WO2009136288A2

WO2009136288A2 - Feeding system for coating multiphase liquids

Info

Publication number: WO2009136288A2
Application number: PCT/IB2009/005810
Authority: WO
Inventors: Dmitry Lekhtman; Jacob Gilan; Fernando De La Vega; Kenichi Nakata
Original assignee: Cima Nanotech Israel Ltd.
Priority date: 2008-05-05
Filing date: 2009-05-05
Publication date: 2009-11-12
Also published as: TW201006561A; WO2009136288A3

Abstract

A system and process are disclosed for the continuous coating of an emulsion onto a moving substrate that allows the homogenized emulsion to be coated at a relatively constant pre-determined flow rate without significant destabilization of the emulsion. The system and process are particularly suited for use with emulsions containing fine conductive particles.

Description

FEEDING SYSTEM FOR COATING MULTIPHASE LIQUIDS

RELATED APPLICATIONS

This application claims priority under 35 USC §119(e) to U.S. Provisional Application Serial No. 61/050,347, filed on May 5, 2008, the entire contents of which are hereby incorporated by reference.

TECHNICAL FIELD

The present invention relates to systems and methods for processing and delivering emulsions for continuous coating on a substrate.

BACKGROUND

United States Patent Applications 2005/215689, 2005/238804, and 2005/214480 describe nanoparticle emulsions that self-assemble into conductive transparent coatings when applied to a substrate. The emulsion is preferably a water- in-oil emulsion in which the nanoparticles are dispersed in the organic phase of the emulsion.

The aforementioned emulsions as well as other emulsions, with and without dispersed particulate material, can be susceptible to phase separation which can interfere with the formation of a uniform coating of the emulsion. In particular, when coating an emulsion in a continuous, roll-to-roll coating process, it is desirable to maintain the stability of the emulsion throughout the process in order to maximize the quality of the coating and minimize defects in the resulting film.

SUMMARY

The present invention provides a system for the continuous application of an emulsion to a substrate comprising: (a) a tank having a mixing mechanism, the tank being adapted for receiving and processing at least two immiscible liquids to produce a mixture of the immiscible liquids; (b) a high shear homogenizer adapted to receive and process the mixture to produce or maintain an emulsion comprising a continuous phase of one of the immiscible liquids and a discontinuous phase comprising droplets of the other immiscible liquid, the droplets being substanitailly uniform and within a predetermined size range; (c) a dispensing mechanism adapted to deliver the emulsion from the homogenizer to a surface of the substrate comprising (i) at least one dispensing tank and (ii) a driving mechanism for driving the emulsion to the substrate; and (d) a controller, operatively associated with the driving mechanism to (i) provide a residence time of the emulsion within the dispensing mechanism that is less than the destabilization time of the emulsion and (ii) control the flow rate of the emulsion through the dispensing mechanism within a predetermined range.

In one embodiment, the average droplet size of the emulsion produced by the homogenizer is below 20 microns, often below 10 microns, and even below 5 microns. In one embodiment, the control of the flow rate of the emulsion out of the dispensing tank is performed by a controller such as a programmable logical controller (PLC) preferably adapted to receive process inputs (flow rate, pressure, etc.) and make any necessary adjustments to the dispensing means to bring the flow rate within the predetermined range. It is generally desirable to control the flow rate to within 5 percent (e.g., of a flow rate set point or of an average measured flow rate), preferably within 3 percent and most preferably within 2 percent. The flow rate of the emulsion through the dispensing mechanism may be controlled by means well known to those skilled in the art such as the use of control valves, and/or by controlling the pressure in the dispensing tank and/or adjusting the speed of a pumping device. In one embodiment of the system, two or more dispensing tanks are present, configured to operate in parallel fashion, such that continuous coating can be carried out by switching the emulsion delivery from a first dispensing tank to a second dispensing tank without interrupting the flow of the emulsion. During emulsion delivery from the second dispensing tank, the first dispensing tank is prepared for another cycle of emulsion delivery.

In another embodiment of the system, a single dispensing tank is present along with a pump to control the feed rate of the emulsion to the coating apparatus.

In one embodiment of the system, the residence time of the emulsion within the dispensing mechanism is less than 75 percent of the destabilization time of the emulsion.

In one embodiment, the system further includes a variable pressure-drop element such as a filter disposed along the dispensing mechanism downstream of the dispensing tank. Such filters are particularly useful if the emulsion contains dispersed particulate material above the desired particle size. The controller and dispensing mechanism are adapted to effect precise control of the emulsion flow rate, even during changes in head downstream, for example, due to partial blockage of a downstream filtration unit. Thus, even when the variable pressure-drop element causes a variation in the pressure differential between pressure in the dispensing tank and atmospheric pressure, the controller and the dispensing mechanism can maintain the flow rate within 5 percent (e.g., of a flowrate setpoint), typically within 3 percent, and often within 2 percent, while substantially maintaining the physical and chemical properties of the emulsion. Flow rate can be measured in terms of volume or weight parameters.

