Note: Descriptions are shown in the official language in which they were submitted.
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This invention relates to electrical devices
comprising conductive polymer PTC elements.
It is known that PTC conductive polYmer compositions
[i.e. compositions which comprise a polymer and conductive
particles dispersed therein and which exhibit positive
temperature coefficient (PTC) behaviour] can be used in a
variety of electrical devices (see for example J. Pol. Eng.
and Sci, 14, 706 (1974), U.S.Patent Nos. 3,351,882,
3,858,144 and 3,914,363 and German Offenlegungschriften Nos.
2,543,314.1, 2,755,077.2, Z,755~,076.1, 2,821,799.4 and
P2,903,442.2, and the applications fiied contemporaneously
with this appLication coresponding to U.S. Serial Nos.
965,344 and 965,345). Such devices often comprise a jacket
of a polymeric material which insulates the device electric-
ally and also provides physical protection.
We have now discovered that such devices have improved
electrlcal stability if access of oxYgen to the PTC element
is restricted.
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In one aspect the invention provides an electrical
device which comprises
(1) a PTC element which is composed of a
composition which exhibits PTC behavior with
a switching temperature Ts and which
- comprises a polymer and conductive particles
dispersed therein;
(2) at least two electrodes which can be
connected to a source of electrical power and
which, when so connected, cause current to
flow through said PTC element; and
(3) an oxygen barrier which, when the device is
in air at standard temperature and pressure,
restricts access of air to the PTC element so
lS that the rate at which the PTC element
absorbs oxygen is less than 10 6, preferably
less ~han 4 x 10 7, especially less than
3 x 1~ 7, particularly less than 2 x 10 7
cc/sec/gram.
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Especially when the device is one which is
expected to operate in such a way that the barrier is
maintained at elevated temperatures, it is preferred that
the barrier should remain an effective oxygen barrier at
elevated temperatures and thus prevent any excessive change
in the resistance of the device. It is, therefore,
preferred that if the resistance of the device at a
temperature TC is RT, and the resistance of the device at
the same temperature after an active aging treatment as
defined below is RT~, then RT~ is from 0.5 RT to 3RT,
preferably from O.SRT to 2RT, at at least one value of T
which is between (Ts-110)C and Ts and preferably is between
(Ts-60)C and Ts, especially at all values of T between
(TS-60)C and Ts, particularly at all values of T between
(Ts-110)C and Ts. The active aging treatment just referred
to consists of passing current through the device for a time
t hours, the current being such that I2R heating of the
device maintains the PTC element at a temperature between Ts
and (Ts + 50)C, and t being 100 hours, preferably 250
hours, particularly 500 hours, especially 1000 hours. For
many devices, some or all of ~hese criteria of resistance
change will be met if RTA is from 0.5RT to 3RT, preferably
0.5RT to 2RT, at T = 25C.
The PTC compositions used in the present invention
may be any of the PTC conductive polymers disclosed in
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Applicant's earlier patent applications and in the prior art.
The conductive particles preferably comprise carbon black, but
other conductive particles, e.g. metal powders, metal oxides,
inorganic salts and graphite, can be used. Preferred composi-
tions comprise an organic polymer (the term polymer being used
to include mixtures of polymers) having at least 10%, prefer-
ably at least 30%, crystallinity and having dispersed therein a
conductive carbon black having a particle size of 20 to 250
millimicrons. The PTC composition may further comprise a non-
conductive inorganic filler, e.g. zinc oxide, antimony trioxideor clay, and/or an antioxidant (e.g. a hindered phenol such
as those disclosed in U.S. Patent No. 3,986,981 - ~.J. Lyons,
issued l9th October, 1976 and those manufactured by Ciba Geigy
under the trade name "Irganox"~ or any other additive which will
s-tabilise the composition against thermo-oxidative degradation,
the amount of such additive generally being 0.005 to 10%, for
example 0.01 to 6%, preferably 0.5 to 4%, by weight based on
the weight of the polymer. Some materials which are generally
useful as antioxidants for polymers can have an adverse effect
on electrical stability~, but suitable antioxidants can readily
be selected on a trial-and-error basis.
