Nanoparticles

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Serial No. 61/952,347 filed March 13, 2014. This disclosure of the prior application is considered part of (and is incorporated by reference in) the disclosure of this application.

TECHNICAL FIELD

This invention relates to preparing conductive coatings by applying metal nanoparticle-containing compositions to a substrate.

BACKGROUND

Electrically conductive coatings on substrates have been prepared using metal nanoparticle-containing coating compositions. Typically, the surface of the substrate is treated first with a primer, followed by application of a metal nanoparticle-containing coating composition. Examples of suitable coating compositions include the metal nanoparticle-containing emulsions described in U.S. 7,601,406. Nanoparticle-containing inks, in which the metal nanoparticles are dispersed in a liquid carrier, have also been used. The coated substrate is dried and sintered, e.g., by exposure to heat and/or a chemical reagent such as an acid, to at least partially join the nanoparticles together and form electrically conductive metal traces.

SUMMARY

A process for preparing an electrically conductive article is described. The process includes applying a pre-treat composition to a surface of a substrate to form a pre-treated surface. The pre-treat composition, in turn, includes (i) a polymerizable priming agent, (ii) a first sintering agent comprising a halide salt, and (iii) a second sintering agent comprising a protonic acid. The pre-treated surface is exposed to radiation, heat, or a combination thereof to polymerize the polymerizable priming agent and form a primed surface. Next, a coating composition is applied to the primed surface. The coating composition includes metal nanoparticles in a liquid carrier. Following coating, the liquid carrier is removed to form an article comprising electrically conductive metal areas on the primed surface. It is not necessary to expose the article to a chemical sintering agent after applying the metal nanoparticle-containing composition to the primed surface. The process thus eliminates the need for a separate sintering step following deposition of the nanoparticle-containing composition in order to form electrically conductive metal areas, thereby simplifying the process.

In some embodiments, the polymerizable priming agent includes a polymerizable (meth)acrylate. The halide salt may be a chloride salt. The protonic acid may be selected from carboxylic acids, oxyacids of sulfur, oxyacids of phosphorus, and combinations thereof. Polyacids (i.e. polymers with acid functional groups) may also be used. The metal nanoparticles may be silver nanoparticles. The substrate may be in the form of a sheet or roll. The electrically conductive metal areas may be in the form of an interconnected network of metal traces.

The coating composition may be in the form of an ink in which the metal nanoparticles are dispersed in the liquid carrier. Alternatively, the coating composition may be in the form of an emulsion in which the liquid carrier comprises (a) water, a water-miscible organic solvent, or combination thereof and (b) an organic solvent that evaporates more quickly than the water, water-miscible solvent, or combination thereof.

The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and from the claims.

DETAILED DESCRIPTION

A pre-treat composition is applied to a substrate. Examples of useful substrates include glass, paper, metal, ceramics, textiles, printed circuit boards, and polymeric films or sheets. The substrate can be flexible or rigid, and can be in the form of rolls or sheets. Suitable polymeric films can include polyesters, polyamides, polyimides (e.g., Kapton ^® by Dupont in Wilmington, Delaware), polycarbonates, polyethylene, polyethylene products, polypropylene, polyesters such as PET and PEN, acrylate-containing products, polymethyl methacrylates (PMMA), epoxy resins, their copolymers or any combination thereof.

The pre-treat composition includes agents having a priming function and agents having a sintering function. Priming agents can be used to pre-treat substrates for subsequent coatings and can improve, e.g., wetting or adhesion of the coating by modifying the substrate surface. Priming agents may also be used to alter mechanical properties of substrates, e.g., to improve abrasion resistance or chemical barrier properties. Priming agents can be radiation cured, e.g., UV, visible, or e-beam radiation, thermally cured, or solvent coated. Priming agents useful in the present invention include polymerizable components, such as polymerizable urethanes, epoxies, cellulosic derivatives, and (meth)acrylates (i.e. acrylates or methacrylates), and combinations thereof. Priming agents may be monofunctional or multifunctional. In addition, combinations of priming agents may be used. The pre-treat composition can also include particles, such as small sized inorganic particles, which may alter the surface texture or optical properties of the coating. Examples of small sized particles include micron- or submicron-sized particles of silica, zirconia, alumina, or titania. Preferred primer compositions can be transparent.

