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Description  |
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FIELD OF THE INVENTION
This invention relates to a pressure sensitive electro-conductive material
which becomes more conductive, that is, less resistant to electrical
current, when pressure, i.e. a force, is applied to the material.
BACKGROUND OF THE INVENTION
A number of prior art products have been made which are conductive and
flexible. These products include materials made by drying and polymerizing
dispersions of conductive carbon in a binder of elastomer. In a number of
the prior art products, the carbon is wetted and ground to a fine paste
which is mixed with a polymeric binder. The resulting composition is dried
and cured to form a conductive, flexible material. The conductive carbon
is ground to submicroscopic size using a high shear methods. The bulk of
carbon is reduced to a size below 0.1 micrometers. Such finely ground
carbon appears as a brown haze in the microscope. The carbon "grind"
prepared by conventional mixing is considered unsatisfactory because the
carbon particles are intimiately adsorbed to the binder and conductivity
is achieved only with an excess of carbon resulting in a randomly mixed
bulk composition of poor pressure-conductive properties. In an alternative
prior art process, the carbon particles are dispersed dry in a semi-solid
prepolymer or monomer under high shear by milling action, and the mixture
is cured and solidified to form a conductive rubber which show
conductivity but poor pressure-conductive characteristics.
The prior art conductive rubbers require a high carbon loading and
sufficient binder to maintain an integral structure of the conductive
rubber. Silicon rubber with dispersed conductive carbon is an example of
such a conductive rubber. Because of the required high carbon loading,
conventional conductive rubbers do not possess strong integrity and are
cast into thin sheets. It is especially difficult to coat and difficult to
obtain pressure sensitive coatings with prior art conductive rubbers.
Most of the conventional conductive rubbers upon the application of
pressure or mechanical force do not exhibit a significant, if any, change
in electrical resistance. Such material is treated and used as a fixed
resistance material. Expensive shaping and specially designed electrodes
are required to produce pressure sensitive electro-conductive devices from
conventional conductive rubber. Thus, direct application of the
conventional conductive rubbers does not result in a useful force
discriminating sensor which can sense beyond opened/closed positions.
Moreover, the conventional conductive rubbers cannot be used in touch
feed-back systems and directly monitored switches which indicate closed
circuits with open switches. Where a surface is roughened and formed into
irregular geometry, the function of sensitivity with pressure is limited
and difficult to control.
My U.S. Pat. No. 4,054,540 is directed to an electric resistant element
sensitive to pressure comprising a substantially discontinuous phase of
metallic conducting particles in a matrix of a cured elastomeric resin.
The metallic conducting particles are coated with a deformable,
semi-conducting compound. The element has a high loading of metal
conducting particles to resin of from 75:100 to 110:100 by weight.
My U.S. Pat. No. 4,120,828 is directed to finely divided metal particles
coated with a deformable, electrically semi-conductive compound. The
particles can be employed in an electric resistant element which is
sensitive to pressure.
U.S. Pat. No. 4,258,100 is directed to a pressure sensitive
electric-conductive sheet material comprising at least one layer of
rubbery elastic material and an adhesive layer disposed on at least one of
the surfaces of the sheet. Both layers having substantially uniform
distributed fine particles of electric conductive metal. The particle size
of the fine metal particles is from 10-1000 mesh and the loading of the
sheet material of metal particles to the rubbery elastic material is
10:100 to 800:100 by weight.
SUMMARY OF THE INVENTION
The present invention is directed to a deformable pressure sensitive
electro-conductive switch comprising first and second electrodes and a
deformable pressure sensitive conductive film sandwiched between the first
and second electrodes. The film comprises an elastomeric composition
impregnated with electrically conductive microagglomerates of finely
divided unbound carbon particles.
The electrically conductive micro-agglomerates of unbound finely divided
carbon particles are enclosed in a matrix of finely divided carbon
particles bonded together by an elastomeric composition. The
micro-agglomerates are roughly spherical shaped and have a maximum
dimension of between about 0.1 and about 10 microns; preferably between
about 0.3 and 2 microns.
The deformable pressure sensitive conductive material is prepared by a
process comprising the steps of:
(a) preparing a solvent system comprising water, a water-miscible,
carbon-wetting organic solvent and a surfactant,
(b) dispersing finely divided carbon into the solvent system to form a
uniform slurry,
(c) allowing the slurry to soak until the external surface of substantially
all the carbon particles are wetted by the solvent system to form a
pre-agglomeration composition
(d) ultrasonically dispersing the pre-agglomeration composition into an
elastomeric-carbon composition to form an elastomeric compositon
containing electrically conductive micro-agglomerates.