Generally, the barr;er will be such that, when the
device is placed in air, the only oxygen which can contact
at least 95%, prefera~ly substantially 100%, of the surface
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of the PTC element is oxygen which has passed through the
barrier. The barrier is preferably composed of a material
having an oxygen permeability rate at 25C of less than
5xlO 9, especially less than 10 9, cc(STP)/cm2/mm/sec/cm Hg,
as measured by ASTM D 1434-75. Especially when the device
is one which is expected to operate in such a way that the
barrier is maintained at an elevated temperature, the
physical properties of the barrier, including its oxygen
permeability, at elevated temperatures are preferably such
that the barrier retains its structural integrity and the
device has the desired electrical properties after active
aging as defined above. The thickness of the barrier should
be sufficient to restrict the access of air to the PTC
element to the desired extent and to prevent the formation
of pinholes, e.g. at least 1 micron, and for polymeric
materials is generally 0.0025 to 0.25 cm, preferably 0.013
to 0.13 cm, especially 0.025 to 0.075 cm. The barrier
preferably protects the device against mechanical abuse, and
for this reason is preferably composed of a material having
a Young's Modulus greater than 7,000 kg/cm2. When using
such a barrier, it is preferred, in order to avoid any
danger of the barrier constricting the PTC element and thus
changing the ~lectrical performance of the device, that the
barrier is separated from the PTC element by an air gap or a
layer of another material of Young's Modulus less than 7,000
kg/cm .
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Suitable materials for the barrier include metals
and polymeric compositions based on, for example, one or
more polymers selected from polyvinylidene chloride,
polyvinyl fluoride, polyethylene terephthalate, rubber
hydrochloride, polychlorotrifluoroethylene, phenol-
formaldehyde resins, polyamides, epoxy resins,
styrene/acrylonitrile copolymersl polycarbonates,
polystyrene, isobutylene/isoprene copolymers, polyethylene,
ethyiene/tetrafluoroethylene copolymers, vinylidene
fluoride/hexafluoropropylene polymers and fluorinated
ethylene/propylene copolymers. The continuous surface
temperature of the polymer should preferably exceed the Ts
of the PTC element. These polymeric compositions can
contain conventional additives, but should preferably not
comprise materials which will migrate into the PTC element
and have an adverse effect on its properties.
In one preferred embodiment of the invention, the
device is a circuit control device and the barrier is in the
form of a self-supporting container, through whose walls the
electrodes pass (via suitably sealed orifices) and within
which the remainder of the device is supported or suspended
out of contact with the walls of the container. The
container preferably does not contain any oxygen; for
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example it may be evacuated or filled with an inert gas such
as argon or nitrogen. Typically the container will
principally be made of metal, with the electrodes passing
through a wall composed of a ceramic or rigid plastics
material. In another preferred embodiment, the device is a
heater or a circuit control device and the barrier is in the
form of a layer of polymeric composition which surrounds the
remainder of the device, with the volume enclosed by the
layer being substantially free from voids. The barrier may
be composed of a single material or two or more materials,
either mixed together or as discrete components of the
barrier, e.g. a laminate. One or both of the electrodes may
be par~ of the barrier. The barrier should not of course
provide an electrical connection between the electrodes.
.
The electrodes are generally composed of metal or
some other material having a resistivity of less than 0.1
ohm. cm. Each of the electrodes may be in physical contact
with the PTC element or wholly or partially separated
therefrom by electrically conductive material, e.g. a
conductive polymer composition which exhibits relatively
constant wattage behavior~ i.e. which does not exhibit PTC
behavior at temperatures below the Ts f the PTC element.
~lternatively the electrodes can be sandwiched between the
PTC element and a relatively constant wattage conductive
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polymer composition. Preferably at least the outer surface of
each of the electrodes is composed of a metal which does not
catalyse degradation of the conductive polymer which it con-
tacts. Thus the electrodes are preferably composed of nickel,
tin, silver or gold, or one of these metals coated onto copper
or another metal. When a planar electrode is required, elect-
rodes in the form of an expanded metal or wire mesh are pre-
ferred. Other electrodes which can be used include solid
wires, stranded wires and braids. When using stranded wire
electrodes or other electrodes which contain voids, care should
be taken to ensure that these voids do not provide a passageway
for air to enter the device, e.g. by filling the voids or by
sealing any exposed portions thereof. In preparing the device,
care should be taken to minimize contact resistance.
The devices of the invention include circuit control
devices, and seIf-limiting heaters, including strip heaters.
In one clas~s of dev;:ces according to the invention,
generally circuit control dev;~ces, the PTC element is of
reIatively small size, having a volume of for example less
than 2~ cc., often less than la cc. or even
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smaller such as less than 5 cc or 1 cc., and the resistance
of the device at 25C is relatively small, for example less
than 50 ohms, preferably less than 10 ohms., or even smaller
such as less than 1 ohm. or 0.5 ohm.
