Note: Descriptions are shown in the official language in which they were submitted.
CA 02509608 2005-06-09
AQUEOUS PRINTABLE ELECTRICAL CONDUCTORS
Description
The present invention relates to the preparation of electrical conductors
which can be printed on
substrates and used as electrical circuits in intelligent and active
packaging, sensors, RFID antennae, etc.
These electrical conductors are identified as Aqueous Printable Electrical
Conductors or "APEC's".
Background of the Invention
Printable inks that can be used in different applications, such as those
envisaged herein are well
known. Such inks are set forth, for example, in US Patent No. 6,379,745 of
Parelec and in a US Patent
Application of Flint Ink (No. 2003/0151028).
Such inks have serious limitations which limitations are discussed
hereinbelow.
The printable electrical conductive materials of the '745 patent are
comparable, and in some
cases superior with regard to conductivity, to aqueous printable electrical
conductive materials. However,
they are not aqueous and the substrates with which such inks are used are
generally limited to plastics,
which are heat resistant and are not recyclable. In addition, these printable
electrical conductive
materials require high temperature curing, thus using more heat energy. They
require a special heating
chamber, which produces gas emissions during heating.
The Flint Ink materials are not able to reach the conductivity of the
technology described herein,
although they are also aqueous electrical conductive materials (i.e. inks) and
are designed to print both
on paper and cardboard. They are not printable on plastics. The Flint Ink
conductive ink materials are
less shelf-stable, separate easily, require heat for dying and curing, are
more difficult to control during
printing press applications, and are much more sensitive to pH variations.
Neither of the aforementioned applicants describes how they could provide a
commercially
applicable means to ensure electrical conductivity using an aqueous media
combined with ambient
temperature air curing, resulting in the production of industry-useable
devices on a number of existing
printing press set-ups common in world markets.
Other patents, such as US 5,492,653; 5,286,415; 4,715,989 etc., describe
aqueous silver, other
metallic and carbon flakes-based compositions as well as water-based
conductive thick film inks that are
mostly used as coating compositions with primary applications being sprays,
paints or printable screens.
They are always used with organic co-solvents with possible uses in EMI
(electromagnetic interference)
and RFI (radio frequency interference) shielding. Their electrical
conductivity is much lower than that
which is described by the methodology in this patent application and they are
not applicable in the ways
so described for aqueous printable electrical conductors.
CA 02509608 2005-06-09
Summary of the Invention
The present invention involves a dispersion comprised of powder or flakes of
highly conductive
material suspended in an aqueous acrylic, urethane/acrylic mix, or other
vehicle. This dispersion is
capable of being deposited onto a substrate using different printing methods
for production of various
devices. The preferred morphology of the conductive material is flaky. The
metal of choice is silver (Ag)
with particle sizes ranging from 0.001Nm to 50Nm (normal is 1-8pm). The
surface of the metal flakes is
preferably chemically treated but this does not exclude untreated flakes in
which case the surface
treatment is performed in-house as a first step before preparation of the
dispersion. Fatty acid treated
flakes from large batch producers currently work the best.
The first step of the method for making APEC's is preparation of the vehicle
(i.e. the printing
media) for the metal powder or flakes. The vehicle for this type of printing
application has surface
properties that are different when compared to conventional printing ink
vehicles. The requirement for
different surface properties is the result of the aqueous printing processes
needing higher than normal
surface pressures, lower resin content, low molecular weight polymers, and the
use of polymers not
capable of intensive cross-linking, which characteristic minimizes the
influence of dielectric binders and
increases electrical conductivity. Additionally, the preparation of APEC's
requires very low or no use of
additives (e.g. adhesion promoters, antifoaming agents, waxes, etc.). The
dispersion used to make
APEC's retains acceptable printing characteristics but requires more frequent
on-press additive addition
in negligible amounts (less than 1 %) compared to conventional printing inks.
An APEC requires 2 to 4
times more frequent addiction of adjusters. The APEC's also possess mechanical
resistance, shelf life
and other required properties without compromising the conductivity of the
deposited electrical conductor.
