Note: Descriptions are shown in the official language in which they were submitted.
~25'79~
D~SCRIPTION
ENVIRONMENTAL INDICATOR DEVICE AND M~T~IOD
.
BACKGROUND OF THE ~NVENTION
1. Field of the Invention.
This invention relates to a method of measuring the
incremental environmental exposure of an indicator
device that includes a tuned circuit and an element that
is sensitive to the environmental exposure.
2. Backyround of t_e Invention
.
Several patents have disclosed methods for
measuring environmental exposure.
~ .S. Patent 4,189,399, issued February 19, 1980, to
Patel, discloses co-crystallized acetylenic compounds
useful in measuriny time-temperature or radiation-dosage
history of an article by a color change.
U.S. Patent 4,212,153, issued July 15, 1980, to
Kydonieus et al.~ discloses a two-layer time-temperature
indicator that changes color as the interior layer
migrates to the outer surface of the exterior layer.
European Patent Application, Publication No.
0117390, published September 5, 1984, discloses a
process for measuring environmental exposure of indi-
cator devices that comprise a composition whose optical
reflectivity changes incrementally with incremental
environmental exposure.
A characteristic of these prior art methods is that
the indicator reacts to the environmental exposure with
a change in color and/or optical reflectivity. A
quantitative measure of the exposure requires an optical
detector that can "see" the indicator. Thus, if the
indicator is monitoring the exposure of a product, it
must be at the surface of the product in order to be
read. If the product is packaged, the prior art methods
measure the environmental exposure of the outer surface
of the packaging, which may be substantially different
from the exposure of the product.
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SUMMARY OF_T~IE INVENTION
In accordance with the present invention, an indi-
cator device comprises a target that has a tuned
electrical circuit and that includes an element having
an electrical property that, in response to a particular
environmental parameter, changes in a predetermined
fashion, whereby the response of the target to an
interrogation signal, having a Erequency in the
microwave range or lower, can be related to the exposure
of the target to the parameter. In one embodiment of
the invention, the target also comprises an electrical
shield, which includes the element and which is posi-
tioned so that the response of the target to the inter--
rogation signal depends on the electrical properties of
the element.
In operation, an incremental environmental exposure
is measured by:
(a) measuring a first response to an interrogation
signal of a target that
(i) comprises a tuned electrical circuit and
(ii) includes an element having an electrical
property that changes in response to the
environmental exposure,
(b) measuring a second response, after the incre
mental environmental exposure of the target, and
(c) calculating the incremental environmental
exposure by using a pre-established relationship between
the response of the tuned circuit and the environmental
exposure.
The devices of the present invention find applica-
tion in monitoring the exposure of products to environ-
mental parameters, such as temperature, combined time-
temperature, humidity, radiation, a particular fluid
(gas, vapor or liquid), and mechanical shock.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a schematic of a system that incorporates
a device of the present invention.
Fig. 2 shows a modification of the system of Fig. l.
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Fig. 3 is a graph of the response of a time-
temperature indicator of the present invention.
Fig. 4 is a graph of the response of another time-
temperature indicator of the present invention.
DETAILED DESCRIPTION OF THE INVENTIOM
This invention provides a method for measuriny
environmental exposure by making use of an element whose
electrical properties change in response to the environ-
mental exposure. The element can be part of a tuned
circuit or it can be part of a shield for a tuned cir-
cuit. In either case, the tuned circuit is interrogated
before and after an incremental environmental exposure,
and the change in response can be related to the
exposure.
The indicator devices of this invention can operate
in either a go, no-go mode or in a quantitative mode.
In the go, no~go mode, the devices provide only two
responses - a null response or a positive response,
depending upon whether or not the environmental exposure
has exceeded a specified limit. In the quantitative
mode, a continuous change results from increasing
exposure to an environmental variable. The devices are
particularly useful for monitoring product quality. If
the effects of an environmental parameter on both
product quality and indicator response are known, a
quantitative measure of the effect of the parameter on
product quality can be deduced from a measurement of the
indicator response. Alternatively, operation in the go,
no-go mode can be used to indicate that exposure to an
environmental parameter has exceeded a critical value;
for example, that the exposure has been sufficient to
make the product unusable.
