HK1191664A - Pickering emulsion for producing electrically conductive coating and process for producing a pickering emulsion - Google Patents

Description

Pickering emulsion for producing electrically conductive coatings and method for producing pickering emulsion

The invention relates to a method for producing a Pickering emulsion comprising water, a water-immiscible solvent and preferably sterically hindered, stabilized silver nanoparticles for producing electrically conductive coatings. The invention also relates to a method for completely or partially coating a surface, in particular with a pickering emulsion according to the invention, wherein the coating obtained has in particular a high electrical conductivity and can advantageously be transparent.

Plastic components generally have good mechanical properties and in part also have additionally good optical properties, such as transparency of polycarbonate. However, most engineering plastics are electrical insulators.

The association of mechanical properties (such as stability), optical properties (such as transparency) and electrical properties (such as conductivity) in the case of transparent plastics is desirable for many applications and can bring great advantages. In this context, the transparency of the component is to be emphasized, which should be as high as possible in many fields of application, for example for the production of automobiles or for the glazing of buildings, or for the viewing windows in devices which are to be connected to extended electrical applications, for example electrical heating, shielding against electromagnetic radiation or conduction of surface charges. At the same time, in most cases, the mechanical stability of the base material and the design freedom in terms of shaping should be as high as possible. The use as highly conductive electrical conductors in the field of solar cell technology (photovoltaic devices) is also desirable.

In the processing of silver or other metals, it is known to disperse stabilized nanoparticles in organic solvents or water, and subsequently to apply this formulation to a substrate and dry it. But generally require relatively high temperatures to sinter the stabilized nanoparticles. However, this is not tolerable for all substrates, especially not for many plastic substrates (e.g. those made of polycarbonate).

Xia et al, adv.Mater.,2003,15, No.9,695-one 699, describe the preparation of stable aqueous dispersions of silver nanoparticles with poly (vinyl pyrrolidone) (PVP) and sodium citrate as stabilizers. Xia thus obtains a monodisperse dispersion with silver nanoparticles having a particle size of less than 10nm and a narrow particle size distribution. The use of PVP as a polymeric stabilizer here results in steric stabilization of the nanoparticles to prevent aggregation. Such sterically hindered polymeric dispersion stabilizers may optionally reduce the direct contact of the particles with each other and thus the conductivity of the coating by surface coverage of the silver particles in the resulting conductive coating. It is not possible according to Xia to obtain such stable monodisperse dispersions without using additional PVP as a stabilizer.

EP 1493780a1 describes the preparation of a conductive surface coating using a liquid conductive composition composed of a binder and silver particles, wherein the above silver-containing particles may be silver oxide particles, silver carbonate particles or silver acetate particles, which may each have a particle size of 10nm to 10 μm. The binder is a polyphenol compound or one of a plurality of resins, i.e. in each case at least one additional polymer component. According to EP 1493780a1, an electrically conductive layer is obtained by heating after application of such a composition onto a surface, wherein the heating is preferably carried out at a temperature of 140 ℃ to 200 ℃. The conductive composition described according to EP 1493780a1 is a dispersion in a dispersant selected from alcohols such as methanol, ethanol and propanol, isophorone, terpineol, triethylene glycol monobutyl ether and ethylene glycol monobutyl ether acetate. In this case, it is stated in EP 1493780a1 that the silver-containing particles are preferably dispersed in a dispersant by adding dispersion stabilizers such as hydroxypropyl cellulose, polyvinylpyrrolidone and polyvinyl alcohol to prevent aggregation. These dispersion stabilizers are also polymeric components. Thus, the silver-containing particles are sterically stabilized in the dispersant by preventing aggregation at all times due to the above-mentioned dispersion stabilizer and the binder as the dispersion stabilizer.

Methods for preparing transparent, electrically conductive, metal nanoparticle-based coatings are disclosed in WO2006/13735a2 and US 7,566,360B 2. Here, the nano-metal powder is first processed into a uniform mixture with various additives, such as a surface active substance, a binder, a polymer, a buffer, a dispersant and a coupling agent, in an organic solvent. The nano-metal powder may also be silver nanoparticles. This homogeneous mixture is then re-mixed with water or a water-miscible solvent to obtain a water-in-oil (W/O) emulsion. The emulsion is applied directly by spraying, printing, spin-coating or dip-coating onto the surface to be coated, the solvent is removed and the coating is sintered, wherein a conductive and transparent coating or structure is obtained. The formation of networks from metal nanoparticles is also described.