The second step in creating APEC's is selecting the correct metal powder or
flake. The metal's
surface treatment needs to be selected to ensure compatibility of the metal
with the vehicle system, thus
ensuring dispersion stability. Fatty acid treated particle surfaces are
preferred. The treatment of the
metal particle's surface ensures a very thin, low molecular weight layer that
does not affect the
conductivity significantly. The surface treatment also regulates the surface
pressure of the metal
particles. Surface treatment substances migrate towards the film part of the
deposited conductor as part
of the film-forming process. The surtace treatment substances are critical to
ensuring the printability of
conductive traces for different applications.
The third step is the process of dispersing the metal into the printing
vehicle. This process
requires slow mixing of the metal with the selected vehicle. A dispersion
mixer with specially shaped
mixing heads is used to ensure good and visually laminar flow. In preparing
conventional inks, grinding
aids and surfactants are added during this process to reduce the surface
pressure between mixing
surfaces. To avoid possible influences on the conductive properties, such
additives are not used in the
current process. Generally, the quantity of metal added is two to four (or
more) times the weight of the
vehicle. As a result of this high load, heat is produced during the mixing
process because of friction and
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CA 02509608 2005-06-09
increased mixing speed. This is avoided by regulating the mixing speed and
adding small aliquots of the
vehicle during mixing. When the metal has been incorporated into the base, the
dispersion is mixed at a
higher speed for a short time with care being taken not to introduce air into
the mixture. The temperature
of the dispersion should not exceed 30-35°C during mixing. In some
cases, the final product will need to
be filtered using an appropriate sized silk mesh filter. The mixing process
produces a visually
homogeneous mixture in liquid form that is stored in sealed bottles at ambient
and room temperature (5°C
to 30°C).
The viscosity of the APEC's varies inversely with the metal load. As an
example, for flexography
process printed applications, the viscosity ranges from 25 to 85 seconds
measured with a Zahn 3 cup
(approximately 500 to 3600 centipoises, based on the APEC's specific gravity
of 2.4-4). Gravure, screen,
and some other dry offset applications can be achieved by adjusting the
viscosity of the base
composition.
The viscosity and printing properties of APEC's should be adjusted prior to
printing on the press
and held constant during printing through the appropriate use of additives.
This can be achieved by
adding ammonia water and minimal amounts of specially selected antifoaming
agents if needed.
Depending on the printing method and duration of the print run, the addition
of 1.0 - 10% ammonia water
and 0.01 - 0.2% antifoaming agent (both by volume) may be necessary. Such
addition should be done
immediately prior to printing and the combination mixed well. The amounts and
frequency of later
additions are dependent on the ink properties, structure of the press, exposed
surfaces of the emulsion,
ambient temperature, humidity, printing speed and other factors. Closed ink
box systems, coupled to a
slow ink circulating pump, are best for keeping the physical properties of the
printed electronic conductors
constant.
Detailed description of the process
The conductivity of APEC's depends on a number of factors. The most important
is the position,
arrangement, and physical connection between the powder or flake metal
particles that are deposited
onto the printed film. Conductivity and stability are influenced by the film
drying process, the origin of the
heating/curing media, and external treatment processes such as applied
pressure and high energy light
treatments.
It is desirable to achieve proper orientation and close positioning between
the metal particles
without the formation of a thick polymer particle surface-wetting layer. This
is accomplished by selection
of raw materials and correct preparation of the vehicle. It is also necessary
to prepare the mixing surface
through the use of techniques such as particle surface treatment.
It is well known that the proper orientation of the particles is facilitated
by surface property
adjusters. These include: (1) surfactants, (2) adhesion promoters, and (3)
stabilizers. These chemicals
generally result in good particle wetting and stability of the dispersion
process. However, they also cause
a stable surface layer to form on top of the particles (metal particles in the
present case) thereby
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decreasing electrical conductivity. A trade-off exists when using surface
property adjusters. For these
additives to be used, they must be highly efficient and work in extremely low
quantities. It is also
preferable to use non-ionic stabilizers to reduce the forces of the particle
surface layers. The particle
stabilization process, including stabilization layer charging, increases the
shelf life of the dispersion but
reduces conductivity. The advantage of the conductive dispersion is also
related to the conductive
properties of water, which allows for the elimination of the electrical
charges resulting from friction created
by the mixing process.