The sintering agents included in the pre-treat composition facilitate low temperature sintering (e.g., fusing or joining) of metal nanoparticles to form a continuously conductive coating or network. A variety of chemical sintering agents have been described in WO 2011/110949 and also US 2012/0168684. Sintering agents can include protonic acids, such as inorganic acids, e.g., sulfuric, phosphoric, or hydrochloric acid. Protonic acid sintering agents can also include organic acids such as carboxylic acids and oxyacids of phosphorus or sulfur having organic moieties. Examples of carboxylic acids include octanoic acid, lauric acid, and (meth)acrylic acids. Examples of oxyacids of sulfur having organic moieties include sulfonic acids, e.g., 4-dodecylbenzene sulfonic acid. Organic acid sintering agents can also be present as polyacids, e.g., polymers having acidic functionalities, such as polystyrene sulfonic acid or

poly(meth)acrylic acids. Sintering agents can also include halide salts, such as chloride, fluoride, or bromide salts, or combinations thereof. Preferred compositions include a combination of acidic and halide sintering agents, and can include both organic acids, e.g., polysulfonic acids and halide salts, e.g., chloride salts.

The pre- treat composition can further include a liquid carrier, e.g. a solvent or mixture of solvents. The solvent or mixture of solvents is selected to dissolve both the priming agents and the sintering agents to form a homogeneous composition. In some cases, the priming agents can be dissolved in a first solvent selected to be compatible with the priming agents, the sintering agents can be dissolved in a second solvent selected to be compatible with the sintering agents, and the two solvents can be combined to form a homogeneous composition.

Examples of conductive metal nanoparticles include conductive metals or mixtures of metals including metal alloys selected from silver, gold, platinum, palladium, nickel, cobalt, copper, and combinations thereof. The term "nanoparticles," as used herein, refers to fine particles small enough to be dispersed in a liquid to the extent they can be coated onto a substrate and form a uniform coating. This definition includes particles having an average particle size less than about three micrometers. For example, in some implementations, the average particle size is less than one micrometer, and in some embodiments the particles measure less than 0.1 micrometer in at least one dimension. Nanoparticles may be spherical or may have high aspect ratios such as nano wires.

Conductive metal nanoparticle compositions can be dispersions of the

nanoparticles in a liquid carrier such that the composition can be applied to a substrate in the form of a continuous or discontinuous (e.g., patterned) coating. Known nanoparticle dispersions include conductive inks, which may be applied using coating or printing techniques. If a discontinuous nanoparticle dispersion is applied as, e.g., a pattern, the resulting conductive film can be transparent if the pattern occupies less than about 50% of the surface area of the film.

Conductive metal nanoparticle compositions can also be liquid emulsions that, when coated on a substrate, self-assemble to form a network-like pattern of electrically conductive traces defining cells that are transparent to light at wavelengths in the range of 370 to 770 nm. The emulsion can include a liquid carrier having an aqueous phase and an oil phase. One of the phases forms a continuous liquid phase. The other phase forms dispersed domains within the continuous liquid phase. In some implementations, the continuous phase evaporates more quickly than the dispersed phase. One example of a suitable emulsion is a water-in-oil emulsion, where water is the dispersed liquid phase and the oil provides the continuous phase. The emulsion can also be in the form of an oil-in-water emulsion, where oil provides the dispersed liquid phase and water provides the continuous phase. An example of such a composition and coating is described in US 7,601,406, which is incorporated by reference. Preferably, a pre-treat composition used with such a self-assembling network is chosen such that the coated pre-treat layer permits self-assembly to proceed, i.e. the surface energy of the pre-treat layer is compatible with the self-assembly process.