The disadvantage of the above-mentioned water-in-oil emulsions is, inter alia, that they have to be washed at least twice with water before they can be used, so that the desired transparent, electrically conductive coatings based on metal nanoparticles can be prepared therefrom.

Self-organized into honeycomb structures by means of emulsion techniques CNT (carbon nanotubes) or SWNT (single-walled carbon nanotubes), described by N.Wakamatsu et al in Ad.Funct.Mater.2007,19, 2535-.

The self-organization of silver nanoparticles in polymeric systems is described in the publication Polymer40(1999)6169 to Minzhi Rong.

There is still a need for alternative methods for coating surfaces with electrically conductive coatings using dispersions comprising silver nanoparticles, wherein short drying times and sintering times and/or low drying temperatures and sintering temperatures can be used, so that temperature-sensitive plastic surfaces can also be coated. Also desirable are alternative coating agents for producing highly conductive coatings which in particular can also have good transparency, and particularly low-cost and simple methods for producing them, which for example enable complicated washing or cleaning of the emulsion to be discarded before it can be used.

According to the invention, a method for producing pickering emulsions for producing electrically conductive coatings is therefore proposed, in which

a) Mixing an aqueous dispersion comprising, inter alia, sterically hindered, stabilized silver nanoparticles and water, with at least one water-immiscible solvent, and subsequently dispersing into an emulsion, wherein the content of stabilized silver nanoparticles is between 0.5 and 7% by weight, based on the total weight of the emulsion obtained, and

b) subsequently separating the emulsion obtained in (a) during a standing time into an upper concentrated emulsion phase and a lower predominantly aqueous phase by forming an emulsion concentrate, and

c) separating the obtained upper concentrated emulsion phase, wherein the emulsion phase has a silver nanoparticle content of up to 7 wt. -%, preferably up to 4.5 wt. -%, based on its total weight.

In other words, the initial emulsion in step (a) according to the invention is prepared by dispersing stabilized silver nanoparticles (e.g. a silver nanoparticle sol), water and a water-immiscible solvent in one or more aqueous dispersants. The initial emulsion may preferably be an O/W emulsion. In this case, the oil phase of the O/W emulsion is formed by one or more water-immiscible solvents.

The silver nanoparticles cover the surface of the oil droplets and stabilize the oil droplets in the emulsion. The silver nanoparticle content given in% by weight refers according to the invention to the stabilized silver nanoparticle content, that is to say silver nanoparticles covered on their surface with a dispersion stabilizer.

The aqueous dispersant or dispersants is/are preferably water or a mixture comprising water and an organic, preferably water-soluble, solvent. Particularly preferably, the liquid dispersant(s) is/are water or a mixture of water with alcohols, aldehydes and/or ketones, particularly preferably water or a mixture of water with monohydric or polyhydric alcohols having up to 4 carbon atoms (e.g. methanol, ethanol, n-propanol, isopropanol, ethylene glycol), aldehydes having up to 4 carbon atoms (e.g. formaldehyde), and/or ketones having up to 4 carbon atoms (e.g. acetone or methyl ethyl ketone). A very particularly preferred dispersant is water.

Silver nanoparticles within the scope of the present invention are understood to have a d of less than 100nm, preferably less than 80nm, measured by means of dynamic light scattering₅₀Silver nanoparticles of value. Suitable for measurement by means of dynamic light scattering are, for example, the Zetaplus Zeta potential analyzer from Brookhaven Instrument Corporation.

The stabilization of the silver nanoparticles in the aqueous silver nanoparticle dispersion used (for example silver nanoparticle sols) is preferably carried out according to the invention with sterically hindered dispersing aids, for example polyvinylpyrrolidone, block copolyethers and block copolyethers with polystyrene segments, very particularly preferably Disperbyk190 (BYK-Chemie, Wesel).

The silver nanoparticle sol used for preparing the pickering emulsion according to the invention has a high colloidal chemical stability by using a dispersing aid. The choice of dispersing aids also allows the optimum particle surface properties to be adjusted. The dispersion aid attached to the surface of the particles may, for example, impart a positive or negative surface charge to the particles.

In principle, it is also possible, however, according to the invention that the silver nanoparticles are electrostatically stabilized. For the electrostatic stabilization of the silver nanoparticles, at least one electrostatic dispersion stabilizer is added during the preparation of the dispersion. Electrostatic dispersion stabilizers in the sense of the present invention are understood to be those which, owing to their presence, provide a repulsive force to the silver nanoparticles and, on the basis of this repulsive force, no longer tend to aggregate. Thus, due to the presence and action of the electrostatic dispersion stabilizer, there is an electrostatic repulsive force between the silver nanoparticles that resists the van der waals forces that cause the silver nanoparticles to aggregate.