To achieve improved conductivity with lower mixing time, low particle surface
wetting is used.
Low particle surface wetting also allows mixing to be achieved by shaking.
Shaking is limited to small
volumes and is not necessarily scalable for commercial serial printing.
Thin surface layers are required to ensure a long shelf life. With the current
process, shelf lives
of several months (e.g greater than 6 months) are achieved. Incompatibility
exists between the ammonia
water or any water media and the fatty acids coating the metal particles. This
incompatibility prevents the
metal particles from being completely stable. To achieve the greatest
conductive properties, the metal
particles must not be permanently wetted within the vehicle polymer layers.
The formation of a polymer layer on top of metal particle surfaces is
necessary to achieve proper
particle deposition during the printing process. The polymer layer also
facilitates adherence of the
particles to the substrate material, and enhances inter-particle consolidation
after the drying process.
APEC's tend to dry quickly because of the low percentage of liquid in the
metal dispersion. The
pH of the APEC is typically 7.5 to 8.5. The pH drops when the ammonia water
compound evaporates.
During this process the polymer system goes from a water-soluble state to a
water-insoluble state. The
drying process can be facilitated by forced convection with ambient or
slightly elevated temperature air.
This process plays a major role in determining the conductive properties of
the final product. While the
APEC is drying the polymers shrink. This is occurs when the polymers lose
their water bridges, which are
composed of hydrogen bond chemical connections.
The post printing drying process continues for 24 hours at ambient
temperature, until most of the
liquid has evaporated. This process results in a conductivity increase of up
to 50 percent. Several sub
processes take place during the drying process. Oxidization polymerization may
result in the formation of
oxygen cross-linking bridges. Other sub processes are characterized by
additional cross-linking of bonds
between the reactive polymer chains in the vehicle. Further cross-linking of
bonds results from Diels-
Alder reactions but only when conjugated double bond structures exist within
the vehicle system.
The final process of chemical drying results in a significantly reduced volume
of the printed film,
which affects film thickness and metal particle orientation. The metal
particles move closer to each other
due to the reduced volume of the printed film. At the same time, more cross-
linked polymers increase
the dielectric properties, thereby decreasing conductivity. The largest effect
is due to the orientation of
the metal particles, resulting in an overall increase in conductivity. Post-
drying or post-drying under
pressure gives increased conductivity, resulting in stable resistance readings
and a film that is less
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dependent on ambient temperature and humidity. These post-printing operations
should only be applied
in special cases, as they are not part of the standard printing process.
Normally, with flexographic printing, APEC's do not require special heating.
Room temperature
blown air is enough to obtain usable prints. However, with gravure and screen
printing, APEC's require
extra heat for thorough curing and better performance, especially when
materials of differing viscosities
are used in the printing process.
The preferable heating and curing methods employ IR heat sources that have the
ability to deliver
energy into the printed layer allowing drying to commence not from the surface
(as is the case when
using hot air to dry) but from within. Although these APECs are not UV
curable, application of UV drying
during flexographic printing offers two benefits: (1) UV sources produce
additional heat, and (2) UV light
leads to a slight destruction of binder polymers which increases electrical
conductivity.
A major advantage of APEC's is their ability to be printed on paper
substrates. Pricing benefits
(compared to printing applications requiring other substrates) and ecological
gains (via de-inking
procedures and recycling benefits) are other advantages of APEC's.
EXAMPLES
Example 1. APEC formulated as an ink for flexography to print UHF antennae.
1 ) Predominantly oleic fatty acid treated silver (particle size 1 to 5 pm
flakes shaped) is combined
and blended in a regular open air mixer with aqueous solution of 50% solids
acrylic resins in water (with
ammonia traces to keep pH in the pH window of 7.5-8.5) in a proportion 3.4 to
1 weight portions with no
additives.