For purposes of this invention, the term "environ-
mental exposure" is to be interpreted broadly to include
temperature, time, time-temperature (i.e., the combined
effect of the two parameters), humidity, exposure to
actinic radiation, mechanical shock, exposure to speci-
fied fluids (gases, vapors, or liquids), etc.
--4--
The impedance of an electrical circuit depends on
its inductance, resistance, and capacitance and on the
driviny frequency. The current in a passive circuit
(i.e., one without an associated power supply) is a
maximum when the driving frequency has a particular
value, the "resonant frequency," and the circuit may be
said to be "tuned" to that frequency. If a target is
irradiated with an electromagnetic signal (an "interro-
gation signal") that includes the resonant frequenc~,
then the presence of the "tunecl circuit" in the target
can be readily detected by an appropriately situated
antenna to yield an output signalO If the target
includes, in addition to the tuned circuit, a shield in
the path of the interrogation signal, then the resultant
output signal depends on the extent of the shielding
eEfect, which, in turn, depends on the electrical con-
ductivity of the shield. If the impedance of the cir-
cuit changes, then the resonant frequency changes, and
the same incident interrogation signal yields a dif-
ferent response and different output signal.
Thus, any electrical property that affects the
circuit impedance can serve as the electrical property
whose change, in response to an environmental parameter,
underlies this invention. For example, the electrical
property can be capacitance, inductance, or conductance
(electronic or ionic). Similarly, if a shield is
present, a change in its electrical conductivity yields
a different response and different output signal, even
if the impedance of the tuned circuit remains
unchanged. Thus, by interrogating a target repeatedly
and monitoring the resultant output signal induced in an
antenna, a change in an electrical property of an ele-
ment of the target can be detected. The element may be
a part of a tuned circuit or a shield for a tuned cir-
cuit.
Fig. 1 depicts a schematic of a target of the
present invention as it would be used in the method of
this invention. Interrogation signal 10, which would
~.2~;7~Z~
typically be in the radio or microwave frequency range,
is incident on target 11. Target 11 comprises a tuned
circuit 12, which, in one embodiment of the invention,
includes an element having an electrical property that
changes in response to a particular environmental
parameter. In another embodiment, the target includes
shield 13. In that embodiment, shield 13 includes an
element sensitive to the environmental parameter, but
circuit 12 need not. In either case, the signal 14 that
emanates from target 11 is converted by an antenna 15
into an electrical current. Preferably, both the source
of the interrogation signal 10 and the antenna 15 are in
a single instrument package. As is shown in Fig. 2,
antenna 15 can be the source of interrogation signal 10.
Tuned circuits of the type suitable for use in this
~invention have been disclosed in U.S. Patent 4,321,586,
issued March 23, 1982, to Cooper et al., and in earlier
patents cited there. Cooper et al. used tuned circuits
in systems for theft detection, where the concern is
merely to distinguish betwen the presence and absence of
the tuned circuit. In those systems, the tuned circuit
is either removed or totally deactivated, by rupturing a
fusible link for example (see U.K. Patent Application
GB2 105 952 A, published March 30, 1983)~ These tuned
circuits, in the form of antitheft targets, are commer-
cially available. (Suppliers include Checkpoint
Systems, Inc., Thorofare, NJ, and Sensormatic Elec-
tronics Corp., Deerfield ~each, FL) Also available are
detection systems, which are used with the targets and
which typically respond to the amplitude or frequency of
an incident signal. Although the detectors generally
operate in the go, no-go mode, they can be readily
modified to provide a continuously variable measure of
the circuit parameter that is variable. A variety of
suitable commercially-available theft-detection devices
were disclosed in High Technology, pp. 16, 17
(September/October, 1983).
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If the elernent whose electrical properties change
is a shield, then a sheet containing that element is
situated in the path of the signal from the interro-
gation system. The shield may be either conducting
initially, and made non-conducting, or vice versa. The
signal detected decreases as shield conductivity
increases.