Particularly preferred electrostatic dispersion stabilisers are citric acid and/or a salt of citric acid, for example lithium citrate, sodium citrate, potassium citrate or tetramethylammonium citrate. In aqueous dispersions, salt-like electrostatic dispersion stabilizers are present predominantly dissociated into their ions, with the individual anions serving for electrostatic stabilization.

In step (b), the initial emulsion from step (a) undergoes formation of a cream. Here, the initial emulsion separates during the standing time into an upper concentrated emulsion phase and a lower (mainly aqueous) emulsion phase. According to the invention, the upper concentrated emulsion phase is also referred to as the emulsion phase or the emulsion layer. In other words, the emulsion phase advantageously contains a higher concentration of droplets of the oil phase, as the oil droplets rise during the resting time.

In step (c) of the process according to the invention, the emulsion phase is subsequently separated. Here, the emulsion phase may comprise stabilized silver nanoparticles in a content of up to 7 wt. -%, preferably of up to 4.5 wt. -%, based on the total weight of the isolated pickering emulsion.

In other words, the pickering emulsion according to the invention is formed by the emulsion phase. Here, the oil droplets in the emulsion phase are also covered on their surface by silver nanoparticles, so that a sufficient concentration of silver is advantageously achieved in the emulsion phase. It is thus ensured according to the invention that the coating material obtained is suitable for the production of electrically conductive coatings.

The concentration of silver nanoparticles in the emulsion phase relative to the concentration in the initial emulsion may also advantageously be increased according to the invention when the initial concentration of silver nanoparticles in the initial emulsion is relatively low. This is advantageous in particular in terms of a cost-effective coating process, since then a suitable coating material for producing an electrically conductive coating can be obtained with relatively little use of silver nanoparticles according to the invention.

The process according to the invention for preparing the coating agent (i.e. the pickering emulsion) is simple and inexpensive. It was surprisingly found that the pickering emulsions provided with the process according to the invention are furthermore particularly stable and can be stored for example for several days. The process of the invention is further advantageous in that the pickering emulsion obtained in step (c) is suitable in particular as a coating agent for producing electrically conductive, in particular also transparent, coatings on substrates.

Furthermore, in the case of the process according to the invention, it may be advantageous to dispense with the use of additional additives, such as binders, dispersing aids and film formers, which slow down the drying and/or sintering of the surface coating obtained from step (c) from the pickering emulsion according to the invention, or even require elevated temperatures until drying and/or sintering and thus the conductivity of the surface coating occurs by sintering of the silver particles.

In a preferred embodiment of the process, provision is made for the standing time of (b) to be from 1 hour to 5 days, preferably from 6 hours to 3 days, particularly preferably from 12 hours to 36 hours, for example 24 hours. This standing time proved to be particularly suitable for forming stable pickering emulsions with good properties for the preparation of electrically conductive coatings.

In another preferred embodiment of the process, the content of silver nanoparticles of the initial emulsion of (a) is preferably between 0.7% and 6.5% by weight, particularly preferably between 0.7% and 3.0% by weight, based on the total weight of the initial emulsion obtained in (a). By adjusting the silver nanoparticle content within this preferred range, pickering emulsions can be obtained and isolated in subsequent steps (b) and (c), which emulsions exhibit particularly advantageous properties for forming electrically conductive coatings. After the pickering emulsion separated in (c) is applied as a coating agent, the self-organization of the silver nanoparticles into a network structure, for example a honeycomb structure formed by the nanoparticles in such a coating, is also promoted, in particular by adjusting the silver content of the initial emulsion within this preferred range. It is furthermore possible that the pickering emulsions obtained from these preferred initial emulsions according to the invention are also rich in silver nanoparticles and can have a higher content of silver nanoparticles than the initial emulsion.

With regard to further features of the process according to the invention, explanations relating to the pickering emulsion according to the invention and the use according to the invention are explicitly mentioned here.

According to the invention, for the purpose of solving the problem according to the invention, a pickering emulsion for producing electrically conductive coatings is also proposed, wherein the emulsion comprises stabilized silver nanoparticles, water and at least one organic, water-immiscible solvent, wherein the stabilized silver nanoparticles are present in an amount of 0.5 to 7 wt. -%, preferably 0.7 to 6.5 wt. -%, particularly preferably to 5 wt. -%, for example to 3.5 wt. -%, based on the total weight of the emulsion.