2) After 20 minutes mixing with an average mixing speed of 1500 rpm with Hi-
Vane mixing head
(which does not allow the heat to exceed 30°C) 10% pure ammonia
preliminary diluted in water is added
via continuous mixing in 1.7 wt. % to the ink. The dispersion is mixed for 5
more minutes. Oleic fatty
acids react with ammonia to form ammonia soap according to the following basic
chemical reaction:
CH3(CHz)~CH=CH(CHZ)~COOH + NH40H = CH3(CHz)~CH=CH(CH2)~COONH4 + H20
The reaction is similar for other oils included with oleic acid - linolic and
linoleic acids respectively
with two and three double bonds. It is widely known that unsaturated 2 and 3
double bond fatty acids are
responsible for the formation of an elastic film after printing and oleic acid
does not dry at all. That acid
forms a nano layer on the top of the metal particles, helping to make the
silver flakes compatible with the
alkyl parts used in resins added to dispersions to provide stability and
printability. Such oleic acid nano
layers reduce the conductivity of the APEC, however, when oleic acid is
removed from the surface during
exposure to heat shock or other drying methods the printed structure will
exhibit an increase in sheet
conductivity.
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CA 02509608 2005-06-09
3) The obtained ammonia soap is an active surfactant which helps achieve
partial wetting of the
silver particles by washing part of the oil layer off the top of the metallic
flake particles. The reaction
essentially produces a lubricant that eases mixing.
Further, acrylic resins experience simultaneously the well-known influence of
ammonia on poly
acids to regulate viscosity (change from coil polymer chains to stretched
chains), allowing adjustment of
pH and drying time and thus contributing to final conductivity.
4) A freshly prepared dispersion of the APEC is printed on semi-gloss paper
and the resulting
prints tested for conductivity, printing properties, adhesion, and other
properties, with the following results:
Before any heat treatment the measured conductivity is 0.4 Ohms/square on a
fresh printed
sample (3-4 Nm thick films) and increases to 0.2 Ohms/square over the
following 24 hours. After a heat
shock is applied the conductivity is increased by 100% to less than 0.1
Ohms/square at the same
thickness.
To print UHF RFID antenna patterns using the APEC described in this example, a
Mark Andy
2200 flexography press is used with 13 BCM Praxair anilox drum to print by
flexography method with Du
Pont Cyril photopolymer plates (hardness 42) at speeds up to 350 feet per
minute with preliminary
addition of 0.01 % antifoam just before the press starts printing. The results
are kept stable with the
addition of ammonia water to re-establish the initial volume every 20 minutes.
Stable in-line conductivity
values are verified with the proprietary high-speed in-line resistance
feedback device. This process
results in fully working RFID transponder labels made in-line on a standard
flexogrpahic press with a chip
attachment device, using chips from Texas Instruments and Alien Technologies.
The transponders
provide read ranges of 14-25 feet.
Example 2. A nanotechnology for increasing APEC conductivity.
The process comprises:
1) The ink is made as described in Example 1, but without or with only a
minimal amount of
binder resin and 80-100 % water emulsion of pre-reacted fatty acids with
different metal salts ( Ag, Pd, Pt,
Au, Cr, Ni, Cu, Na, K, Mg etc). to form metal soaps and corresponding
surfactants. The main reaction is:
CH3(CHZ)~CH=CH(CH2)~COOH + AgCI = CH3(CHZ)~CH=CH(CHZ)~COOAg + HCI
This is a method for making silver soap of oleic acids. The process is similar
for other fatty acids.
At high energy treatment (e.g. thermal shock at 200° C for 1-3 seconds)
fatty acid chains are destroyed
but the Ag metal remains to form a nano dispersion. Nano dispersed metals (Ag
in the example) create
conductive bridges between the silver flakes, thus increasing the conductivity
of the printed traces. A
further benefit of this process is that some hydrochloric acid (HCI) is also
released. HCL reduces the pH,
which helps keep the polymer at low viscosity (coil configuration) at the same
solids content.