In one embodiment, the dispensing mechanism further comprises a coating apparatus for applying the emulsion to the substrate. In a preferred embodiment the coating apparatus comprises a die, most preferably a slot die.

In one embodiment, the emulsion applied to the substrate contains dispersed particles, preferably conductive particles such as silver, a silver-copper alloy, carbon black or graphite. Such particles generally have an average particle size below about 3 microns in at least one dimension, preferably below about 1.0 micron, more preferably below about 0.5 micron and most preferably below about 0.1 microns (100 nanometers). In the preferred emulsions, the particles are dispersed in the organic phase of a water-in-oil emulsion. Such emulsions are used to form self-assembled transparent conductive coatings on substrates as described in United States Patent Applications 2005/215689, 2005/238804, and 2005/214480, incorporated herein by reference.

The present invention also provides a process for continuously forming a transparent conductive coating on a substrate comprising (a) forming a mixture of at least two immiscible liquids and fine conductive particles; (b) subjecting the mixture to high shear homogenization to produce or maintain the mixture as an emulsion comprising a continuous phase comprising one immiscible liquid and a discontinuous phase comprising droplets of the other immiscible liquid, the droplet size being within a predetermined size range; (c) continuously dispensing the homogenized emulsion to a coating apparatus at a substantially constant pre-determined flow rate that results in coating of the emulsion onto the substrate before destabilization of the emulsion; (d) continuously coating the homogenized emulsion onto a substrate; and (e) evaporating the liquid from the emulsion to form a transparent conductive coating on the substrate wherein the coating is in the form of a network- like pattern of interconnected traces of the fine particles that define randomly shaped voids on a surface of the substrate. In one embodiment of the process, the conductive particles are particles of silver, silver-copper alloy, carbon black or graphite. The average particle size of the conductive particles is generally below about 3 microns, preferably below about 1.0 micron, more preferably below about 0.5 micron, and most preferably, below about 0.1 micron (100 nanometers). The preferred emulsion is a water-in-oil emulsion, preferably formed from a dispersion of conductive particles in the continuous, organic phase of the emulsion. The droplet size of the discontinuous phase after homogenization is preferably less than 20 microns. The flow rate is generally controlled within 5 percent, preferably 3 percent, and most preferably 2 percent of a set flow rate. The system and process are especially suitable for thixotropic fluids, shear- thinning fluids, and unstable fluids.

The system and process are especially suitable for emulsions containing dispersed particles, preferably fine metal particles, and most preferably nano-sized metal particles. The system and process can be employed to attain a number of advantages.

Depending on how those practicing it choose to configure it, the system and process make possible the construction and operation, at reasonable costs, of emulsion (with or without dispersions) production and delivery systems having one or more of the following advantages: (1) good tolerance to changes in head upstream (e.g., due to the mixture height in the mixing tank) and downstream (e.g., due to various changes in a downstream filtration unit); (2) ability to operate for lengthy periods while maintaining substantially constant flow rates over a wide range of coating speeds and consistent emulsion properties (density, viscosity, droplet size, etc.); and (3) robust process control. Other features and advantages will become apparent from the drawings and detailed description below.

DESCRIPTION OF THE DRAWINGS

Figure 1 is a schematic flow diagram of the process of the invention.

Figure 2 is a schematic diagram of an embodiment of the system of the invention.

Figure 2a is a schematic diagram of the system of the invention with an alternative dispensing mechanism.

Figure 3 is a schematic of a central processing unit that controls the flow rate of the emulsion from the dispensing tank to the coating apparatus. Figure 4a is an illustration of a slot coater die for applying the emulsion to a substrate.

Figure 4b illustrates the application of the emulsion through the die onto the substrate.

Figure 5 is a graph of the emulsion flow rate vs. time with the system of the invention for Coating Run 7 described in the Examples below.

Figure 6, 7 and 8 are graphs of the emulsion flow rate vs. time of three representative coating runs using the system of the invention.

DETAILED DESCRIPTION

The principles and operation of the system and process of the present invention may be better understood with reference to the drawings and the accompanying description.

Before explaining at least one embodiment of the invention in detail, it is to be understood that the system and process are not limited in their application to the details of construction and the arrangement of the components set forth in the following description or illustrated in the drawings. The system and process are capable of other embodiments or of being practiced or carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein is for the purpose of description and should not be regarded as limiting. As used herein in the specification, the term "emulsion" refers to a suspension of small droplets or globules of one liquid (discontinuous phase) in a second liquid (continuous phase), the liquids being substantially immiscible. An emulsion may also contain fine particles dispersed therein.