The pickering emulsions provided according to the invention are suitable as coating agents for producing electrically conductive structures, in particular for forming reticulated honeycomb structures by self-organization of silver nanoparticles, and can also be advantageously used for producing transparent electrically conductive structures, in particular for producing continuously connected transparent electrically conductive networks. The self-organization of silver nanoparticles into a honeycomb structure is advantageous in that no complex printing processes or expensive techniques are required to obtain a conductive structure. In addition, the honeycomb structure is transparent and/or helps to improve the transparency of the resulting structured coating.

As already stated above within the context of the description of the production process, the Pickering emulsion according to the invention as coating agent preferably comprises small, sterically stabilized silver nanoparticles which essentially have a d of approximately 80nm₅₀And is stable in a colloidal state in the silver nanoparticle sol used. The stabilized silver nanoparticles have a low concentration according to the invention of from 0.5% to 7% by weight, preferably from 0.5 to 5% by weight, particularly preferably to 4.5% by weight, for example to 3.5% by weight, without additional dispersing assistants being included in the pickering emulsion. It is estimated that also due to the low concentration being sufficient to achieve a low post-treatment temperature of 140 c,to achieve a surprisingly high electrical conductivity of the formed structure after the pickering emulsion has been applied as a coating agent on a substrate and dried.

In a preferred embodiment of the pickering emulsion according to the invention it is therefore provided that the emulsion does not comprise additional surface-active compounds, binders, polymers, buffers, film formers or dispersants. Advantageously, the pickering emulsions according to the invention are therefore free of substances which might additionally reduce the conductivity of the coating produced thereby.

This means that no additives, in particular no additional surface-active compounds, binders, polymers, buffers or dispersants, have to be added to obtain a pickering emulsion suitable for preparing electrically conductive coatings. The pickering emulsions according to the invention therefore have the additional advantage that they are not only inexpensive but also easier to prepare than coating agents with additional additives, such as surface-active compounds, other dispersing assistants or polymers. It is also advantageous that the steric hindrance of the silver particles is reduced due to these additional additives and that good electrical conductivity of the coating produced from the pickering emulsion according to the invention can be ensured, in particular even at relatively low post-treatment temperatures.

In a preferred embodiment of the pickering emulsion according to the invention, the organic solvent is at least one linear or branched alkane, optionally alkyl-substituted cycloalkane, alkyl acetate or ketone, benzene or toluene. Examples of suitable organic solvents according to the invention are cyclohexane, methylcyclohexane, n-hexane, octadecane, ethyl acetate, butyl acetate, acetophenone and cyclohexanone, where this list is not to be understood as exhaustive. The use of these solvents as oil phase makes it possible to prepare stable pickering emulsions according to the invention which are particularly well suited as coating agents for producing electrically conductive structures, in particular for forming reticulated honeycomb structures by self-organization of silver nanoparticles.

In a further preferred embodiment of the pickering emulsion according to the invention, it is provided that the organic solvent and water are preferably contained in the emulsion in a ratio of 1:4 to 1:2, for example in a ratio of 1:3 (in% by weight).

Within the scope of a preferred embodiment of the pickering emulsion according to the invention, the silver nanoparticles introduced in the pickering emulsion, for example in the form of a silver nanoparticle sol, are sterically stabilized by a dispersing assistant. The dispersing assistants used for the steric stabilization according to the invention are preferably selected from the following series: polyvinylpyrrolidone, block copolyethers, and block copolyethers having polystyrene blocks. Particular preference is given to using polyvinylpyrrolidones having a molar mass of about 10000amu (e.g.PVP K15 from Fluka) and polyvinylpyrrolidone having a molar mass of about 360000amu (e.g.PVP K90 from Fluka); and particularly preferably block copolyethers with polystyrene blocks, which have 62% by weight, based on the dried dispersing assistant, of C₂Polyether, 23% by weight of C₃Polyether and 15% by weight of polystyrene, where C₂Polyethers and C₃The ratio of the polyether block lengths is 7:2 units (for example Disperbyk190, BYK-Chemie, Wesel).

Preferably, the dispersing aid is present in an amount of up to 10 wt%, preferably 3-6 wt%, based on the silver content of the particles. By selecting such a concentration range, it is ensured on the one hand that the particles are covered so extensively with the dispersing aid that the desired properties, such as the stability of the emulsion, are ensured. On the other hand, excessive coating of the particles with dispersing aids is thus avoided according to the invention. An unnecessary excess of dispersing aid can adversely affect the properties of the pickering emulsion to be produced and the coating produced therefrom in an undesirable manner. Furthermore, too much dispersing aid may be detrimental to the colloidal stability of the particles and may hinder optional reprocessing. Moreover, an excess of dispersing aid reduces the conductivity of the coating produced by the pickering emulsion, or even causes insulation. All the above mentioned disadvantages are advantageously avoided according to the present invention.