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Example 3. APEC formulated as an ink for flexography to print smart packages
1) Predominantly oleic fatty acid treated silver (particle size 1 to 5 pm
flakes shaped) is combined
and blended in a regular open air mixer with aqueous solution of 50% solids
acrylic and resins water
emulsion in a proportion 2.4 to 1 by weight with 0.1 %
polyethylene/polypropylene wax, a silicon based
adhesion promoter 1 %, an antioxidant 0.1 % and antifoam 0.01 %. A plasticizer
is added if needed as half
the minimal amount recommended by the manufacturer.
2) Mixing at an average speed of 1500 rpm with a Hi-Vane mixing head (which
does not allow the
heat to exceed 30° C) results in an APEC with a minimum shelf life of
six months.
Fatty acids react with ammonia to form ammonia soaps as described in Example
1. No additional
ammonia is added when making smart packaging ink, but it is necessary to add
ammonia on the press to
recover the initial volume. The initial ammonia comes from the ammonia
existing in the pH adjuster of the
acrylic resins. It is well known that unsaturated 2 and 3 double bond fatty
acids are responsible for the
formation of an elastic film after the printing. There is no requirement for
special heating above room
temperature blown air. If extra heat is applied the unsaturated fatty acids
polymerize to form a thin
dielectric layer on top of the prints, reducing conductivity. If reduction of
resistance is desirable a
combination of IR, UV and/or heat shock can be used. However this must be
applied under strict control
to avoid the opposite effect of the heat treatment, which can release
dielectric compounds into the APEC
structure and increase resistance.
3) The small amount of ammonia soap is still an active surfactant that
lubricates surfaces to ease
mixing and improves printability. The acrylic resins experience simultaneously
the well-known influence
of ammonia on poly acids to regulate viscosity (change from coil polymer
chains to stretched chains), and
to adjust pH and drying time. This also contributes to the final conductivity.
4) A dispersion of the APEC is printed (see below) on label stock 55 Cast
Gloss Elite (Avery
benison) and the prints tested for conductivity, stability on bending,
printing properties, adhesion, and
other properties, giving the following results:
The measured conductivity is 1.5-2 Ohms/square on a fresh printed sample (1-2
Nm thick films)
and this increases over the subsequent 24 hours to 0.8-1.0 Ohms/square.
Commercial example: A SOHN 4 colour 8 inch flexography label printing press is
used with 10.9
BCM anilox drum to print by flexography method with Du Pont Cyril photopolymer
plates (hardness 42) at
speeds up to 100 feet per minute. Results are kept stable by the addition of
extra ink and extender every
1 hour of printing. A lamination and die cutting station is used in-line to
create pharmaceutical smart
package inlays (commercially produced under the Med-ic~ trademark) with an
additional RFID sensor
chip attachment. Such printed smart packages are able to record removal of
medication doses from a
blister package and display the results on a computer screen using an
associated RFID reader and
software.
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Similar technology but with different viscosity and resin binders is used to
make gravure, screen,
dry offset etc. APEC's.
In summary of the foregoing it will be seen that the present invention
provides a number of
significant advantages and improvements having regard to the known prior art
and the general practice of
preparing electrically conductive inks for printing on different substrates.
Thus the present invention may
be seen to contemplate the following:
1. A method for creating aqueous printable electrical conductors (APEC's)
which includes
preparation of a relatively stable dispersion but with intentionally partially-
wetted silver flake particles to
increase conductivity. To achieve the greatest conductivity, the metal
particles should not be
permanently wetted within the vehicle polymer layers. The basic chemistry of
the process is
demonstrated (example 1 ).
2. A method for making (APEC's) which includes (as a means to increase the
electrical
conductivity) a vehicular system using (a) low molecular weight polymers, (b)
polymers incapable of
intensive cross-linking and (c) other polymer systems that minimize the
influence of dielectric binders.
