Electrically Conductive Printable Inks and Methods of Manufacture Thereof

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims priority to United Kingdom Patent Application No. 1005977.2 filed April 9, 2010, the contents of which is incorporated herein by reference.

FIELD OF THE INVENTION

[0002] The present invention relates to printable inks and methods of manufacture thereof. Of particular interest are printable inks comprising metals such as copper, gold and silver. Silver is the preferred metal. The present invention relates to inks which comprise nanoparticles of such metals and in particular silver. The nanoparticles can be used as a conductive filler in printable inks such as ink-jet printable inks.

BACKGROUND OF THE INVENTION

[0003] Metal particles and in particular silver nanoparticles have been used as conductive fillers in printable inks such as ink jet printable inks. However, there are issues with their use. For example, when formulating an ink it is typical to take nanoparticles which have already been prepared for example from a commercial supplier. One such commercial supplier is

Nanodynamics of 901 Fuhrmann Blvd. Buffalo, New York, 14203. Typically, the silver particles of a desired size are precipitated from a solution comprising a silver salt and in the presence of other agents such as a reducing agent, stabiliser and base. One typical size of particles is approximately 30 nm.

[0004] However, after formation of the nanoparticles, there is typically

precipitation/agglomeration of the silver particles. This occurs because there is often removal of all other materials involved in the production process such as waste products/by-products.

Agglomerations of up to 5 μπΐ in size can occur. Such agglomerations are however not suitable for use in applications which are desirable in the present invention, in particular, ink-jet printing processes.

[0005] Typically high energy is required to break the agglomerations apart. Also, the agglomeration tends to reoccur. Dispersion of nanoparticles which have already been agglomerated in this way is typically difficult. This is because the metals in question, and in particular silver, have a high Hamaker constant that causes the particles to agglomerate under the influence of Van der Waals attraction forces. In particular, it is difficult to achieve a stable dispersion which comprises substantially non-agglomerated nanoparticles, i.e. with an average particle size substantially the same as the originally formed nanoparticles.

[0006] It is known that dispersants can be employed to prevent agglomeration. Typically the dispersant is employed by adding it to a suspension of particles and then subjecting the dispersion to an energetic dispersion process. High-energy dispersion can be applied using a high-energy source such as an ultrasonic disperser or a micro-fluidiser. Such dispersants typically provide one or both of an electrostatic or steric barrier towards agglomeration.

[0007] Typically in print inks, such as those including silver, the metal is provided to confer electrical conductivity on the ink. The problem therefore that arises is that dispersants can interfere with the formation of metal-to-metal bonds and there is an adverse effect on the overall conductivity of the printed material.

[0008] There is a requirement therefore to carefully choose the amount and type of dispersant employed to ensure that it does not deleteriously affect the printed material.

SUMMARY OF THE INVENTION

[0009] The present invention provides inks suitable for printing, and methods for forming same, which confer good properties on the printed material. In particular, with the present invention it is possible to include a dispersant which has good efficacy in dispersing which additionally has compatibility with the printing solution, yet which does not interfere with the properties, in particular the conductivity, of the printed material.

[0010] One particular end use application of the present invention is to provide for re-dispersion of agglomerated nano-silver particles within a process for the preparation of electrically conducting ink-jet printable inks.

[0011] The present invention provides a printable ink comprising:

(i) a liquid vehicle;

(ii) nanoparticles of metal wherein the metal is selected from the group consisting of silver, copper and gold, or an alloy thereof and wherein the nanoparticles are dispersed in the liquid vehicle; and

(iii) a dispersant component comprising a co-polymer of maleic acid and polyisobutylene. [0012] Such inks are very useful in the printing of conductive films. Desirably the metal is silver. The expression "or an alloy thereof does not limit the alloy to including all of silver, copper or gold - it includes an alloy of any of those. Combinations of metals/alloys may be employed. Desirable alloys include those where at least two of silver, copper or gold are present.

[0013] The liquid vehicle is desirably a water-based or alcohol-based liquid vehicle or combinations thereof. Such a liquid vehicle is environmentally friendly. Where an alcohol-based liquid vehicle is employed it may be an ethanol-based liquid vehicle.