With regard to the other features of the pickering emulsion according to the invention, explanations relating to the method according to the invention and the use according to the invention are explicitly mentioned here.

The invention also relates to a method for coating a surface in full or partial area, wherein

AA) applying the pickering emulsion according to the invention to the surface over the entire or partial area,

AB) the surface thus coated is subsequently covered with a covering such that water and solvent can evaporate,

AC) drying the thus covered, coated surface at least one temperature below 40 ℃ in order to remove water and organic solvents,

AD) the coating thus dried is subsequently sintered in the presence or absence of a covering.

The pickering emulsion according to the invention is applied in step (AA), for example by spraying, dipping, flow coating or knife coating. For example, the pickering emulsion may also be applied by means of a pipette. Surprisingly, when applying the pickering emulsion according to the invention, it has been shown that oil droplets encapsulated with silver nanoparticles are still present, which thereby advantageously influences the self-organization of the silver nanoparticles and the formation of a honeycomb structure. The good self-organization of silver nanoparticles enables the formation of continuous structures, wherein the use of complex printing processes or expensive techniques for producing them can advantageously be dispensed with.

By means of the covering placed on the surface coated with the pickering emulsion in step (AB), the drying rate of the wet layer can advantageously be influenced in a targeted manner, so that a continuous network can be formed from the silver nanoparticles. It was furthermore surprisingly established that the covering additionally promotes the self-organization of the silver nanoparticles, in particular the formation of a honeycomb structure from the silver particles. By forming the honeycomb structure from silver particles, good transparency can advantageously be achieved according to the invention in addition to good electrical conductivity.

The drying in step (AC) is carried out at least one temperature lower than 40 ℃ in order to remove water and organic solvents. It has been shown that under these drying conditions, in particular with a low thermal load, and the concomitant slow evaporation of water and organic solvent, particularly good conditions can be created for the formation of the desired honeycomb structure by the silver nanoparticles. Furthermore, the drying conditions described are also suitable for plastic substrates.

Within the scope of a preferred embodiment of the process, the drying in (AC) is carried out at least one temperature below 35 ℃, particularly preferably at room temperature. The drying step is thus very gentle and the formation of a continuously connected reticulated honeycomb structure is advantageously achieved by the silver nanoparticles.

In another embodiment of the method, the drying in (AC) may be carried out for a time of 15 minutes to 36 hours.

Within the scope of a further preferred embodiment of the method, the covering can be a glass or plastic plate, a plastic film or a synthetic nonwoven or nonwoven, preferably a water-and solvent-permeable covering.

If a water and/or solvent impermeable cover is used, the water and/or solvent may evaporate, for example, through the edge region between the substrate and the cover. An example of this is to cover a glass slide coated with pickering emulsion with another glass slide. If such a substrate covering arrangement is used, this is also referred to as a sandwich method according to the invention.

In a preferred embodiment according to the invention, the water and/or solvent permeable cover may be a porous filter cloth. For example, Monodur Polyamide (PA) filter cloth (VERSEIDAG Co.) can be used. It is commercially available in various mesh sizes and can be used adaptively according to the solvent used. Such a filter cloth is advantageous in that drying can be performed more uniformly on the surface of the base material than on the impermeable cover. Furthermore, the drying time can be shortened. Here, it is advantageously possible to achieve as good or even better results as are achieved in the sandwich method with regard to a suitable network or honeycomb structure of the self-organization of the silver nanoparticles. Furthermore, silver nanoparticles adhere relatively poorly to such a covering, so that the risk of damage to the already formed, optionally not yet sintered, silver nanoparticle structure can be significantly reduced.

In a further preferred embodiment of the method according to the invention, provision is made for the surface to be coated in full area or in part area, the surface being a surface of a glass substrate, a metal substrate, a ceramic substrate or a plastic substrate. The plastic substrates can be composed, for example, of Polyimide (PI), Polycarbonate (PC) and/or polyethylene terephthalate (PET), Polyurethane (PU), polypropylene (PP), which can optionally be pretreated with the pickering emulsion according to the invention and/or provided with a primer, for example, in order to ensure sufficient wettability. Preferably, the substrate may additionally be transparent.