3. A method for making APEC's using ammonia treatment during dispersion
preparation and
during printing on commercial presses. Although chemically treated flakes
(e.g. with fatty acids and salts)
are generally known to be incompatible with ammonia, in the proposed method
this is used to create
partial silver particle wetting. This is unrelated to the common use of
ammonia as a pH and viscosity
adjuster for water based printing inks (see example 1 ).
4. A method for producing APEC's that requires little or no use of additives
(e.g. adhesion
promoters, antifoaming agents, waxes, etc.) during preparation but which
achieve and maintain desirable
printing properties including but not limited to mechanical resistance, shelf
life, etc. without compromising
the conductivity of the deposited electrical conductor.
5. A method for creating APEC's comprising a way to achieve proper orientation
and close
positioning between the metal particles without the formation of a thick
polymer particle surface-wetting
layer. This is accomplished by special selection of raw materials and correct
preparation of the vehicle. It
is also necessary to prepare the mixing surface through the use of techniques
such as particle surface
treatment.
6. A method for making an APEC that cures at ambient temperature with or
without air blowing
(depending on the printing speed) when printed in the thickness range of 1-8
Nm using flexography
process.
7. A method for creating an APEC suitable for thick film printing (> 8 Nm) by
screen or gravure
that is cured with a combination of infrared (IR) and ultraviolet (UV) light
sources to achieve optimum
conductivity. IR drying is more uniform than hot air drying because the
process does not start at the
surface but from the bulk of the film and UV light sources are also heat
sources whereby the UV light
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CA 02509608 2005-06-09
leads to destruction of oil films and sometimes of dry polymer chains in the
binder, giving increased
conductivity.
8. A method for creating an APEC suitable for application to substrates by
different printing
processes such as flexography, gravure, screen, or dry offset printing, and to
substrates including but not
limited to coated paper, uncoated paper, and plastics with treated and
untreated surfaces.
9. A method for making APEC's optimized to print on paper substrates and
having the highest
quality and pricing benefits compared to existing inks and coatings and with
ecological gains (via de-
inking procedures and recycling benefits).
10. A method for increasing the conductivity of an APEC by exposing the
printed surfaces
(preferably on the printed side) to a thermal shock during printing (e.g. by
touching the surface for 1 to 3
seconds with a hot metal drum at 120°C - 300°C mounted on the
printing machine). This increases the
conductivity of the printed matter more than 50 percent while reducing the
film thickness by 25 percent or
more depending on the thickness of the initially printed film. Although such
shock drying is not necessary
to cure thin-film inks (1-8 pm thickness), it can contribute to maximizing the
conductivity of the APEC.
11. A method for increasing the conductivity of an APEC by exposing the
printed surfaces to the
combined effect of ultrasound and heat to further increase the conductivity of
the printed traces.
12. A method for creating an APEC that facilitates chemical control over the
conductive properties
by means of selecting the type of fatty acids to treat the particle surface
(forming nano layers) and
removing these layers partially and in controlled fashion via chemical
reactions through soap (surfactant)
forming and subsequent suppression of the foam.
13. A method for creating an APEC that allows increasing the conductivity
chemically by means
of forming metal soaps of fatty acids that are susceptible to destruction by
oxidation and high temperature
shock, releasing a metal in nano dimensional form. This process creates extra
bridges between silver
flakes and increases conductivity.
14. A nanotechnology for increasing the conductivity of APEC's by forming
nanoscale dispersed
metals (Ag in the example) to create conductive bridges between the main
components (silver flakes).
15 A device and method for controlling the mechanical lay-down of APEC's on a
printing press by
utilizing an array of devices controlled by an in-line resistance measurement
toot (taking and reporting
between 10,000 and 1 million resistance measurements per second) connected to
a network of pump,
injector, pressure, speed and drying equipment controllers in real time (while
printing).
16. A graphical display that gives immediate feedback to the press operator of
improper APEC
laydown and thus allows both manual and automatic correction of the printing
process. Poorly printed
APEC structures can also be marked (e.g using ink jet markers or printers
inline) to avoid them being
utilized downstream (e.g. to skip chip attachment if the APEC structure is an
antenna).
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