The process of forming a substrate having a conductive coating includes the application of a pre-treat composition onto the substrate. The pre-treat composition includes both priming agents and sintering agents combined in a liquid carrier, e.g., a solvent or combination of solvents. The pre-treat composition can be applied by coating techniques such as bar spreading, immersing, spin coating, dipping, slot die coating, gravure coating, flexographic plate printing, spray coating, or any other suitable techniques. The pre-treat composition can be applied to a wet thickness of, e.g., 1-100 μηι. After drying, the pre-treat composition can be further cured using techniques such as radiation curing or thermal curing, thus forming a pre-treated substrate, e.g. a roll or sheet, which can be immediately coated with a conductive metal nanoparticle

composition, or can be reserved for coating at a later time or shipped for coating in a different location.

Once the pre-treat layer has been formed on the substrate, the conductive metal nanoparticle composition can be applied onto the pre-treat layer. The nanoparticle composition can be applied by coating techniques such as bar spreading, immersing, spin coating, dipping, slot die coating, gravure coating, flexographic plate printing, spray coating, or any other suitable techniques. The nanoparticle composition can be applied to a wet thickness of, e.g., 1-100 μηι. Once applied, the nanoparticle layer can form by evaporation of the liquid carrier, thus forming a conductive nanoparticle layer. In the case of certain emulsions, the nanoparticles may self-assemble to form a conductive network, as described in US 7,601,406.

Including the sintering agent with the priming agent in the pre-treat composition eliminates the need for a separate sintering step following application and drying of the nanoparticle composition.

EXAMPLES

Glossary

Component Function Chemical description Source

CN968 Monomer Aliphatic urethane hexaacrylate Sartomer Co., oligomer Exton, PA

SR9020 Monomer 3 mole propoxylated glyceryl Sartomer Co.

triacrylate

SR610 Monomer Polyethylene glycol (600) diacrylate Sartomer Co.

CN373 Coinitiator Reactive amine coinitiator Sartomer Co.

CD553 Monomer Monofunctional methoxylated PEG Sartomer Co.

550 aery late monomer

Esacure TZT Photoinitiator Mixture of 2,4,6- Lamberti trimethylbenzophenone and 4- S.p.A., Italy methylbenzophenone

Genocure Photoinitiator 1 -hydroxy-cyclohexylphenyl ketone Rahn USA CPK Corp., Aurora,

IL

Silica Anti-blocking 10 wt. % dispersion of Aerosil R974 Evonik Solution Agent Hydrophobic Fumed Silica in MEK Industries,

Germany

MEK Solvent Methyl Ethyl Ketone Sigma- Aldrich, St. Louis, MO

Ethanol Solvent Ethanol Sigma- Aldrich

PSS Sintering agent Poly(4-styrenesulfonic acid) Sigma- Aldrich

Octanoic Acid Sintering agent Octanoic Acid Sigma- Aldrich

Laurie Acid Sintering agent Dodecanoic Acid Sigma- Aldrich

NaCl Sintering agent Sodium chloride Sigma- Aldrich

BYK-410 Liquid Solution of a modified urea BYK USA, rheology Wallingford, additive CT

K-Flex A307 Flexibility Linear, saturated, aliphatic polyester King

modifier diol with primary hydroxyl groups Industries,

Norwalk, CT

Span 60 Sorbitan monostearate Sigma- Aldrich

Nacure 8924 Blocked acid Amine blocked sulfonic acid catalyst King catalyst Industries

Nacure 2501 Blocked acid Amine blocked toluenesulfonic acid King

catalyst Industries

BYK-348 Silicone Polyether modified BYK USA surfactant polydimethylsiloxane

P204 Silver Silver nanoparticle powder Cima

nanoparticles Nanotech,

Inc., Israel

Q4-3667 fluid Silicone polyether (glycol) copolymer Dow Corning,

Midland, MI

Ethyl Sigma- cellulose Aldrich

Synperonic Polyethylene glycol nonylphenyl Fluka, Sigma- NP-30 ether Aldrich

Cymel 303 Cross-linking Hexamethoxymethyl melamine Cytec

agent Industries,

Woodland Park, NJ

Test methods

Sheet resistance (Rs) was measured using a Loresta-GP MCP T610 4 point probe (Mitsubishi Chemical, Chesapeake, VA).