Within the scope of a further preferred embodiment of the coating method according to the invention, the sintering in (AD) is carried out at least one temperature of more than 40 ℃, preferably at least one temperature of from 80 ℃ to 180 ℃, very particularly preferably at least one temperature of from 130 ℃ to 160 ℃, for example at 140 ℃. It is also possible, advantageously by means of the relatively low aftertreatment temperatures, that the coating method according to the invention can also be used for producing transparent, electrically conductive structures on temperature-sensitive substrates, for example polycarbonate films. The invention is also advantageous in that even at low thermal loads, it is possible to obtain very well adhering, electrically conductive structures on substrates, such as glass supports, and polycarbonate films, for example. The electrically conductive structure is in particular a structure having a resistance of less than 5000 Ω/m. Furthermore, the coating obtained according to the invention may be transparent, which is particularly advantageous for various applications.

Another subject of the invention is a conductive coating obtained by the process according to the invention, wherein the conductive coatings have the additional advantage that they are transparent. Such a conductive transparent coating can form, for example, a conductor circuit, an antenna element, a sensor element or a connection to a semiconductor module contact.

The transparent and electrically conductive coating according to the invention can be used, for example, as transparent electrodes for displays, screens and touch panels, as electroluminescent indicators, as transparent electrodes for touch switches, as transparent shields for electrodes and auxiliary electrodes, for example for solar cells or in OLEDs, in the application of plastic spectacle lenses, as transparent electrodes for electrochromic layer systems, or as transparent electromagnetic shields. This advantageously replaces or supplements expensive layers and structures made of tin-doped indium oxide (indium tin oxide, ITO).

The following examples are intended to illustrate the invention without being understood to be limiting thereto.

Examples

The apparatus used for the analysis:

1. with the aid of a solid balance: ag concentration determination by METTLER TOLEDO HG53 halogen moisture analyzer

2. Determination of the rheological characteristics and the stability of the milk layer:

the measuring instrument is as follows: MCR301 SN80118503

The measurement system comprises: CC17-SN 7448; d =0mm

And (3) measuring distribution: 21 measurement points;

measurement point duration 30 … 2s log

25℃

d (γ)/dt =0.1.. 1E + 31/s log; i slope | =5 points/dec

3. The droplet size distribution was determined and the emulsion stability was checked by means of an optical microscope:

LEICA DMLB optical microscope

4. With the aid of a ring tensioner: 20002901 determination of surface tension

5. Dispersing equipment: ULTRA-TURRAX (IKA T25 digitalULTRA-TURRAX)

6. Ultrasonic probe of dispersion equipment (G.Heinemann, ultrashall undLabortechnik)

7. And (3) measuring the resistance: a multimeter: METRA Hit 14A

UV-VIS spectrophotometer:

HEWLETT 8452 A

PACKARD diode array spectrophotometer

Example 1: preparation of silver nanoparticle Sol (Nano silver Dispersion)

0.054 mol of silver nitrate solution was mixed with a mixture of 0.054 mol of sodium hydroxide solution and the dispersing assistant Disperbyk190 (manufacturer BYK Chemie, Wesel) (1 g/l) in a volume ratio of 1:1 and stirred for 10 minutes. To the reaction mixture was added 4.6 moles of aqueous formaldehyde solution with stirring to make Ag⁺The ratio to the reducing agent was 1: 10. The mixture was warmed to 60 ℃ and held at this temperature for 30 minutes, and then cooled. The particles are separated from the unreacted reactants in a first step by diafiltration. The sol was then concentrated, for which a 30000 dalton membrane was used. A colloidally stable sol with a solids content of 21.2% by weight (silver particles and dispersing assistant) was produced. The proportion of Disperbyk190 after membrane filtration according to elemental analysis was 6% by weight, based on the silver content. The effective particle size was found to be 78nm by means of laser correlation spectroscopy.

Example 2: preparation of Pickering emulsion

The silver nanoparticle sol from example 1, water and organic solvent were mixed and treated with an ultrasonic probe at 50% amplitude for 3 minutes to prepare an O/W emulsion. After standing for 24 hours, the emulsion phase was characterized.

The droplet size distribution, viscosity, surface tension, and solid content of the stabilized silver nanoparticles of the pickering emulsion were determined. The results are summarized in Table 1.