As used herein in the specification, the term "thixotropic", with respect to a fluid or emulsion, refers to a fluid or emulsion that, like some non-Newtonian pseudoplastic fluids, exhibits a time-dependent change in viscosity: the longer the fluid or emulsion undergoes shear stress, the lower its viscosity. A thixotropic fluid takes a finite amount of time to attain equilibrium viscosity when introduced to a step change in shear rate. Many gels and colloids are thixotropic materials, exhibiting a stable form at rest but becoming fluid when agitated.

It is important to note the distinction between thixotropic fluid and shear- thinning fluid. Thixotropic fluids display a decrease in viscosity over time at a constant shear rate, while shear-thinning fluids display decreasing viscosity with increasing shear rate. As used herein, the term "destabilization time", with regard to the emulsion, refers to the maximum time until a change in a physical property of the emulsion occurs, wherein the change in that physical property affects a desired property of the coated emulsion in an adverse way. Furthermore, destabilization time with regard to emulsions used to form transparent conductive coatings such as those described in United States Patent Applications 2005/215689, 2005/238804, and 2005/214480 refers to the maximum time after emulsion formation within which the emulsion can be coated onto a substrate to provide a suitable transparent and conductive network- like pattern.

Determination of destabilization time must correlate with the properties of the resulting coating. In some instances, measurable physical properties of the emulsion fluid may not seem to exhibit significant change over time, yet the properties of the resulting coating are not desirable. This case also indicates that the processing time is above a maximum desired time which is the destabilization time.

As used herein in the specification, the term "unstable emulsion" refers to an emulsion having a destabilization time of less than 20 minutes. Some unstable emulsions processed with the systems and methods of the present invention have destabilization times of less than 10 minutes, and in some cases, less than 7 minutes. For various applications in which the emulsions are fed to a die, the emulsions often have destabilization times of less than 5 minutes, and typically, about 2-5 minutes. In all cases, the residence time of the emulsion in the dispensing mechanism is less than the destabilization time of the emulsion.

As used herein, the time that the emulsion resides in the system between the discharge from the homogenizer and the discharge from the system via the coating apparatus (the "dispensing mechanism"), is referred to as the "residence time". In most cases, the portion of the residence time during which the emulsion is in the dispensing tank, termed "dispensing tank residence time", is nearly identical to the residence time because once the emulsion is discharged from the dispensing tank, it is quickly applied to the substrate by the coating apparatus.

As used herein in the specification, the term "flow rate", with respect to an emulsion transported through the system between the dispensing tank and the coating apparatus, refers to a measured flow rate that is sampled within a time interval of at least 1-5 seconds.

Referring now to the drawings, Figure 1 is a schematic block diagram of one embodiment of the process that is used to prepare transparent conductive coatings as described in United States Patent Applications 2005/215689, 2005/238804, and

2005/214480 referred to above and incorporated herein by reference. In step 1, which may be combined with step 2, a fine powder such as a fine metal powder and a liquid carrier are mixed together to form a dispersion. In step 2, the dispersion (or liquid and powder) is combined with a second liquid that is substantially immiscible with the liquid carrier of the dispersion (or first liquid) to form a mixture, generally, but not necessarily in the form of an emulsion at this stage. Preparation of suitable dispersions and emulsions for forming transparent conductive coatings is described in aforementioned patent applications. The mixture may be continually mixed to maintain homogeneity. Subsequently, the mixture undergoes high-shear homogenization (step 3), to produce or maintain an emulsion having a continuous phase and a discontinuous phase comprising droplets within a relatively narrow, predetermined size range. The emulsion may be a water-in-oil emulsion or an oil-in- water emulsion, but a water-in-oil emulsion is preferred, and it is preferred to have the powder dispersed in the oil phase. The homogenized emulsion is continuously dispensed (step 4) to a coating apparatus at a substantially constant pre-determined flow rate that results in coating of the homogenized emulsion onto the substrate before destabilization of the emulsion. The emulsion is continuously coated (step 5) onto the substrate, and liquid from the emulsion is evaporated (step 6) such that the fine powder, dispersed in step 1, is formed on the surface of the substrate in a network- like pattern of interconnected traces of the powder that define randomly shaped voids.

This process will be further elaborated in the context of the system of Figure 2 and Figure 2a. Figure 2 is a schematic drawing of system 100 for emulsion preparation and handling, and delivery of the emulsion to a die to produce a coating. The emulsion may contain finely dispersed solids. The major unit operations performed in system 100 include mixing of the emulsion components, production or maintenance of the emulsion using a high-shear homogenizer, and precise/accurate delivery of the emulsion to a coating die or other downstream apparatus for applying the emulsion to the surface of a substrate. The main equipment for effecting these unit operations may include an agitated feeding tank 40, a high-shear homogenizer and/or emulsifier 50, at least one dispensing tank 60 illustrated as dispensing tanks 60a and 60b, and a die 70.