The pressure sensitive electro-conductive material has a relatively high
resistance (or low conductance) at rest, that is, when not pressed or
subject to a force, and a lower resistance when subject to pressure. The
material is sensitive to forces as low as one ounce per square inch or
less and as high as 100 pounds, or higher, per square inch. For example,
the material has been used to detect the removal or placement of a quarter
coin and the encroachment of pets and adults on a 3 square foot area.
The pressure sensitive electro-conductive material of the present invention
can be utilized to make pressure sensitive switches for alarm systems,
detection systems, counting systems, safety systems and the like. For
example, a switch can be made by sandwiching the material between two
electro-conductive electrodes attached to a detection system having a
voltage source, and signalling unit such as a light, bell, horn or the
like. The switch could be applied to a floor or platform to detect the
presence of an object, such as a person, vehicle, cart or box, when the
object encroaches, rolls over, or rests on the switch to complete the
circuit between the electrical supply and the signalling element.
Similarly, the switch can be applied to dangerous areas around machinery
and connected to a shut-off device for the machinery. In the event someone
encroaches a danger area, the weight of the person closes the switch, that
is, makes the switch more conductive, to complete the circuit between the
switch and the shut-off device to stop the machinery. Similarly, the
switches can be used to determine when a door is closed or opened by
placing a switch between the hinge plates of a door to compress or squeeze
the switch when the door is closed to complete the circuit in a detection
system. The switch can also be used as a transducer in a weighing device
since conductivity of the switch changes with the applied force over a
wide range of force.
The material has a threshold pressure at which point its conductivity will
increase with increasing pressure placed on the material. The responsive
characteristics of the material has an upper conductivity limit. When the
upper conductivity limit is reached, further pressure on the material will
not increase the conductivity. The conductivity range is relatively broad
and the material can be calibrated to function as a transducer for
weighing systems. In addition, the material can be utilized as a variable
resistor, the resistivity of which can be altered by applying or removing
force from the material. Thus, the material can be employed as a variable
resistor in a wheatstone bridge type circuit to alter the response range
of the circuit.
The pressure sensitive electro-conductive switches can be utilized as a
control means in an electrical apparatus for carrying out a pre-determined
operation that is at least partically controlled by a pressure sensitive
electro-conductive switch comprising:
electrical powered output means for powering a system of said apparatus;
voltage source for energizing said electrical powered output means;
pressure sensitive electro-conductive switch means connected to said
electrical powered output means and said voltage source to switch the flow
of electrical current from said voltage source to said electrical powered
output means to carry out a pre-determined operation, said switch means
comprising first and second electrodes, and a deformable pressure
sensitive electro-conductive material sandwiched between said first and
second electrodes, said material comprising a matrix of an elastomeric
material and electrically conductive micro-agglomerates, wherein the
micro-agglomerates comprise unbound finely divided electro-conductive
carbon particles enclosed by the elastomeric material and finely divided
electrically-conductive carbon particles bound together by the elastomeric
materials.
BRIEF DESCRIPTION OF THE DRAWINGS
These and other features, aspects and advantages of the present invention
will be more fully understood when considered with respect to the
following detailed description, appended claims and accompanying drawings,
wherein:
FIG. 1 is a schematic cross-section of the pressure sensitive
electro-conductive material of the present invention;
FIG. 2 is an enlarged cross-section of the pressure sensitive
electro-conductive material of the present invention;
FIG. 3 is a schematic cross-section of a switch employing the pressure
sensitive electro-conductive material of the present invention;
FIG. 4 is a schematic cross-section of an alternate embodiment of the
electro-conductive material of this invention;
FIG. 5 is a schematic plan of a circuit employing a switch of this
invention;
FIG. 6 is a graph depicting the resistivity of a pressure sensitive
electro-conductive material of the present invention with low resistivity
under different pressures (pounds per 4 square inches and 6 square
inches):
FIG. 7 is a graph depicting the resistivity of a pressure sensitive
electro-conductive material of the present invention with intermediate
resistivity under different pressures (pounds per 1 square inch and 4
square inches); and
FIG. 8 is a graph depicting the resistivity of a pressure sensitive
electro-conductive material of the present invention with high resistivity
under different pressure (pounds per 1 square inch and 4 square inches).
DETAILED DESCRIPTION
Referring to FIG. 1, a pressure sensitive electro-conductive material 10
provided in accordance with principles of this invention is shown.
The term "pressure sensitive electro-conductive" as used herein means that
the material is less conductive in the normal state, i.e. the non-press
state, than when a force or pressure is applied thereto. The material 10
comprises a plurality of micro-agglomerates 12 of unbound finely divided
carbon particles dispersed in a layer of rubbery elastomeric material 14.