The signal that is detected in response to interro-
gation of a target of the present invention depends on
the relative locations and orientations of the interro-
gation source, the targetl and ~the detector. It is
important to insure that the "geometry" dependenc~ does
not mask the changes that result from environmental
exposure of the target. Consequently, if successive
measurements are to be made, the relative locations and
orientations of the source, target, and detector should
remain constant throughout. Alternatively, if in
addition to the indicator target, an inert target is
used, and the orientation of the two targets relative to
each other remains constant, then the response of the
"true" indicator target can be inferred from the
response of the inert target.
A key to the present invention is identifying an
element having an electrical property that changes in
response to the particular parameter of interest. A
charge-transfer complex whose electrical properties
change with time, depending on its temperature, is
suitable for use as an element of a time-temperature
indicator of the present invention.
Often, it is necessary to incorporate in the target
a substance that combines with the element to effect the
electrical property change in response to the environ-
mental parameterO The element and substance may be
spaced-apart in a matrix through which one of them can
diffuse. The matrix may be the same as the element or
substance or it may be a third material. Porous paper
and plastic film are examples of suitable matrices for
time-temperature indicators. The times and temperatures
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required to reach a specified indicator response level
can conveniently be varied by varying the diffusion
distance (for example, the thickness of a barrier
plastic film) or the porosity of the matrix. Similarly,
response characteristics can be varied by choosing
matrices having different diffusion coefficients ~or the
substance whose diffusion provides the device
response.
When the environmental parameter is the combined
effect of time and temperature, a preferred embodiment
has an element that is an undoped polymer and a
substance that is a dopant vapor. In that case, a
suitably chosen polymer may show a larye increase in
conductivity as the dopant concentration increases (for
example, by dopant diffusing through a matrix) with the
passage of time. The rate of conductivity increase is
generally higher at higher temperatures. Among the
polymers that are suitable elements are polymers having
conjugated backbones. Polyaniline, polyacetylene,
polycarbazoles, polypyrrole, polythiophenes, and
polyisothianaphthalene are preferred. Suitable dopants
are acceptor dopants such as AsF5, I2, 2~ HCl, H2SO4,
FeC13, SbF5, and salts containing NO~, NO2+ or Fe III
and also containing BF4 , PF6 , or perchlorate. Other
suitable polymers and dopants include those disclosed in
R. H. Baughman et al., Chem. Rev. 82, 209 (1982).
In another embodiment of the present invention, the
element whose electrical property changes with time-
temperature exposure comprises an absorbent substrate,
such as paper, and the substance that combines with the
element to cause the change is a salt solution.
Salt solutions that are suitable include both
inorganic and organic salts in aqueous or nonaqueous
solution. Specially preferred are inorganic salts such
as alkali or alkaline earth halides in aqueous solu-
tion. In general, electrical properties change abruptlyat the freezing temperature~ The freezing temperature
of an aqueous salt solution can be varied from about 0C
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to about -50~C by varying the salt concentration and the
type of salt used. For example, the lower limit can be
reached by using aqueous solutions containing about 32
wt. % CaC12. Other salt/water combinations and concen-
trations use~ul for this invention are described in "The
Handbook of Chemistry and Physics," 65th edition, pages
D-222 to D-274.
A convenient salt form with melting point near room
temperature is sodium sulfate decahydrate (m.p. 32C),
which, admixed with urea, has a melting point of 18-
22C.
An activatable target of the present invention
encapsulates the element or the substance, whose com- -
bination causes a change in an electrical property of
the element. Preferably, the capsules are microcap-
sules, with diameters in the range from a few microns to
several thousand microns. The preparation of such
capsules is described in "~icroencapsulation: Processes
and Applications," Edited by J. E. Vandegaer, Plenum
Press, New York (1974). The use of capsules is
particularly convenient for the activation of time-
temperature indicator devices to be used at low tempera-
tures, since the activation option eliminates the need
to maintain the indicator at low temperatures from the
time of manufacture to the time of application. Speci-
fically, rupture of a capsule, mechanically or by
freezing, can activate the indicator device by
initiating the time-temperature dependent combination of
substance and element that results in the electrical
property change of the element.
The present invention is suitable for determining
exposure to temperature, without regard to time. In
such an embodiment, for example, the element may undergo
an irreversible change in an electrical property at a
particular temperature. Depending on the range of tem-
peratures to be monitored and the particular applica-
tion, the particular temperature may mark either a lower
limit (freezing, for example) or an upper limit
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(melting, for example).