The components of the emulsion are introduced to an agitated feeding tank 40 which is designed to produce, or substantially maintain, macroscopic homogeneity of the mixture that includes an oil-containing phase and an aqueous phase that are substantially immiscible. For emulsions containing finely dispersed solids, it may be advantageous, or even necessary, to disperse the solids in one of these phases (liquids) before being introduced to feeding tank 40.

Feeding tank 40 may advantangeously be equipped with a mixer 42 such as a low-power, low shear mixer having an impeller 44 such as an anchor-type impeller.

Mixer 42 is driven by a motor 46, such as an air motor disposed above feeding tank

40.

The mixture produced in agitated feeding tank 40, which may or may not be an emulsion, depending on the particular application, is preferably delivered continuously to high- shear homogenizer 50. In some cases it may be advantageous to reduce the viscosity of the liquid in feeding tank 40 prior to applying high shear in homogenizer 50.

Homogenizer 50 serves to maintain and/or produce emulsions having an average characteristic droplet diameter below 20 micrometers, and often, below 10 micrometers or even below 5 micrometers and to maintain the droplet size within a narrow range. If a solid material is present in the mixture received from feeding tank

40, high-shear homogenizer 50 is adapted to maintain and/or to better produce dispersions of fine solid particles in the emulsion.

Although various types of high-shear homogenizer 50 may be apparent to those skilled in the art, a particularly suitable high- shear homogenizer includes a conical or disk rotor that is separated from a complementary, liquid-cooled stator by a closely-controlled rotor-stator gap. As the rotor rotates at high rates, it pumps fluid between the outer surface of the rotor and the inner surface of the stator, and shear forces generated in the gap process the fluid. While various types of homogenizers may be suitable for use as homogenizer

50, the high-RPM, in-line, rotor-stator types have been found to be particularly suitable for maintaining and/or producing emulsions having an average droplet size of below 20 micrometers, and even below 10 micrometers, or 5 micrometers or less.

One particularly suitable, commercially-available high-shear homogenizer is MEGATRON^® MT 3000 (Kinematica, Switzerland).

It is often advantageous to recirculate a portion of the emulsion (product stream) produced by homogenizer 50 to feeding tank 40. The remaining portion of the emulsion is delivered downstream.

In many applications, the discharging of the emulsion by homogenizer 50 is effected at a flow rate that is sufficiently constant to suit the process requirements. Often, however, process requirements dictate that the flow rate of the emulsion supplied to the coating apparatus be maintained within very narrow limits. Homogenizer 50 is generally unsuited to meet such requirements, inter alia, because the flow rate delivered by homogenizer 50 is strongly dependent on the upstream head and the downstream head. By way of example, the level of fluid in feeding tank 40 influences the upstream head. The downstream head may vary greatly, depending on the specific application. If the emulsion discharged by homogenizer 50 is fed directly to a die, the die may contribute most of the downstream pressure drop, such that small changes in the die operation can effect profound changes on the emulsion flow rate. Similarly, when the discharge of homogenizer 50 passes through a filter, the pressure drop may be substantial, and more importantly, the pressure drop generally increases throughout the run, as the filter becomes clogged and less filtration area is available for the emulsion to pass through. Channeling effects due to channels or cracks in the filter bed may also cause changes in the pressure drop over the course of the run. Thus, the system advantageously provides additional components between the homogenizer 50 and the coating apparatus for applying the emulsion to the substrate to insure continuous, high-precision coating of the emulsion. In an intermediate step, the discharge of homogenizer 50 is introduced to at least one dispensing tank 60. In Figure 2 two dispensing tanks 60a and 60b are illustrated. A multi-component fluid dispensing system is used to deliver the emulsion from homogenizer 50 to a downstream coating apparatus such as die 70 and then to the surface of the substrate. Often, the emulsion is first passed through a filtration unit 65 to remove any large, undesirable particles.

Associated with each respective dispensing tank 60a, 60b is a driving mechanism 61a, 61b for driving the emulsion out of the respective dispensing tank. In one preferred embodiment, driving mechanism 61a, 61b includes a supply of pressurized gas (preferably having a pressure of up to 25 psi above atmospheric pressure) that communicates with the respective dispensing tank so as to provide the dispensing tank 60 with a super-atmospheric pressure (preferably, but not limited to, 5 to 15 psi above atmospheric pressure)

Downstream from dispensing tanks 60a, 60b are disposed valves 62a and valve 62b, respectively, which are preferably control valves. When valve 62a is open, driving mechanism 61a applies a pressure to the top of the emulsion in dispensing tank 60a, driving the emulsion out of tank 60a, via a delivery line 63, to a downstream application that may fluidly communicate with delivery line 63 and dispensing tanks 60a, 60b.

Often, the downstream application has a variable linear pressure-drop as a function of length and conduit dimensions for a given flow rate. In other applications, a variable non-linear pressure-drop element such as filtration unit 65 is disposed on- line between dispensing tanks 60a, 60b and the downstream application.

