Note: Descriptions are shown in the official language in which they were submitted.
~N ELE~ L~ CONDU~rI~L ~
The present invention relates to an electrically conductive
ink comprising a binder and an electrically conductive
constituent, in particular to a conductive polymer, and to a thick
film electrically resistive track made from the same.
Conductive polymer inks are known in the art. Japanese
Patent Application No. 60-218689 laid open under No. 62-7495
(Nihon Shashin Insatsu~ di6closes examples of an electrically
conductive ink in which the binder i~ a resin chosen Erom a group
termed 'heat -reaisting engineering plastics' and in which the
electrically conductive powder is silver. I'hi6 publication
further dicloses that tests have ahown that gold, silverl copper,
nickel and carbon are 6atlsfactory as the electrically conductive
powder used in the ink and accordingly claims an electrically
conductive ink characterized by the fact that the electrically
conductive powder is at least one of the powder~ cho6en from the
group consisting of gold, silver, copper, nickel and carbon.
As disclo6ed by N. Nazarenko and C.N. Lazaridis 'Polymer
Thick Film Conductor6 and Dielectrics for Membrane Sw~tches and
~0 Flexible Circuitry' (Publication reprinted from the Proceedings o
the 1982 ISHM Symposium) 5 it is known that products baving a low
resistance can be made from silver-based conduct~ve ink~.
However, in view of the cost of silver, there iB a need or
conductive inks containing less ~ilver. Figure 1 which is
taken from that publication, ~ho~s the variation of resi~tivity
with proportion by wei~ht oE blends of silver and a second
con~tituent. Curve 2 is for a silver/graphite blend system;
Curve 4 is for a blend system containing a silver-based polymeric
ink and a non conductive aluminium based polymeric ink. As can be
seen from the Figure, the resistivity of the inlc increases with
the proportion of the 6econd constituent.
It is an obJect of the present invention to provide a low
re~i~tivity e}ectrically conductlve lnk suitable for producing a
~ilm-type heating track.
According to the present invention there is provided an
electrically conductive ink comprising a binder and an
electrically conductive constituent, wherein the electrically
conductive con6tituent includes a dendritic metal. Thus, by
incorporating a dendritic metal into the electrically conductive
constituent, an ink of particularly low re~istivity, a9 compared
with the prior art i8 achieved.
Preferably, the electrically conductive constituent comprises
a mîxture of dendritic metal and carbon. The inventors have found
that an ink of low resitivity can be achieved by a mixture of a
dendritic metal and carbon, which is a contradistinction to the
teachings of the prior art, as shown in Figure 1.
Preferably, the dendritic metal consists of nickel.
Electrlcally conductive inks provided in accordance with the
present invention are particularly suitable for the manufacture of
heating elements for large areas such as door mlrrors in motor
vehicles. Heating elements so produced compare favourably in both
C05t and heating effect with prior art heating elements used for
6uch applications.
An embodiment of the present invention will now be described,
by way of example only, and with reference to the accompanying
drawings of which:
Figure 1 ~hows variation in re~itivity with relative
proportions of constituents ln prior art inks;
and figure 2 shows variation in resistivity with relative
proportions of nickel and carbon in an ink provided in accordance
with the present invention.
It ha~ been found that a method of manufacturing the
electrically conductive which employs a powdered dendritic
metal produces an ink with high electrical conductivity as
compared to the prior art.
A dendritic metal is one in which the surface of the metal
appears spiky, and in powderetl form, the granules are ~enerally of
equiaxial ~hape.
:3:
A method of maklng an electricaLly conductive ink provided in
accordance with the present invention is described below.
a) A stock solution of the binder is formed by dis~olving 20 g
of poly (vînyl butyral) granules (Butvar B76) in lO0~ of
N-methyl-2-pyrrolidone. Thifi is mixed using a high apeed stirrer
for 1 hour and then allowed to ~ettle until the air disper~es.
b) 55 ml of carhon black (type 40220) is baXed for l hour at
125C to remove any moisture. This is used with an equal
~olume of dendritic metal powder, nickel hafi been found to be
particularly suitable for obtaining a high-conductivity ink,
untilan homogenous blend is obtained. This form~ the electrically
conductive con~tituent.
c) A screen printing medium i3 produced by combi~ing 50 g of
Blythe medium type 63/182 wlth 5g of unaccelerated unsaturated
polye~ter in styrene ~anufactured by Scott Bader).
SOg of the binder and 100 g of the conductive con~tituent are
added to 55g of the screen printing medlum. The mixture i6 then
passed through a triple roll mill until the rheological properties
~uitable for screen printing known to those ~killed in the art are
obtained. The resulting ink is screen printed in the de~ired
pattern onto a aub3trate and cured at a temperature of 80C.
The ink exhibits good adheaion to a variety of types of
substratefi, including alumina, polyester, glass and painted
~urfaces. The re6i6ti~1ty of tracks made from thi~ ink wa~ in the
rflnge of from 1.5 to 2.0 ohm~ per square.