A salt deposited on filter paper impregnated with
frozen water can be used to show that a product has
warmed above the ice point. The salt should not be very
hygroscopic and should dissolve in water formed by
melting at the upper temperature to cause an irre~
versible change in an electrical property.
A particularly convenient type of indicator for
demulsification resulting from freezing (freeze
indicator) uses the product to be monitored as the ele-
ment. ~'or example, if the element comprises salad dres-
sing or a similar emulsion, demulsification can cause
the electrical property change that is detected.
Polymer/dopant systems, discussed above, can serve
as fluid sensors; i.e., sensors for gases, vapors, and
liquids. For example, alkali-metal doped polyacetylene
or alkali-metal doped poly(p-phenylene) provide the
basis for very sensitive integrating devices for the
detection of trace oxygen or water vapor. Less
sensitive elements for the detection of oxygen can be
made from acceptor-doped polymers, such as acceptor-
doped polyacetylene.
Doped or undoped conjugated polymers can be used to
detect the release of toxic donor or acceptor chemicals
either by the formation or the destruction of conducting
charge transfer complexes as a consequence of exposure
to these chemicals. For example, doped or undoped
polyacetylene can provide the element for a detector for
iodine, ammonia, sulfuric acid, H2S, hydrazine, SO2, and
the like.
Humidity is another environmental parameter that
can be monitored by the present invention. One tech-
nique uses an element comprising a moisture-sensitive,
highly-conducting charge-transfer complex, such as a
doped polymer. However, the preferred element comprises
a deliquescent salt deposited on a porous substrate,
such as filter paper. To increase the sensitivity of
such a device, one would ~o from less deliquescent to
~L2~;79%~
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more deliquescent salts. A preferred salt for use in
the element of a humidity indicator is CaC12. Depending
upon the choice of element, the humidity indicator can
be either integrating or nonintegrating. For example,
the use of a very deliquescent salt protected by a
polymer membrane that is semipermeable to water can
provide an integrating humidity indicator, since a very
hydroscopic salt will not lose its hydration in the
application temperature range. By selecting a salt of
suitable deliquescence and the optional use of
semipermeable moisture barriers of selected polymer type
and thickness, the response of the element can be made
to replicate the effect of humidity on the quality of a
product. Depending upon the device construction,
humidity response will depend on temperature to a
greater or lesser extent, since diffusion and hydration
processes generally depend upon temperature. Analogous
dependence on the combined effects of humidity and
temperature are found for many products, such as tobacco
products.
Exposure to actinic radiation can be measured by
the device and method of the present invention. For
that purpose, the element may be a polymer that includes
a photochemical that generates a dopant for the polymer
on exposure to the radiation. The dopant causes a large
increase in the electrical conductivity of the polymer.
Suitable polymers are, for example, poly(p-pheny-
lene), polyacetylene~ poly(p-phenylene sulfide), poly(p-
phenyl vinylene), polyaniline, polypyrrole, polycarba-
zoles, polythiophene, and polymeric sulfur nitride.
Suitable photochemicals include triarylselenonium
salts, triarylsulfonium salts, and diary]iodium salts of
the form:
Ar Alr
1,l n I " n or (Ar2I Y
Ar Ar
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, where MXn is BF4 , PF6 I SbF6 , etc, Y is Cl , Br ,
I , or MX n~ and where Ar, Ar', and Ar" are phenyl or
substituted pheny], (see "Photoinitiated Cationic
Polymerization by Triarylselenonium Salts," J. V.
Crivello and JO H. W. Lam, Journal of Polymer Science:
Polymer Chemistry Edition Vol. L7, 1047-1057 (1979)).
Halocarbon acceptors, such as CC14, CBr4, and CI4, are
other suitable photochemicals (see D. C. Hofer et al.,
Appl. Phys. Lett. 37, 314-316 (L980)).
. ................. _
The principal restriction on the combination of
particular photochemicals and polyrners is that the
dopant generated by the radiation must have suEficient
electron affinity or the polymer have sufficiently low
ionization potential so that charge carriers are
generated in the polymer when the two combine. For
example, iodine is a good choice for polyacetylene, but
not for poly(p-phenylene sulfide), because iodine has
relatively low electron affinity, and the ionization
potential of poly(p-phenylene sulfide) is much higher
than that of polyacetylene.