Associated with each respective dispensing tank 60a, 60b is at least one weighing element such as load cells 64a, 64b preferably adapted to continuously weigh the respective dispensing tank. Load cells 64a, 64b preferably produce an output, corresponding to or associated with a weight of the respective dispensing tank, which is conveyed to a central processing unit (PLC) 80, shown in Figure 3.

PLC 80 is preferably programmed to determine a maximum fill weight for dispensing tanks 60a, 60b, based on inputs such as the destabilization time and the specific gravity of the particular emulsion. In all cases, the residence time of the emulsion in the dispensing tank is set to be less than the destabilization time of the emulsion, and is preferably less than or equal to 75% of the destabilization time of the emulsion.

Alternatively or additionally, PLC 80 is preferably programmed to determine a maximum fill height of dispensing tanks 60a, 60b, based on inputs such as the destabilization time and the specific gravity of the particular emulsion, and the dimensions of the tanks. In this case, the tanks may advantageously be equipped with at least one level sensor such as differential pressure cells. Other types of suitable sensors and the like will be apparent to those of ordinary skill in the art.

Using such sensors and inputs as described hereinabove, PLC 80 may be programmed to operate dispensing tanks 60a, 60b in parallel fashion. A run can be effected continuously by controlling/switching the emulsion delivery (e.g., by means of PLC 80) from a first dispensing tank such as dispensing tank 60a to a second dispensing tank, such as dispensing tank 60b.

During emulsion delivery from the second dispensing tank, the first dispensing tank is prepared for another cycle of emulsion delivery. The preparation may include flushing of the tank via controlled line 67a having control valve 69a, or via controlled line 67b having control valve 69b, with any remaining material in the tank being discarded, or returned to the process, e.g., to agitated feeding tank 40, via a controlled return line 48.

Dispensing of the emulsion may be performed through the dispensing tanks by pressure, or by a suitable pump or other suitable means, to provide a continuous flowrate of the emulsion out of the dispensing tanks. The flow rate is consistently controlled within 5 percent (e.g., of a flow rate setpoint), typically within 3 percent, and often within 2 percent, while maintaining the continuous flow rate of the emulsion, even when the variable pressure-drop element causes a variation in the pressure differential between the tank pressure and atmospheric pressure. Significantly, this consistent control of the flow rate through the delivery line downstream from the dispensing tanks (and optionally through the variable pressure- drop element) is effected during long-term operation, for at least 1.5 times the residence time of the emulsion, preferably at least 2 times the residence time of the emulsion, and typically, at least 4 times the residence time of the emulsion. In various runs, the continuous flow rate of the emulsion was maintained within 5% (and often within 2%) of the flowrate setpoint for more than 20, and even more than 40 times the residence time of the emulsion, even during periods in which the emulsion delivery is switched from one dispensing tank to a second dispensing tank. Moreover, the continuous flow rate of the emulsion was maintained while substantially maintaining the physical and chemical properties of the emulsion.

Operation of driving mechanism 61a, 61b may be advantageously linked to a flow rate of emulsion through delivery line 63. For example, driving mechanism 61a, 61b may be responsive (e.g., via PLC 80) to an output from flow meter or flow indicator 66 disposed on delivery line 63, such that a decrease in the measured flow rate of emulsion may result in an increase in the driving force of driving mechanism 61a or 61b, or vice versa.

Thus, when a driving force for driving mechanism 61a, 61b includes the pressure of the respective dispensing tank, a decrease in the measured flow rate of emulsion may result in an increase in the pressure exerted within the dispensing tank by means of the pressure source, such as a super-atmospheric gas (air, nitrogen, etc.) fluidly communicating with the dispensing tank, so as to make a correction in the flow rate of the emulsion.

Alternatively, the increase in the pressure exerted within the dispensing tank may be achieved by means of a floating piston or membrane (not shown) associated with the dispensing tank, wherein an external pressure source, such as a super- atmospheric gas or liquid, associated with the dispensing tank, acts upon the piston or membrane to increase the pressure within the dispensing tank.

The above-described driving mechanisms are particularly advantageous in that pressure is exerted in a substantially even fashion across the entire surface area of the emulsion in the tank. This minimizes various deleterious effects with respect to the emulsion, including local coalescence of the discontinuous phase and/or coagulation of aggregated dispersed particles.

Moreover, many emulsions are sensitive to irregularities in the surfaces they contact (e.g., screws of various types). The above-described driving mechanisms have no contact area, or relatively little contact area, compared to the contact area of various conventional emulsion delivery mechanisms.

When a variable pressure-drop element such as filtration unit 65 is disposed on-line between dispensing tanks 60a, 60b and the downstream application, it may be particularly beneficial to measure the pressure drop across the variable pressure-drop element. In Figure 2, the pressure-drop measuring instrument includes a pressure transducer 68 disposed on delivery line 63, on both sides of the variable pressure-drop element.