% Transmittance (%T) is the average percent of light that is transmitted through a sample. Measured by Nippon Denshoku NDH5000 (Nippon Denshoku Industries Co., Japan), ASTM D1003 method.

Haze (%H) was measured by Nippon Denshoku NDH5000, ASTM D1003 method.

Adhesion was measured by the change in sheet resistance after application and delamination of Intertape 51596 Electronic Tape (Intertape Polymer Group, Canada). Sheet resistance of a sample was measured and the area of measurement was marked. Intertape 51596 was applied to the marked area and immediately removed by peeling at an angle of 180 degrees. The resistance of the marked area was re-measured and adhesion was calculated as percent change from the original measurement (Adhesion =

[(RSafter - RSoriginal)/ RSoriginal)] X 100%). Examples 1-13

A polyester (PET) film substrate (Melinex X6667 available from Dupont Teijin Films, Wilmington, DE) was primed with solutions described in Table 1. The components listed in Table 1 were added together and mixed until uniform, the PSS and NaCl aqueous solutions being added as the final components. The primer was coated onto the substrate using a Mayer rod to a wet thickness of 6 um, dried for 1 min. at 50 deg. C, and UV cured by passing through a system having an F300S UV curing lamp with an H+ blub on an LC6B Conveyor (Fusion UV Systems Inc., Gaithersburg, MD) at a speed of 15 meters/min.

Table 1 - Primer Components (Example number, wt. %)

C indicates comparative examples Next, the components shown in Table 2 were mixed in the following manner. All of the components except the D.I. (i.e. deionized) water were mixed until uniform using an ultrasonic homogenizer to form a dispersion. Then the D.I. water, premixed with BYK348 and Nacure 8924, was added and the composition was mixed using an ultrasonic homogenizer to form a uniform emulsion.

The uniform emulsion was coated onto the primed PET film using a Mayer rod at 30 microns wet thickness. The coated films were dried 1 min. at 50 deg. C, followed by 2 min. at 150 deg. C, during which time the conductive network self-assembled and dried.

Table 2 - Emulsion components

The films thus formed were tested and the results are shown in Table 3. It is believed that the relatively high Rs tested for Examples 5, 6, 8, and 10 was caused by incomplete formation (e.g. self-assembly) of conductive traces from the coated emulsion. Table 3 - Results

*Not tested

Example 14

The primer of Example 11 was coated onto a PET substrate as described in Examples 1-13. Next, the emulsion listed in Table 2 was coated onto the primed substrate as described in Examples 1-13, except the wet thickness used was 24 um. The testing results were: Rs 12.9 Ohms/square, %T 86.26 %, Haze 2.44%. Adhesion was not tested.

Example 15

The primer of Example 11 and the emulsion of Examples 1-13 was coated onto a PET substrate as described in Examples 1-13. Triplicate samples had Rs of 9.5, 10.0, and 10.0 Ohms/square. One of the samples was then washed by spraying acetone over the surface, followed by draining and drying at 150 deg. C for 1 min. The acetone-washed sample had an Rs of 7.5 Ohms/square. Adhesion was not tested.

Example 16

A sample was prepared as described for Example 15, except that a first washing was done by spraying water over the surface, followed by draining, then a second washing was immediately done with acetone as described in Example 15. This washed sample had an Rs of 8.5 Ohms/square. Adhesion was not tested.

Example 17

A sample was prepared as described for Example 9, except that 0.9 wt. % octanoic acid in ethanol was substituted for 0.9 wt. % PSS in DI water. The testing results were: Rs 10 Ohms/square, %T 79.76%, Haze 3.66%. Adhesion was not tested.

Example 18

A sample was prepared as described for Example 9, except that 0.9 wt. % Laurie acid in ethanol was substituted for 0.9 wt. % PSS in DI water. The testing results were: Rs 10 Ohms/square, %T 78.47%, Haze 3.96%. Adhesion was not tested.

A number of embodiments of the invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. Accordingly, other embodiments are within the scope of the following claims.