Generally, the conductivity change that provides
the device response need not result from the doping of a
polymer, but can instead arise from the formation of a
conducting charge-transfer complex. An example is the
ionization of tetrathiafulvalene by bromine to produce a
highly-conducting charge-transfer complex. In this case
the element whose electrical property changes comprises
tetrathiafulvalene and the substance that combines with
the element is bromine, generated when CBr4 is exposed
to actinic radiation. Also, the radiation-sensitive
chemical can produce a compensating agent that reacts
with a dopant that is already part of a conducting
charge-transfer complex. The reaction of the radiation-
generated compensating agent with the dopant initially
in the charge-transfer complex (e.g., reaction of
electron donor with an electron acceptor doping agent)
can lead to a decrease of electrical conductivity.
~L2S79%~
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Suitable actinic radiation includes ~-rays, x-rays,
electrons and other particle beams (proton, alpha
particles, etc.), and ultraviolet and visible light. A
particular photochemical may be used to monitor more
than one of these types of radiation; thus, if the
effect of a single type is to be monitored, it is
sometimes necessary to mask the target from the
extraneous types.
It is often important to know whether an article
has been subjected to mechanical shock, and the present
invention provides a device and method for determining
that. When the environmental parameter being rnonitored
is mechanical shock, the device preferably comprises an
element and a substance that can combine with the ele-
ment (as a result of mechanical shock) to change the
electrical property. The element may comprise a conju-
gated polymer, such as polyacetylene, and the substance
may comprise a solution that can cause a large increase
in polymer conductivity, such as an aqueous solution of
KI3. Alternatively, the element can comprise a salt
dispersed on filter paper and the substance a solvent
for the salt. In a preferred embodiment, the element
comprises absorbent paper and the substance is a salt
solution contained in a frangible capsule.
Capsules suitable for the present invention may be
of glass, polymer, thin metal foil, etc., and may
include means for breaking the capsule by mechanical
shock, such as a ball bearing within the capsule.
Alternatively, the capsule may be sandwiched near the
pivot point of a movable lever. The sensitivity of the
indicator device to mechanical shock can be controlled
by varying the mechanical strength of the capsule
(changing wall thickness and/or the mechanical
properties of the capsule material) and, for example,
changing the weight and size of the ball bearing. Cap-
sule size is preferably in the range of about l mm3 - 1
c~3 volume.
~L2S~79:21
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Indicator devices that denote that a shipping
package has not been held riyht-side-up during shipping
can be constructed analogously to the mechanical shock
indicators, except that instead of a liquid being
released by breaking a capsule, the liquid pours out of
a tilted holder.
The devices of the present invention find their
primary application in monitoring the environmental
exposure to which a product has been subjected and/or
its effect on the product. That purpose may be
accomplished by first attaching a device of this
invention to the product and then determining the
exposure to which the product has been subjected from a-
calculation of the exposure to which the device has been
subjected. The device (and product) may be in a sealed
package and can be interrogated without opening the
package. Of course, the packaging material must not
interfere with the interrogation. Prior art devices, on
the other hand, generally must be located at the surface
of the package.
The following examples are presented in order to
provide a more complete understanding of the inven-
tion. The specific techniques, conditions, materials,
and reported data set forth to illustrate the principles
and practices of the invention are exemplary and should
not be construed as limiting the scope of the invention.
EXAMPLE_l
Time-temperature indicator
Approximately 80% of one side of an rf-antitheft
target (38 mm x 38 mm in size, from Check Point Systems,
Inc., Thorofare, New Jersey), which operates at a
resonance frequency of 8.2 MHz, was covered with a piece
of undoped 0.1-0.2 mm polyacetylene film (prepared by
the method of Ito, Shirakawa, and Ikeda, J. Polym. Sci.,
12, 11 (1974)).
The laminated target was then encapsulated in a
polyethylene case under argon together with 200 ~1 of an
aqueous KI3 Solution in a breakable capsule. When
~LZ~;79Z~
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interrogated with a signal at or near the resonance
frequency, the target emitted a signal that was detected
as 7.5 V on a conventional radio-frequency detector 6 cm
from the target.