The system of Figure 2a is similar to that of Figure 2 except an alternative dispensing mechanism is illustrated. Agitated feeding tank 40, mixer 42, impeller 44, motor 46, controlled return line 48, and high-shear homogenizer/emulsifier 50 are as described above with respect to Figure 2.

In Figure 2a, the discharge of homogenizer 50 is fed to a single dispensing tank 60a through an ON/OFF input valve 69a. Dispensing tank 60a preferably has a relatively small work volume to allow the residence time of the emulsion in the tank to be relatively short thereby reducing sedimentation, he inside of the dispensing tank 60a is preferably coated with Teflon and equipped with a mixer (not shown). Feeding tank 60a is placed on load cell 64a.

Dispensing tank 60a is not equipped with mechanical dispensing means as in the dual dispensing tanks 60a and 60b shown in Figure 2. Rather, the emulsion flows through to pump 72 after it exits dispensing tank 60a through controlled outlet ON/OFF valve 62a. Dispensing tank 60a is preferably elevated from pump 72 as illustrated. Both input valve 69a and output valve 62a are controlled by the central process unit. Pump 72 provides precision emulsion feeding to the coating die 70 via delivery line 63. While various pumps may be used, it is desirable to select a pump that has the least impact on the stability of the emulsion. One example of a suitable pump is a gear pump such as the commercially-available Cole Parmer Digital Gear Pump (model 75211-35, 60-3600 RPMs). Another example of Pump 72 is a screw pump such as a NEMO pump. In the case of the NEMO pump, it is preferable to place it on a slanted table to eliminate air bubbles and sedimentation.

Both the gear pump and the screw pump provide non-pulsating constant flow proportional to rotating speed. These pumps handle the emulsion without destabilizing it, and provide constant flow rates even at low coating speeds. One or more filters 65 may be included in the flow line between pump 72 and the coating die 70. Also, one or more flow meters 68 and pressure transducers are included to provide input to PLC 80. It may also be desirable to include a static mixer (not shown) in the system just before the coating die to provide final homogenization of the emulsion prior to application to the substrate. Figure 3 is a schematic block diagram showing examples of analog and digital inputs to PLC 80, and examples of analog and digital outputs from the PLC 80.

Various fixed input parameters to PLC 80, may include the following: flow rate set point; feeding tank (such as tank 40) initial weight; feeding tank minimum weight; pressure tank (such as tanks 60a, 60b) maximum weight; pressure tank minimum weight; outlet valve time during switching between tanks (such as tanks 60a, 60b); initial set point starting pressure; maximum process pressure level (safety shut-off criterion); "on" criterion of the high-shear homogenizer (such as homogenizer 50) prior to refilling the pressure tank (based on pressure tank minimum weight); change in pressure criterion for switching filter (such as filter 65 ); internal PLC instructions such as data logger recording time. Input parameters to PLC 80 for the system illustrated in Figure 2a include a set point for the desired flow rate of the emulsion exiting the pump 72.

An exemplary commercially-available PLC that is suitable for use in various systems and processes of the present invention is Unitronics V280 (Quincy, MA). The systems and methods of the present invention may be efficacious for a wide variety of emulsions, including water-in-oil emulsions, oil-in- water emulsions, thixotropic emulsions, shear-thinning emulsions, and emulsions having a wide range of transport properties.

With regard to viscosity, the systems and methods of the present invention may be particularly efficacious in producing, maintaining, and transporting emulsions having a viscosity below approximately 1000 centipoise (cp), and more typically below 100 cp or 50 cp. They are also particularly appropriate for emulsions having viscosities on the order of 10 cp and less.

As described briefly hereinabove, the emulsion is ultimately delivered to a downstream application such as a coating apparatus including die 70. The coating apparatus may be any of a variety of contact or non-contact coaters known in the art, such as comma coaters, die coaters, gravure coaters, reverse roll coaters, knife coaters, rod coaters, extrusion coaters, curtain coaters, or any other coating or premetering device. The slot die coater is a preferred application. As shown in Figure 4A, a slot die includes a cavity that receives the emulsion and through which the emulsion is delivered to the lip of the die. In slot die processes, the emulsion is squeezed out by gravity or under pressure through a slot and onto the substrate (See Figure 4B).

The emulsion introduced to the die may include a dispersion of fine conductive particles. In a preferred embodiment, the fine conductive particles are, or include, silver, silver-copper alloy, carbon black, or graphite. The conductive particles are dispersed in one of the liquid phases of the emulsion, preferably the continuous oil phase of a water-in-oil emulsion. After the emulsion has been coated onto or otherwise applied to a substrate, the liquids are evaporated to form a network- like pattern of the fine conductive particles on the substrate. It is a particular advantage to produce a self-assembled, network- like pattern of the fine conductive particles.

EXAMPLES

Reference is now made to the following examples, which , together with the above description, illustrate the invention in a non-limiting fashion.