[0014] One advantage of the inks prepared using the present invention is that they have an electrical resistivity which is less than 10 times that of bulk silver. Typical electrical resistivity for inks printed using the present invention is 1 x 10 ^"5 ohm.cm.

[0015] A further advantage of the inks prepared using the present invention is that the

formulated ink has been shown to be stable against agglomeration for a period of at least 50 days when stored at room temperature.

[0016] Another advantage of the present invention is that the printed ink sinters at a low temperature (temperatures below the normal melting temperature of bulk silver). For example, the printed ink can be sintered at a temperature of between 100° C and 260° C, more desirably between 100°C and 200°C, for between 1 minute and 120 minutes more desirably between 5 and 30 minutes. For example the ink will sinter sufficiently when dried at about 150° C for 30 minutes.

[0017] The technology of the present invention is suitable for printing onto many types of substrate including glass, PET and tapes for use in the electronics industry including soldering tape such as Kapton tape.

[0018] The technology of the present invention is suitable for many end-use applications including ink-jet printing, printable conductive inks, the manufacture of printed circuits, flat panel displays and RFID devices.

[0019] Desirably the metal nanoparticles are present in an amount from 10 to 80 wt %, such as 15-50 wt% for example 15 to 25% by weight of the ink composition. Desirably the dispersing agent is present in an amount from 0.25 to 4.5 wt %, desirably 0.4-1.5wt % (0.25 to 4.5%) by weight of the ink composition. Desirably the average nanoparticle size is the range from 5nm to lOOnm. Desirably the liquid vehicle is water or ethanol and combinations thereof. Typically the liquid vehicle forms 20 to 90 wt %, desirably 50 to 85 wt%, for example 47 to 52% by weight of the composition.

[0020] Further desirable components are now discussed:

[0021] Thickener components for example ethylene glycol, glycerol (both of which may also act as a reducing agent) and combinations thereof may be added to the composition. Desirably the thickener component(s) are present in an amount from 5 to 45 %, desirably 8 to 40 wt% for example 10.5 to 28.5% by weight of the composition;

[0022] Wetting agent components and combinations thereof may be employed, for example non- ionic surfactants including a synthetic alcohol ethoxylate, may be employed. An example of a synthetic alcohol ethoxylate which may be employed is sold under the trade name Synperonic 91/6 and combinations thereof. Desirably the wetting agent component is present in an amount from 0.01 to 1 %, desirably 0.1 to 0.66 wt% for example 0.33 to 0.66% by weight of the ink composition.

[0023] The composition of the invention may also include a conductivity-promoting component such as oxalic acid and acetic acid and combinations thereof. Desirably the conductivity- promoting component is present in an amount up to about 0.1 wt %, desirably 0.04 to 0.08 wt% for example 0.045 to 0.07% by weight of the ink composition.

[0024] The invention also thus provides a process for manufacturing a printable ink comprising:

(i) providing agglomerated nanoparticles of metal wherein the metal is selected from the group consisting of silver, copper and gold, or an alloy thereof;

placing the nanoparticles in a suitable liquid vehicle; and

(ii) breaking the agglomerated nanoparticles apart in the presence of a dispersant component comprising a co-polymer of maleic acid and polyisobutylene.

[0025] Desirably the liquid vehicle is a water-based or alcohol-based liquid vehicle or combinations thereof. In one arrangement the alcohol-based liquid vehicle is an ethanol-based liquid vehicle.

[0026] Desirably the metal is silver.

[0027] The ink may be formed directly at the end of this process. There is no requirement for separate steps to additionally formulate an ink. In particular, the re-dispersion step in the presence of the dispersion agent can be done in the presence of all additional components required (and any optional additional components) to (simultaneously) form the ink. There is no requirement for additional formulation work to form the ink. This aspect of the invention can thus be considered a "one-pot" or single step process.

[0028] The ink composition of the present invention is ink-jet printable to give a printed material which when sintered has excellent electrical conductivity.

[0029] It will be appreciated that all optional and/or preferred features of any embodiment of the invention may be combined with optional and/or preferred features of another/other

embodiment(s) of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Additional features and advantages of the present invention are described in, and will be apparent from, the detailed description of the invention and from the drawings in which:

[0031] Figure 1 is a plot of UV -absorbance spectra generated by the experimental work below.