The device was then activated by breaking the
capsule, allowing the solution to dope the polyacetylene
to a conductivity of 100-400 S/cm. At this point the
interrogated target provided a reading of 1.8 V on the
detector. This voltage gradually increased with time as
oxygen diffused through the polyethylene case and
degraded the conductivity of the doped polyacetylene.
To accelerate the degradation process, the polyethylene
case was punctured, and the voltage response of the
target in ambient was monitored over a period of a few
days. The voltage increased slowly ~as shown in Figure
~ 3) as the conductivity of the polyacetylene decayed.
Similar results are achieved with the degradation of
other polymers.
_AMPLE 2
Time temperature indicator
A target like the one used in Example 1 was covered
on one side with a filter paper coated with dry CaC12
and the composite was sealed in polyethylene. At this
point the response of the modified target was identical
to that of an unmodified target in the sense that both
targets provided identical readings on a detector 6 cm
away. A small area of polyethylene covering the filter
paper side of the target was punctured with holes over
which a film of cellulose acetate butyrate was cast.
This modified device was sealed inside another
polyethylene container containing a piece of wet filter
paper. The target response abruptly dropped to zero
after water vapor diffused through the cellulose film
and dissolved the CaC12 to form an ionically-conducting
solution in the filter paper. The onset of change in
target response, as well as the duration of the initial
delay time, depends on the rate of diffusion of water
vapor through the barrier film (cellulose in this case),
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which is a function of temperature. Voltage vs time
profiles for devices constructed as above and held at
6, 12, and 23C are shown in Figure 4.
EXAMPLE 3
Time-temperature indicator
A coating of conductive polyaniline doped with HCl
was applied to one surface of a taryet like the one des-
cribed in Example l. The target at this point is nearly
"invisible" to the detector at a distance of 6 cm.
Exposure of the target to temperatures in excess of lOO-
125C for short times causes the target response to
increase, where the extent of siynal increase depends on
both the exposure time and the temperature. The thermal
treatment above these temperatures causes vaporization
of the dopant, resulting in gradual de-doping and con-
ductivity loss of the polyaniline coating.
EXAMPLE
Freeze/thaw indicator
One surface of a target similar to the one des-
cribed in Example l was covered with a piece of filter
paper soaked in an aqueous solution containing 1~ by wt.
of NaCl (specific conductance of 16 mS/cm) and the whole
device was encapsulated to prevent water evaporation.
The target was completely "invisible" as evidenced by a
zero reading on the detector. On cooling to below about
-0.6C, the aqueous solution froze and the target became
completely "visible" (7.5 V reading on the detector).
On rewarming, the solid phase became fluid again and the
target returned to the "invisible" state (zero
reading). This demonstrates a reversible freeze/thaw
indicator useful for signaling the freezing of items
containing waterO
Besides aqueous solutions, other liquid/ionically-
conductive material combinations with vastly different
freezing points are possible (e.g., sulfuric acid, (m.p.
10C), phosphoric acid (m.p. 290Cj, acetic acid ~m.p.
16.6C), acetonitrile/LiAsF6 (m.p. -~5C and below),
sulfolane/Li+CF3S03 (m.p. 2~C and below), etc.),
~5~
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providing a large family Oe indicator devices useful
over a wide range of temperatures.
EXAMPLE 5
Freeæe indicator
Microcapsules containing an aqueous NaC1 solution
were coated on a piece of filter paper. The coated
filter paper was placed on one surface of a target like
the one described in Example 1. At this point the
target was completely "visible" ~7.5 V reading at a
distance of 6 cm from the receiver). The target
remained visible on cooling to below the freezing point
of the aqueous solution. On rewarming above the thaw-
point, the target became "invisible" (zero volt reading,
6 cm from the receiver). On freezing, the microcapsules
ruptured due to expansion of the aqueous phase. On
thawing, an ionically-conductive solution formed in the
filter paper and caused the attenuation of target
response. Thus, a target invisible at room temperature
indicates that it was subjected to temperatures low
enough to free~e the solution inside the microcap-
sules. As in Example 4, the low-temperature limit can
be selected by proper choice of salt, solvent, and
concentration.