Data was collected from nine coating runs conducted with the system illustrated in Figure 2. The emulsion used for all of the runs was a water-in-oil emulsion containing silver nanoparticles dispersed in the continuous oil phase. The formulation was similar to example 26 of United States Patent Applications 2005/215689, 2005/238894, and 2005/214480. The liquids included toluene (55-60 weight percent), cyclohexanone (4-8 weight percent) and water (29-34 weight percent). The emulsions contained 3.6-5.0 weight percent of silver nanoparticles made in accordance with United States Patent Application 2006/112785, wherein the diameter of 90 percent of the particles is equal to or less than 0.3 microns, and the diameter of 50 percent of the particles is equal to or less than 0.73 microns. The emulsion was coated onto a polyester terephthalate (PET) substrate.

Each run was divided into two zones. The first zone was the time to reach steady state flow. The second zone was the remainder of the run after achieving steady state flow. Eight of the runs were approximately one hour long, and one run was nearly two hours long. Data relating to flow was gathered every second for the one-hour runs and every two seconds for the two-hour run. The source of the data was a turbine -type flow meter with pulse-wave output located after the filter and just before the die (illustrated by reference numeral 66 in Figure 2). The data recorded included the following: the desired flow rate ("Set Point") in ml/minute; maximum flow rate in the steady state flow zone ("Max"); minimum flow rate in the steady state zone ("Min"); average flow rate in the steady state zone ("Average"); standard deviation of flow rates in the steady state zone ("STD"); and the standard deviation of flow rates in the steady state zone as a percent of the average flow rate (STD%"). The temperature of the emulsion was also recorded with a digital thermocouple after the emulsion first exited the die.

Two emulsion parameters were monitored during the coating runs, viscosity (Zahn cup method according to ASTM standard test method D4212 "Viscosity by Dip-Type Viscosity Cups")) and density (weight of a constant volume in g/ml as determined by a pycnometer). Both parameters were measured prior to feeding the emulsion into the system and after exiting the die. Viscosity and density were measured after initial emulsification (prior to feeding into the system illustrated in Figure 2) and then after the die (reference numeral 70 in Figure 2) at 5, 15, 30, and 45 minutes of the one -hour runs. For the two-hour run, these parameters were measured after initial emulsification prior to feeding in the system and then after the die at 5, 30, 60, and 90 minutes of the run and at the end of the run. The flow of the emulsion through the die was also observed visually.

Run 6 contained a thirty-second pause after 15 minutes and 30 minutes of the run. In runs 8 and 9 the filter was changed three times without interrupting the flow of the emulsion.

10 The destabilization time of the emulsions was generally between 2 to 5 minutes, requiring the use of two small dispensing tanks disposed in alternate configuration, as described hereinabove. Consequently, the flow of the emulsion to the die was switched frequently between the dispensing tanks, to ensure that the residence time of the emulsion was well below the destabilization time, and a

15 substantially uniform flow rate to the die was maintained.

Table 1 below provides the emulsion flow data for Runs 1-9.

TABLE 1: FLOW DATA, RUNS 1-9

Note: SS - steady state flow

Table 2 below provides the Zahn cup viscosity of the emulsions for Runs 1-9.

TABLE 2: ZAHN CUP VISCOSITY, RUNS 1-9

Table 3 below provides the density of the emulsions for Runs 1-9.

TABLE 3: EMULSION DENSITY, RUNS 1-9

Figure 5 is a plot of the emulsion flow rate vs. time for the long run (Run 7). 5 In addition, exemplary plots of emulsion flow rate vs. time for emulsions processed using the systems and methods of the present invention are provided in Figures 6 and 7.

The data obtained and the observations made from these runs indicate that long runs using the system are feasible. Good homogenous and continuous flow was 10 achieved through the die. All measurements were within a ± 5% range, and most measurements were inside a ± 3% range. The standard deviation of flow rate was below 1.5%, with the average flow being very near the set point. The system achieves a steady state flow rate in about 2 to 5 minutes. Large area filters improve performance, with low changes in pressure before and after the filter. The density of the emulsion remains constant. Viscosity of the emulsion a determined by Zahn cup measurements is relatively constant during the process.

For the system illustrated in Figure 2a comprising a gear pump, a run of 49 minutes was performed with the same type of water-in-oil emulsion used in the previous examples using a Cole Partner Digital Gear Pump (Model 75211-35, 60- 3600 RPM) for which flow rate was set at 69 ml/min. Figure 8 and Table 4 below provide emulsion flow data for this run. Excellent flow rate control was obtained such that the average flow rate throughout the run was 68,95 ml/min. with a standard deviation of 0.21 ml/min. The standard deviation thus corresponds to 0.3% of the average flow rate. Furthermore, the system achieved a steady state flow rate within 2.5 minutes of beginning the run.