TABLE 1

Example 3: coated substrate

The pickering emulsion prepared from the initial emulsion in example 2 was coated onto a glass slide, and the wet layer formed was covered with another glass slide (sandwich method) or by placing a porous, water and solvent permeable filter cloth and dried at a temperature below 35 ℃. It was confirmed that a honeycomb structure was formed from the silver nanoparticles in the dried film. To achieve the electrical conductivity of the coating with a honeycomb structure, the dried film was sintered at 140 ℃ after 4-12 hours of cover removal. The resistance of the obtained coating was measured on the honeycomb film with a multimeter at a distance of 1cm between two strips of about 0.3cm wide and 1cm long. The transmission is determined by means of a UV-VIS spectrophotometer.

Example 3.1:

500. mu.l of the emulsion phase were applied to a slide [25mm/75mm/1mm (b/l/h) ] by means of an Eppendorf pipette and covered with the same slide as a cover slip (coverstock). The wet film covered with the coverslip was then dried overnight at room temperature. In this case, water and the organic solvent may be volatilized from the edges of the formed slide glass multilayer structure. After removing the cover glass, the resulting dry film was sintered at 140 ℃ for 12 hours. The formation of the honeycomb structure can be observed with an optical microscope.

Determination of the solid content of the stabilized silver nanoparticles in the pickering emulsion and the conductivity and transmission of the resulting coating:

the results are summarized in Table 2.

TABLE 2

Example 3.2:

5000. mu.l of an emulsion phase were applied to a slide [100mm/200mm/4mm b/1/h ] by means of an Eppendorf pipette and covered with the same slide as a cover slip (cover). The wet film covered with the coverslip was then dried at room temperature for five days. In this case, water and the organic solvent may be volatilized from the edges of the formed slide glass multilayer structure. After removing the cover glass, the resulting dry film was sintered at 140 ℃ for 12 hours. The formation of the honeycomb structure can be observed with an optical microscope.

Determination of the solid content of the stabilized silver nanoparticles in the pickering emulsion and the conductivity and transmission of the resulting coating:

the results are summarized in Table 3.

TABLE 3

Example 3.3:

5000. mu.l of the emulsion phase were applied to a glass slide [100mm/200mm/4mm b/1/h ] by means of an Eppendorf pipette and drawn down with a 200 μm wet layer. On the wet layer was placed a filter cloth PA having a mesh size of 100 μm. The wet film covered with the filter cloth was then dried at room temperature for 1 hour. In this case, water as well as the organic solvent may be volatilized through the pores of the filter cloth. After removing the filter cloth, the resulting dry film was sintered at 140 ℃ for 12 hours. The formation of the honeycomb structure can be observed with an optical microscope.

Determination of the solid content of the stabilized silver nanoparticles in the pickering emulsion and the conductivity and transmission of the resulting coating:

the results are summarized in Table 4.

TABLE 4

Example 3.4:

mu.l of the emulsion phase were dropped onto a glass slide [25mm/75mm/1mm (b/1/h) ] by means of an Eppendorf pipette and drawn down with a 50 μm wet layer. On the wet layer was placed a filter cloth PA having a mesh size of 100 μm. The wet film covered with filter cloth was then dried at room temperature for 30 minutes. In this case, water as well as the organic solvent may be volatilized through the pores of the filter cloth. After removing the filter cloth, the resulting dry film was sintered at 140 ℃ for 12 hours.

The test was conducted with cyclohexane once and n-hexane once as the organic solvent used to form the oil phase in the pickering emulsion.

The formation of the honeycomb structure was observed with an optical microscope, respectively.

Determination of the solid content of the stabilized silver nanoparticles in the pickering emulsion and the conductivity and transmission of the resulting coating:

the results are summarized in Table 5.

TABLE 5

Example 3.5:

coating tests similar to the examples described above on untreated polycarbonate films by means of knife coating or spraying were unsuccessful due to poor wetting.

Example 3.6:

1000. mu.l of an emulsion layer were applied to a TiOx-coated polycarbonate film [100mm/100mm/0.17mm (b/1/h) ] by means of an Eppendorf pipette and drawn down in a 100 μm wet layer, wherein good wetting was achieved. On the wet layer was placed a filter cloth PA having a mesh size of 100 μm. The wet film covered with the filter cloth was then dried at room temperature for 1 hour. In this case, water as well as the organic solvent may be volatilized through the pores of the filter cloth. After removing the filter cloth, the resulting dry film was sintered at 140 ℃ for 12 hours. The test was performed with cyclohexane once and methylcyclohexane once as the organic solvent used to form the oil phase in the pickering emulsion. The formation of the honeycomb structure can be observed with an optical microscope.

Determination of the solid content of the stabilized silver nanoparticles in the pickering emulsion and the conductivity and transmission of the resulting coating:

the results are summarized in Table 6.