The micro-agglomerates 12 comprise finely divided carbon particles
enclosed in a matrix of finely divided carbon particles bonded together by
the elastomeric material. The agglomerates can be visualized as very small
voids in the elastomeric composition containing a large number of unbound
finely divided carbon particles. The surface or wall of the void is the
bonded matrix of carbon particles.
Not intending to be bound by theory, it is believed that when a force is
applied to the two opposing greater surfaces 16 and 18 of the pressure
sensitive electro-conductive material, that is, when the matrix is
compressed, the electrically conductive micro-agglomerates 12 are
compressed and thereby deformed forcing the unbound finely divided carbon
particles into close proximity enhancing the conductivity across the
micro-agglomerates. Each micro-agglomerate is in close proximity to at
least one other micro-agglomerate. Thus, when a compressive force is
applied to a portion or all of the pressure sensitive electro-conductive
material, a conductive pathway is established between the two opposing
greater surfaces 16 and 18 of the material. The more pathways that are
established, the greater is the conductivity of the material.
A unique feature of the pressure sensitive, electro-conductive composition
of the present invention is that the resistivity response is both force
and area dependent. For example, the resistivity of a film will be
different for a force of 10 pounds applied to 1 square inch than for a
force of 40 pounds applied to 4 square inches or a force of 60 pounds
applied to 6 square inches (See FIGS. 6, 7 and 8 and Examples 6, 9 and
10). This response is not due to inconsistencies in the film; a unit of
force applied to a unit of area at any location on the film will give
substantially the same change in resistivity. It has been found that for a
given current and applied force, the resistivity decreases with increasing
area (See FIGS. 6, 7 and 8). A discriminating detector element can be
prepared from the composition employing this unique property. The
discriminating detector can discriminate between objects of a given weight
with different base area, such as a 100 pound crate with a foot square
base and a 100 pound table with four legs each having a one square inch
base.
Another unique feature of the present invention is that the resistivity
respone is ampreage dependent. For example, the resistivity of a film is
different for a 100 nanoamp signal than a 100,000 nanoamp (100 microamp)
signal (See FIG. 8 and Example 9). Thus the resistivity response range of
a detector utilizing the composition can be altered by increasing or
decreasing the signal amperage.
Films of the pressure sensitive, electro-conductive composition can conduct
signals having potentials of between about 0.2 and about 25 volts and
currents of between about 10 nanoamps and 1 milliamp. However, the films
can be utilized in circuits having lower or higher signal potentials
and/or lower signal currents. Utilization of signal currents exceeding 5
milliamps is not recommended unless the signals are of short duration
and/or the film is adequately cooled to remove the heat generated in the
film by high current signals, and/or the conduction cross-sectional area
is large, for example 6 square inches per 1 milliamp.
The elastomeric composition is an elastic, rubbery, deformable material
prepared from natural rubbers, synthetic rubbers of synthetic plastic
materials. These materials include natural rubber, isoprene rubber,
styrene butadiene rubber, butadiene rubber, chloroprene rubber, nitrile
rubber, butyl rubber, ethylenepropylene rubber, chlorinated polyethylene,
styrene, butadiene block copolymer, plasticized polyvinyl chloride,
polyurethane and the like. Preferably the elastomeric material is
polyurethane.
The carbon particles making up the electro-conductive micro-agglomerates
are conductive carbon black such as electrically-conductive oil-furnace
carbon black and the like. The carbon particles have a particle size of
about 10 millimicrons to 100 millimicrons, preferably about 15
millimicrons to 75 millimicrons. Conductive carbon black of less than 10
millimicrons can be used; however, conductive carbon particles of such
size are generally not commercially available. Carbon particles larger
than 100 millimicrons have not been found to be satisfactory in the
practice of the present invention because they do not form satisfactory
micro-agglomerates. Conductive carbon blacks are differentiated from other
carbon blacks by their high surface area (about 100 to about 2000 meters
per gram) and low volatile content (about 1.0 to about 3.0 percent by
weight).
The electro-conductive micro-agglomerates are prepared by preparing a
solvent system of water, a surface active agent and a water miscible,
carbon-wetting organic solvent.
The choice of surfactant is not critical to the invention. Water soluble
anionic, cationic, nonionic, or amphoteric surfactants may be employed;
however, nonionic surfactants are preferred since they are more strongly
absorbed on the surface of electro-conductive carbon particles than other
surfactants. Examples of anionic surfactants that can be employed include
the alkylaryl ethers of polyethylene glycol and the pluronic F108 and L62
surfactants of BASF Wyandotte Corporation.