The invention is illustrated in the accompanying
drawings, in which:
Figures 1 to 3 show devices according to
the invention; and
Figure 4 shows the effect of active aging on the
resistance at 25C of various devices of
the invention.
Figure 1 shows a circuit control device which
comprises a PTC element 1 in the form of a round disc having
round mesh electrodes 2 embedded in opposite faces thereof;
leads 4 are attached to the electrodes 2; and barrier layer
3 encapsulates the PTC element 1 and the electrodes 2, with
leads 4 passing through it. The interface between the
barrier layer 3 and the PTC element 1 and the electrodes 2
is free from voids.
Figure 2 shows a strip heater of constant cross-
section comprising solid wire electrodes 2 embedded in PTC
element 1 which is surrounded by barrier layer 3. The ends
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of such a strip heater are preferably covered by an oxygen
barrier, but it is important to note that even if this
precaution is not taken, the absence of voids between PTC
element 1 and barrier layer 3 and in the electrodes 2 means
that only a very limited proportion of the surface area of
the PTC element is exposed to the air. By contrast, if
voids are present between the jacket and the PTC element or
if stranded wire electrodes are used, and the ends of the
heater are not sealed, then even if the jacket is
substantially impermeable to oxygen, air can percolate along
the length of the PTC element and contact a substantial
proportion of its surface.
Figure 3 shows a circuit control device in which
the barrier is formed by a can of generally rectangular
cross-section and having a metal top 1 and a base sealed
thereto. The can is filled with nitrogen. The base
comprises a metal ring 2, which has a peripheral sealing
slot to which the top 1 is sealed, and a disc 4 which is
sealed to the ring 2 and which is composed of glass or an
epoxy resin. Pin leads 3 pass through disc 4 and support
and are connected to rectangular electrodes between which is
sandwiched a PTC element; the electrodes and PTC element are
shown (in outline only) as 5.
The invention is illustrated in the following
Examples, in which parts and percentages are by weight except
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where otherwise noted. In each of the Examples, devices were
prepared and tested by the procedure described below. A PTC
composition was prepared by mixing the ingredients shown in
the Table below (the weight given being in grams); it should
be noted that the polymers used were commercially available
materials which contain a small quantity (about 0.5% by
weight) of an antioxidant. The mixing was carried out at
flux temperature for 5 minutes in a steam-heated Banbury
mixer with a water-cooled rotor. The mixture was dumped from
the mixer, allowed to cool to room temperature and chopped
into small pieces. The chopped material was compression
molded at a temperature of 180C and a pressure of about 70
kg/cm2 for 5 minutes into a slab 0.2 cm. thick. Round discs,
1.9 cm. in diameter, were punched out of the slab. An
electrode was formed on each face of each disk by molding
into it a disc 1.9 cm. in diameter cut from an expanded metal
mesh composed o~ nickel-coated copper. The sample was
irradiated to 20 megarads to cross-link the PTC composition.
20 AWG wire leads were attached to the electrodes. Where
indicated in the Table, preparation of the device was
completed by surrounding the sample with a barrier as
specified in the Table. In Example 2, the sample was dipped
into the epoxy resin composition, which was then cured at
80C or 16 hours. In Examples 3 and 5 the sample was heated
to 110C and then dipped into a fluidised bed of the epoxy
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resin, which was then cured at 110C ~or 16 hours. In
Example 6, the sample was dipped into the silicone resin,
~hich was then cured at 20C for 16 hours. In Examples 2,
3, 5 and 6, the barrier layer was 0.025 cm. thick.
The electrical stability of the devices on active
aging as defined above was tested as follows. The leads of
the device were attached to a variable voltage AC power
supply. The voltage of the supply was maintained at 120
volts except when the device was first connected or
reconnected to the power supply, when the voltage was 30-45
volts for the first 30 seconds and was then increased to 120
volts over a period of 2 minutes. At intervals during the
aging, the device was disconnected from the power supply and
allowed to cool to room temperature for 0.5 hour, and its
resistance at room temperature was then measured.
The room temperature resistance of the devices
after aging as specified above is shown in Figure 5. It will
be seen that the products of Examples 1, 4, 6 and 9, which do
not comprise barriers according to the invention, have poor
electrical stability, whereas the products of Examples 2, 3,
5, 7 and B, which are in accordance with the invention, have
excellent stability.
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The presence of the barrier in the devices of the
invention has the additional advantage that if the device is
subjected to electrical stress which causes breakdown of the
PTC composition, the likelihood of explosive failure or
conflagration is substantially reduced.
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