EXAMPLE 6
~ -radiation dosage indicator
Undoped non-conductive polyaniline powder (prepared
by the method of A. G. Green et al., J. Chem. Sci. 97,
2388-2403 (1910)) in a poly(vinylchloride) matrix was
applied to one side of a target like the one used in
Example 1. At this point, the target was completely
"visible" as evidenced by a reading of 7.5 V on the
detector held 6 cm from the target. The target was
irradiated with q-rays from a cobalt-60 source at a rate
of 1 M rad/hr for 100 hr. The target response (voltage)
was reduced by 20% as a consequence of this radiation
dosage, perhaps because irradiation of the poly
(vinylchloride) released HCl, which doped the
polyaniline to a conducting state.
~:~579~
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_XAMPLE 7
UV doslmeter
A thin coating of tetrathiafulvalene (TTF) (Aldrich
Chem. Co.) wetted with CC14 was applied to one side of a
target identical to the one described in Example 1. At
this point the target response was the same as the
unmodified target (100% active). On exposing the modi-
fied target to UV radiation (256 and 366 nm, ~ 300 )W),
the target response slowly diminished. After an
exposure time of 70 min., the signal strength decreased
by 30~.
A similar device was Eabricated using polyacetylene
as the dynamic element in the target.
A large LC coil used for antitheft detection and
supplied by Checkpoint Systems Inc., was modified by
incorporating a piece of undoped polyacetylene film
(initial conductivity of 10-7 S/cm, 5 mm x ~ 10 mm x
0.13 mm) in parallel with the circuit (contacts were
made using Electrodag~ conductive cement).
The undoped polyacetylene film was covered with a
saturated solution of Ph2I+pF6 in CH2C12. After
evaporation of the CH2C12, the coil was encapsulated in
polyethylene under argon. At this point, the modified
target provided the same response as an unmodified
antitheft target (100~ active). After the polyacetylene
was irradiated with UV light at 254 nm ~or 60 min., the
target response decreased by 85~. Here, the UV
irradiation presumably produces HPF6, which subsequently
dopes the polyacetylene, thereby rendering the polymer
conductive.
EXAMPLE 8
Gas/vapor sensor
Undoped polyaniline (conductivity < 10 7 S/cm) was
applied as a powder to one side of an rf target like the
one described in Example 1. The target remained 100%
active. On exposure to HC1 vapor, the target response
rapidly dropped to 15% of its original value. The
response remained constant when the target was removed
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18-
from the HCl vapors indicating that the response is
permanent.
EXAMPLE 9
Gas/vapor sensor
-
A coating of doped and conductive polyaniline was
applied to one side of a target like the one described
in Example 1. At this point, the target was nearly
"invisible" at a distance of 6 cm. On exposure to
ammonia vapor, the target response rapidly increased,
finally giving a value of ~ 6 V on the detector.
Removing the target from the ammonia vapors caused no
change in target response, indicating that a permanent
change occurred. This demonstrates that such a modified
target can function as a sensor for the release of
reducing agents.
_AMPLE 10
Gas sensor
A 38 mm square of 0.25 mm thick polyacetylene film
was wetted with a degassed 48~ aqueous HBF4 solution and
placed on one side of a target of the type described in
Example 1. The target registered a 6.8 V reading on the
receiver at a distance of 5.5 cm. On exposure to
atmospheric oxygen, the reading gradually decreased to a
limiting value of ~.0 V after 2 days. The target
response results from gradual oxygen doping of the
polyacetylene film. A slower rate of response can be
achieved by encapsulating the target; e.g., in a
polyethylene case.
EXAMPLE 11
Humidity detector
One surface of an rf target similar to the one
described in Example l was coated with dry CaC12 on a
porous substrate and the whole encapsulated in a poly-
ethylene pouch with a number of holes. This device is
completely "visible" to the detector. However, when the
total exposure to environmental humidity exceeds a
certain limiting value, the coating absorbs sufficient
H2O such that it goes into solution and makes the device
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" invisible" to the detector. This type of device
functions as an integrating humidity indicator.