DETAILED DESCRIPTION OF THE INVENTION

[0032] It should be readily apparent to one of ordinary skill in the art that the examples disclosed herein below represent generalised examples only, and that other arrangements and methods capable of reproducing the invention are possible and are embraced by the present invention. Experimental Details

[0033] The following details a process by which the present inventor made "Ink 5", a stable formulated ink-jet printable conductive ink according to the present invention.

[0034] The formulation contains nano-silver as supplied by Nanodynamics, under product code S230W (-45 wt% agglomerated nano-silver in water), ethylene glycol (thickener, reducing agent - medium volatility), glycerol (thickener, reducing agent, - low volatility), water, synperonic 91/6 (wetting agent), oxalic acid (conductivity promoter) and Orotan 731 (dispersant) [Orotan is a co-polymer of maleic acid and polyisobutylene].

[0035] The state of dispersion was monitored throughout by diluting one drop of the dispersion in water and measuring the absorbance in a lxl cm ² cuvette at 300-800nm using an ultra-violet visible spectrometer. The occurance of an absorbance peak at ~410nm is indicative of the presence of nano-particles, a shoulder at wavelengths greater than 410nm is caused by either large particle agglomerates or particles of a different shape.

[0036] Amounts used were as set out in the following Table:

[0037] The films cast with the ink formulation can be heated and generate high conductivity. For example, a film had conductivity of 1.67x10 ^"5 Ω-cm after annealing at 130 °C for 30 min;

another film had conductivity of 1.33xlO ^"5 Ω-cm after annealing at 150 °C for 15 min.

Standard Method to prepare Ink 5

[0038] Step 1 : The S230W and dispersant were added to a glass jar and mixed together, (low shear mixing) after which the UV-vis spectra may be obtained according to the description given below:

[0039] Step 2: The vessel containing the mixture was then transferred to an ultrasonic disperser (Soniprep 150) fitted with an ice cooling bath to maintain the temperature below 40° C. The mixture was then sonicated in a pulsed mode, (10 sec on 10 sec off) tuned at a frequency of 15kHz for a total time of 15 minutes. The UV-vis spectra may be obtained at anytime during the sonication process according to the description given below. This was done at 5 and 15 minutes of the process.

[0040] Step 3 : In a separate vessel all of the remaining components were added and mixed thoroughly (low shear mixing - components are miscible).

[0041] Step 4: The component mixture prepared in Step 3 was added to the silver dispersion and mixed thoroughly. As is advisable, the present inventor re-ran the UV-vis measurement to determine the extent of dispersion at this final stage.

Dispersion Characterisation

[0042] Step 1 : The UV-vis spectrometer was set to cover a range from 300nm to 800nm.

[0043] Step 2: The present inventor diluted a very small sample in deionised water in a 1cm by lcm curvette, placed the cuvette into the UV-vis spectrometer and obtained an absorbance versus wavelength scan. As a guide the sample should be dilute enough to almost see through and run the analysis. Results

[0044] The spectra shown in Figure 1 illustrate the results of the experimental procedure above. The dispersion includes S230W silver and Orotan 731 dispersant, unless noted otherwise. The solvent is water. The concentration of silver is 10% w/w, and Orotan 731 is about 1.4 % w/w. The dispersions were sonicated, unless noted otherwise. The sonication time was 5 min, 15 min, or 30 min respectively. Then the sample which was sonicated for 30 minutes was filtered with filter paper or 0.45 μηι glass fibre syringe. The filtration improved the dispersion quality. For comparison, the spectra of three other samples are also included in the Figure 1. (1) 10% w/w of silver with 1% Orotan 731, sheared with microfluidizer for 30 min; (2) Low concentration of silver dispersion (2% w/w) with 1% w/w of Orotan 731, sonicated for 30 min; (3) Cabot nanosilver ink AG-IJ-G- 100-S 1.

[0045] The words "comprises/comprising" and the words "having/including" when used herein with reference to the present invention are used to specify the presence of stated features, integers, steps or components but do not preclude the presence or addition of one or more other features, integers, steps, components or groups thereof.

[0046] It is appreciated that certain features of the invention, which are, for clarity, described in the context of separate embodiments, may also be provided in combination in a single embodiment. Conversely, various features of the invention which are, for brevity, described in the context of a single embodiment, may also be provided separately or in any suitable subcombination.