Table 4: Emulsion flow rate data for feeding system with gear pump

Although the invention has been described in conjunction with specific embodiments thereof, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, it is intended to embrace all such alternatives, modifications and variations that fall within the spirit and broad scope of the appended claims.

Claims

WHAT IS CLAIMED IS:

1. A system for continuous application of an emulsion onto a substrate comprising: (a) a tank adapted to receive and process at least two immiscible liquids, the tank having a mixing mechanism, the tank and the mechanism adapted to produce a mixture of the immiscible liquids;

(b) a high shear homogenizer adapted to receive the mixture and to act upon the mixture to produce or maintain an emulsion comprising a continuous phase of one of the immiscible liquids and a discontinuous phase comprising droplets of the other immiscible liquid, said droplets being substantially uniform and within a predetermined size range;

(c) a dispensing mechanism adapted to deliver the emulsion from the homogenizer to a surface of the substrate comprising (i) at least one dispensing tank, and (ii) a driving mechanism for driving the emulsion toward the substrate; and

(d) a controller, operatively associated with the driving mechanism to (i) provide a residence time of the emulsion within the dispensing mechanism that is less than the destabilization time of the emulsion and (ii) control the flow rate of the emulsion through the dispensing mechanism within a predetermined range.

2. The system of claim 1 wherein the average droplet size is below 20 microns.

3. The system of claim 2 wherein the average droplet size is below 10 microns.

4. The system of claim 1 wherein the controller is a programmable logical controller.

5. The system of claim 1 wherein the flow rate of the emulsion through the dispensing system is maintained within 5 percent of a predetermined flow rate.

6. The system of claim 5 wherein the flow rate of the emulsion through the dispensing system is maintained within 3 percent of a predetermined flow rate.

7. The system of claim 1 wherein two or more dispensing tanks are present and configured to operate in parallel fashion so that continuous flow of the emulsion through the dispensing mechanism is carried out by switching from one dispensing tank to another without interruption of the flow of the emulsion.

8. The system of claim 1 wherein one dispensing tank is present and the driving mechanism is a pump located between the dispensing tank and the substrate.

9. The system of claim 8 wherein the pump is a gear pump or a screw pump.

10. The system of claim 1 wherein the residence time of the emulsion within the dispensing mechanism is less than 75 percent of the destabilization time of the emulsion.

11. The system of claim 1 wherein the dispensing mechanism further comprises a variable pressure drop element downstream of the dispensing tank.

12. The system of claim 11 wherein the variable pressure drop element is a filter.

13. The system of claim 1 wherein the mixture further comprises dispersed particles.

14. The system of claim 13 wherein the dispersed particles are conductive particles.

15. The system of claim 14 wherein the conductive particles are particles selected from silver, silver-copper alloy, carbon black or graphite.

16. The system of claim 13 wherein the dispersed particles have an average particle size of less than 3 microns in one dimension.

17. The system of claim 16 wherein the dispersed particles have an average particle size of less than 1 micron in one dimension.

18. The system of claim 17 wherein the dispersed particles have an average particle size of less than 0.5 micron.

19. The system of claim 18 wherein the dispersed particles have an average particle size of less than 0.1 micron.

20 The system of claim 1 wherein the dispensing mechanism comprises a coating apparatus for applying the emulsion to the substrate.

21. The system of claim 20 wherein the coating apparatus comprises a die.

22. A process for continuous formation of a transparent conductive coating on a substrate comprising:

(a) forming a mixture of at least two immiscible liquids and fine conductive particles ; (b) subjecting the mixture to high shear homogenization to produce or maintain an emulsion comprising a continuous phase comprising one immiscible liquid and a discontinuous phase comprising droplets of the other immiscible liquid, the droplet size being within a predetermined size range; (c) continuously dispensing the homogenized emulsion to a coating apparatus at a substantially constant predetermined flow rate that results in coating of the emulsion onto the substrate before destabilization of the emulsion;

(d) continuously coating the homogenized emulsion onto a substrate; and

(e) evaporating the liquid from the emulsion to form a transparent conductive coating on the substrate wherein the coating is in the form of a network- like pattern of interconnected traces of the fine particles that define randomly shaped voids on a surface of the substrate.

23. The process of claim 22 wherein the emulsion is a water-in-oil emulsion.

24. The process of claim 23 wherein the fine conductive particles are dispersed in the oil phase of the emulsion.

25. The process of claim 24 wherein the fine conductive particles have an average particle size less than 3.0 microns in at least one direction.

26. The process of claim 24 wherein the fine conductive particles have an average particle size less than 1.0 micron in at least one direction.

27. The process of claim 24 wherein the fine conductive powder has an average particle size less than 0.5 micron in at least one direction.

28. The process of claim 25 wherein the fine conductive particles have an average particle size less than 0.1 micron in at least one direction.

29. The process of claim 22 wherein the fine conductive particles are selected from silver, silver-copper alloy, carbon black and graphite.