company (CHC), and the second overcoat polymer was formulated in-house (HOC3) similar to the polymer described in Example 4. The metal oxide nanoparticles were aluminum oxide nanoparticles ( _Al2O3 ) from both BYK and US- _Nano , or zirconium oxide nanoparticles ( _ZrO2 ) from BYK. All overcoat solutions were applied, dried, and cured as described above. The average size of the nanoparticles was about 20 nm to about 40 nm. The coating solution had about 0.75 wt% polymer and about 0.09 wt% nanoparticles.

Sheet resistance (SR) and optical properties of films formed with and without metal oxide nanoparticles were obtained and are presented in Table 9. Generally, the inclusion of aluminum oxide nanoparticles or zirconium oxide nanoparticles did not significantly increase sheet resistance or decrease total transmittance. In the case of zirconium oxide nanoparticles, haze did not increase and could be slightly reduced. However, in the case of aluminum oxide nanoparticles, haze increased significantly.

Table 10

The foregoing embodiments are intended to be illustrative rather than restrictive. Additional embodiments are within the claims. Furthermore, although the invention has been described with reference to particular embodiments, those skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention. Any incorporation by reference of documents herein is limited such that no subject matter is incorporated that is inconsistent with the express disclosure herein.

Claims (17)

1. An optical structure comprising a transparent substrate and a coating comprising a polymer binder and nanodiamonds, wherein the coating has an average thickness from 50 nm to 25 μm, wherein the nanodiamonds have an average particle diameter of not more than 50 nm, wherein the turbidity of the optical structure is increased by not more than 0.5% relative to an equivalent optical structure without the nanodiamonds, and wherein the coating has a pencil hardness of at least 1H, and the pencil hardness is at least one order greater than that of an equivalent transparent coating without the nanodiamonds. 2. The optical structure according to claim 1, wherein the coating has from 0.01 wt% to 30 wt% nanodiamond. 3. The optical structure according to claim 1 or claim 2, wherein the transparent substrate comprises a polymer film having an average thickness from 5 micrometers to 2 millimeters. 4. The optical structure according to claim 1 or claim 2, wherein the coating has an average thickness from 50 nm to 10 micrometers. 5. The optical structure according to claim 1 or claim 2, wherein the polymer binder comprises polysiloxane, polysilsesquioxane, polyurethane, acrylic resin, acrylic copolymer, cellulose ether and cellulose ester, nitrocellulose, other water-insoluble structural polysaccharides, polyether, polyester, polystyrene, polyimide, fluoropolymer, styrene-acrylate copolymer, styrene-butadiene copolymer, acrylonitrile-butadiene-styrene copolymer, polysulfide, epoxy-containing polymer, copolymer of monomers comprising the above polymers, and mixtures thereof. 6. The optical structure according to claim 1 or claim 2, wherein the coating comprises a sparse metal conductive element. 7. The optical structure according to claim 1 or claim 2, further comprising a sparse metallic conductive layer adjacent to the coating. 8. The optical structure according to claim 1 or claim 2, wherein the coating has a pencil hardness of at least 2 levels harder than that of a transparent coating without filler. 9. An optical structure comprising a transparent substrate, a transparent coating comprising a polymer binder and from 0.01 wt% to 30 wt% nanodiamonds having an average particle diameter of not more than 50 nm; and the transparent coating having a pencil hardness at least one grade greater than that of a transparent coating without filler and a total visible light transmittance reduction of not more than 5% attributable to the transparent coating, wherein the coating has an average thickness from 50 nm to 25 micrometers. 10. The optical structure of claim 9, wherein the transparent coating has an average thickness from 500 nm to 15 micrometers. 11. A solution comprising: Solvent, Optically transparent curable polymer adhesives, and Dielectric nanoparticles comprising nanodiamonds, wherein the nanodiamonds have an average particle diameter of no more than 100 nm. The curable polymer adhesive comprises an acrylic resin, a copolymer thereof, or a mixture thereof, and The dielectric nanoparticles are 0.005 wt% to 5 wt% solids in the solution. 12. The solution according to claim 11, having from 0.005 wt% to 5 wt% nanoparticles. 13. The solution according to claim 11 or claim 12, wherein the nanodiamond has an average particle diameter of not more than 50 nm. 14. The solution according to claim 11 or claim 12, wherein the solvent comprises water, ethanol, isopropanol, isobutanol, tert-butanol, methyl ethyl ketone, methyl isobutyl ketone, cyclic ketone, glycol ether, toluene, hexane, ethyl acetate, butyl acetate, ethyl lactate, propylene carbonate, dimethyl carbonate, PGMEA (2-methoxy-1-methylethyl acetate), N,N-dimethylformamide, N,N-dimethylacetamide, acetonitrile, formic acid, or mixtures thereof. 15. The solution according to claim 11 or claim 12, wherein the curable polymeric adhesive comprises polysiloxane, polysilsesquioxane, polyurethane, acrylic resin, acrylic copolymer, cellulose ether and cellulose ester, nitrocellulose, other water-insoluble structural polysaccharides, polyether, polyester, styrene-acrylate copolymer, styrene-butadiene copolymer, acrylonitrile-butadiene-styrene copolymer, polysulfide, epoxy-containing polymer, copolymer of monomers comprising the above polymers, and mixtures thereof. 16. The solution according to claim 11 or claim 12, having a polymer binder concentration of from 0.025 wt% to 50 wt% and a nanoparticle concentration of from 0.005 wt% to 1 wt%. 17. The solution of claim 11, wherein the solution is non-aqueous and further comprises a crosslinking agent, a wetting agent, a viscosity modifier, a stabilizer, or a combination thereof.