TABLE 6

Example 4: influence of silver concentration

The mixtures listed below were prepared as described in example 2 and subsequently applied, dried and sintered as in example 3. And determining the content of the silver nanoparticles in the emulsion. Two silver dots were then applied with a silver paste on the conductive coating at a distance of 1cm, and the resistance was measured. In addition, the droplet size of the emulsion was determined by an optical microscope. The results are summarized in Table 7.

TABLE 7

Example 5: analysis of silver content in emulsion phase (Pickering emulsion)

First, a Ag sol was prepared analogously to example 1, said sol having a silver nanoparticle solids content of 18.5%.

A pickering emulsion was then prepared analogously to example 2 (ultrasound probe at 50% amplitude for 3 minutes) and the stabilized silver nanoparticle solid content was determined after 20 hours and after 5 days, respectively. It may be shown that a favourable concentration of silver nanoparticles stabilized in the emulsion phase can be obtained at the very initial concentration of the silver nanoparticle sol in the relatively low initial emulsion.

The results are summarized in Table 8.

TABLE 8

Claims (15)

1. A process for the preparation of a pickering emulsion for the preparation of electrically conductive coatings,

a) mixing an aqueous dispersion comprising, inter alia, sterically hindered, stabilized silver nanoparticles and water, with at least one water-immiscible solvent, and subsequently dispersing into an emulsion, wherein the content of stabilized silver nanoparticles is between 0.5 and 7% by weight, based on the total weight of the emulsion obtained, and

b) subsequently separating the emulsion obtained in (a) during a standing time into an upper concentrated emulsion phase and a lower predominantly aqueous phase by forming an emulsion concentrate, and

c) separating the obtained upper concentrated emulsion phase, wherein the emulsion phase has a silver nanoparticle content of up to 7 wt. -%, preferably up to 4.5 wt. -%, based on its total weight.

2. The method according to claim 1, characterized in that the standing time in (b) is 1 hour to 5 days.

3. The process according to claim 1 or 2, characterized in that the silver nanoparticle content of the emulsion of (a) is from 0.7% to 6.5% by weight, based on the total weight of the emulsion obtained in (a).

4. Pickering emulsion for the preparation of electrically conductive coatings, in particular according to one of claims 1 to 3, characterized in that the emulsion comprises stabilized silver nanoparticles, water and at least one water-immiscible organic solvent, wherein the stabilized silver nanoparticles are contained in an amount of 0.5% to 7% by weight, based on the total weight of the emulsion.

5. Pickering emulsion according to claim 4, characterized in that it does not contain additional surface-active compounds, binders, polymers, film formers, buffers or dispersants.

6. Pickering emulsion according to claim 4 or 5, characterized in that the organic solvent is at least one linear or branched alkane, optionally alkyl-substituted cycloalkane, alkyl acetate or ketone, benzene or toluene.

7. Pickering emulsion according to at least one of claims 4 to 6, characterized in that the emulsion comprises an organic solvent and water in a volume ratio of 1:4 to 1: 2.

8. Pickering emulsion according to at least one of claims 4 to 7, characterized in that the silver nanoparticles are sterically stabilized.

9. Method for coating a surface in full or partial area, characterized in that,

AA) applying a pickering emulsion according to at least one of claims 4 to 7 to a surface over the entire or partial area,

AB) the surface thus coated is subsequently covered with a covering such that water and solvent can evaporate,

AC) drying the thus covered, coated surface at least at a temperature below 40 ℃,

AD) the coating thus dried is subsequently sintered in the presence or absence of a covering.

10. The process according to claim 9, characterized in that the drying in (AC) is carried out at least one temperature below 35 ℃, preferably at room temperature.

11. The process according to claim 9 or 10, characterized in that the drying in (AC) is carried out for a period of time of 15 minutes to 36 hours.

12. Method according to at least one of claims 9 to 11, characterized in that the covering is a glass or plastic plate, a plastic film or a synthetic nonwoven or nonwoven, preferably a water and solvent permeable covering.

13. The method according to at least one of claims 9 to 12, characterized in that the surface is a surface of a glass substrate, a metal substrate, a ceramic substrate or a plastic substrate.

14. Method according to at least one of claims 9 to 13, characterized in that the sintering in (AD) is performed at least one temperature above 40 ℃, preferably at least one temperature in the range of 80 ℃ to 180 ℃.

15. Conductive coating produced according to a method according to one of claims 9 to 14, characterized in that it is transparent.