The organic solvent must be miscible in water, soluble in the surfactant,
able to wet the surface of the carbon and able to form a separate phase in
which the carbon remains as a stable agglomerate when the carbon slurry in
dispersed into an elastomeric composition as described herein. Examples of
solvents that can be employed in the present invention include the glycol
ethers, water-soluble esters, water-soluble polyethylene glycols,
water-soluble organic amines and water-soluble polar solvents such as
dimethyl sulfoxide and dimethyl formamide. Examples of glycol ethers that
can be used in the solvent system include methyl, ethyl, butyl, and higher
ethers and dimethyl, diethyl and dibutyl ethers of ethylene glycol,
dipropylene glycol, triethylene glycol, propylene glycol, dipropylene
glycol and tripropylene glycol. Diethylene glycol butyl ether has been the
solvent of choice.
For pH control, a small amount of water soluble basic material may be added
to the solvent system to counteract the pH effect of the carbon particles.
Typical bases that can be employed include sodium metasilicate, methyl
diethanol amine, sodium hydroxide, sodium carbonate and the like.
The solvent system preferably comprises, by weight percent, from about 2.0
to about 15 percent of a water immiscible, carbon-wetting organic solvent,
from about 0.05 to about 1.0 percent of a surfactant and the balance
substantially water. Preferably sufficient organic solvent is employed to
function as film former during the drying stage of the elastomeric-carbon
composition described herein. It has been found that if the solvent system
contains less than 2 percent by weight of an organic solvent, the
formation of micro-agglomerates is adversly affected, and the solvent has
little, if any, film former action. It has been found that if the solvent
system contains more than one percent by weight of a surfactant, the
micro-agglomerates have a tendency to break into a conductive network
during film formation and form a non-pressure sensitive film of the
elastomeric-carbon composition described herein.
After the constituents of the solvent system have been dissolved, the
electro-conductive carbon particles are added to the solvent system to
form an electro-conductive carbon slurry. The slurry can contain from
about 7.5 to about 20% carbon by weight. It has also been observed that if
the solvent system contains less than 0.05% by weight of a surfactant, the
micro-agglomerates are not formed as described herein. The slurry can
contain less than 7.5% by weight carbon; however, a slurry with a low
carbon loading will produce a pressure sensitive electro-conductive
material with a much higher at rest resistance than a material prepared
from a slurry containing between about 7.5 and about 20% by weight carbon.
The slurry is allowed to stand or soak for at least one day, preferably
from about 3 to about 7 days, in order that the external surface of the
carbon particles may be fully wetted by the solvent system to thereby form
a pre-agglomeration composition. To enhance the wetting action, the slurry
can be stirred and/or heated. However, it has been found that the wetting
action will occur with time without stirring or heating. The carbon
particles have a complex surface and to improve control of the surface, a
basic material is added. If the surface is acidic, the pH of the slurry or
paste is adjusted to between about 7 and about 10 by the addition of a
basic material to the solvent system to avoid breaking the binder
emulsion.
The wetting action on the carbon particles is crucial to the preparation of
electro-conductive micro-agglomerates. If the surface of the carbon
particles are not sufficiently wetted, the resulting electro-conductive
carbon slurry, when added to an aqueous elastomeric composition, will not
form the desired electro-conductive micro-agglomerates. The slurry in
uniformly dispersed into the elastomeric composition to form the
electro-conductive carbon micro-agglomerates.
Referring to FIG. 2, which is an enlarged cross section of the
electro-conductive material shown in FIG. 1, it can be seen that the
material is composed of the elastomeric composition 14 impregnated with a
plurality of micro-agglomerates 12. Several micro-agglomerates 12 are
speckled to illustrate the free, unbound, finely-divided carbon particles
contained therein; all micro-agglomerates 12 contain free, unbound, carbon
particles. The elastomeric composition 14 occupies the space between the
micro-agglomerates 12. The micro-agglomerates, which are generally
spherical, have a diameter of from about 0.1 to about 10 microns,
preferably from about 0.3 to about 2.0 microns.
It has been observed that if the micro-agglomerates are larger than 10
microns the agglomerates tend to break when the elastomer-carbon
composition is coated onto a substrate. When the agglomerates break, the
carbon particles within the agglomerate disperse into the elastomeric
composition and, frequently, form conductive pathways between the two
greater opposing surfaces of the film. Such conductive pathways can short
circuit the material. It has been found that if the agglomerates are of
less than 0.1 microns, the material has poor action or pressure
sensitivity, and a low at rest conductance. The best materials prepared
have micro-agglomerates of an average size between about 0.3 and about 2.0
microns.
As explained herein, the size of the micro-agglomerates is primarily
controlled by the surfactant and solvent concentration of the solvent
system and the pH of the carbon slurry. The preferred size of the
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