Note: Descriptions are shown in the official language in which they were submitted.
1~330~5
This invention relates to electrical devices
comprising conductive polymer compositions.
Conductive polymer compositions comprising a
conductive carbon black dispersed in a polymer are well
known. Over recent years, there has been particular interest
in such compositions which exhibit positive temperature (PTC)
characteristics, i.e. which show a very rapid increase in
resistivity over a particular temperature range. Reference
may be made for example to U.S. Patents Nos. 2,978,665;
3,243,753; 3,351,882; 3,412,358; 3,413,442; 3,591,526;
- 3,673,121; 3,793,716; 3,823,217; 3,858,144; 3,861,029;
3,914,363 and 4,017,715; British Patent No. 1,409,695; Brit.
J. Appl. Phys. Series 2, 2 569-576 (lQ69, Carley Read and
Stow); Kautschuk und Gummi II WT, 138-148 (1958, de Meij);
Polymer Engineering and Science, Nov. 1973, 13, No. 6, 462-
468 (J. Meyer); U.S. Patent Office Defensive Publication No.
T 905,001; German Offenlegungschriften Nos. 2,543,314~1,
2,543,338.9, 2,543,346.9, 2,634,931.5, 2,634,932.6,
2,634,999.5, 2,635,000.5, 2,655,543.1, 2,746,602.0,
2,755,077.2, 2,755,076.1, 2,821,799.4 and 2,903,442.2; and
German Gebrauchsmuster 7,527,288.
PTC compositions are useful, inter alia, in
electrical devices comprising a PTC elément in combination
with another resistive element whose resistance remains
relatively constant at least up to the temperature range in
which the PTC element shows a very rapid increase in
resistance, such other element being referred to as a
constant wattage (CW) ~or relatively constant wattage (RCW)]
a- ~
11330~5
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;
element. It is to be noted that the resistance of a CW
element need only be relatively constant in the temperature
range of normal operation; thus it can decrease, remain
constant, or increase slowly in this range, and can eshibit
PTC characteristics above normal operating temperatures of
the device. Such devices are described for example in ~.S.
Patent No. 4,017,715 and German Offenlegungschrift Nos.
2,543,314.1 and 2,903,442.2. In order to obtain the best
results from such devices, it is necessary that the
resistivities of the PTC and CW elements should be correlated
throughout the temperature range of operation and in many
cases that the resistivity/temperature characteristics of the
elements and the contact resistance between the elements
(whether bonded directly to each other, as is generally
preferred, or through a layer of a conductive adhesive)
should not change excessively on storage or in use, eg. due
to temperature variations which take place during operation
of the device. The CW compositions hitherto available are
not fully satisfactory in these respects. For example, it is
well known that certain conductive polymer compositions
comprising an elastomer and a carbon black exhibit CW
behavior, but unfortunately the resistivity of such
compositions is excessively dependent on their thermal
history.
We have now discovered that improved electrical
devices comprise
(a) a CW element composed of a CW composition which
comprises (i) a continuous phase of a first cry-
stalIine organic thermoplastic polymer and (ii)
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dispersed in said first polymer, a first
conductive carbon black having a particle
size (d) which is less than 27 millimicrons
and a surface area (S) in m2/g such the ratio
S/D is at least 12,
(b) a PTC element composed of a PTC composition
which is at least partially contiguous with
~- said CW element and which comprises (i) a
continuous phase of a second crystalline
organic thermoplastic polymer and (ii)
dispersed in said second polymer, a second
conductive carbon black, and
(c) at least two electrodes which are connectable
to a source of electrical power and which are
so placed in the device that, when they are
connected to a source of electrical power,
current flows through the device along a path
which, at least at some temperatures,passes
sequentially through said PTC element and said
CW element.
The CW compositions used in the devices of the
invention contain a carbon black whose particle size (D)
in millimicrons and surface area (S) in m2/g are such
that the ratio S/D is at least 12, especially at least
18. S. and D are measured by methods well known to
those skilled in the art and described in "Analysis of
Carbon Black" by Schuber~, Ford and Lyon, Vol. 9 Page 179,
Encyclopaedia of Industrial Chemical Analysis (1969),
published by John Wiley and Son, New York. D is prefer-
ably less than 27, especially less than 18, particularlyless than 15 millimicrons. Particularly useful CW
.. ~ , ,~
~30~5
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compositions contain carbon blacks having a particle
size of at most 15 millimicrons and a surface of at
least 300, preferably at least 500, especially at
least 700, m2/g. Examples of suitable carbon blacks
which are commercially available include the
following:-
Trade Name S D S/D
Monarch 1300 560 11 51
Raven 8000 935 13 72
10 Super Spectra 742 13 57
Monarch 1100 240 13 18
FW 200 460 13 35
Raven 7000 543 14 39
Raven 3500 319 16 20
15 Royal Spectra 1125 10 112.5
It should be noted that carbon blacks as defined abovehave not previously been recommended for use as con-
ductive blacks, but rather as pigments.
The amount of carbon black used in the CW com-
positions will generally be in the range of 6 to 4~/0 by
weight, with the precise amount required to obtain a
particular resistivity at room temperature being
dependent on the particular carbon black and the method
i used to disperse it in the polymer. The desired resis-
tivity of the CW composition at room temperature will
depend upon the function of the electrical device of
which it is part, from values as high as 10,000 ohm.cm.,
generally 1,000 to 8,000 ohm. cm., for strip heaters,
to values as low as 0.3 ohm. cm. for other devices.
When the carbon black has a particle size greater than
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li~3085
20 millimicrons and a surface area greater than 220
m /g, the resistivity of the composition is preferably
less than 1,000 ohm. cm., particularly less than
900 oh,m. cm., especially less than 750 ohm. cm., e.g.
less than 500 ohm, cm.
In the CW compositions, the ratio of the maximum
resistivity in the temperature range from 25 to a
-- temperature 50C, preferably 40C, below the melting
point of the polymer to the resistivity at 25C is
preferably less than 3, particularly less than 2,
especially less than 1.5, this ratio can be less than
1, i.e. the composition can exhibit a negative tem-
perature coefficient (NTC), but is generally at least
0.9. The teaching of the prior art is that conductive
polymer compositions which are based on thermoplastic
polymers, especially crystalline polymers, and which
have resistivities in the range of 1 to 10,000 ohm, cm.,
will show a sharp increase in resistivity as the melt-
ing point of the polymer is approached, and if the com-
position is not cross-linked, will show a sharp
decrease in resistivity when melting is complete. We
have found that by using carbon blacks as defined
above, the increase in resistivity around the melting
point can be reduced and in some cases can be substan-
tially eliminated. For particularly preferred CW com-
positions, the ratio of the maximum resistivity in the
temperature range from 25C to the melting point of
the polymer to the resistivity at 25C is less than
10, preferably less than 5, especially less than 2
i
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11~30~
The present invention increases the range of base
polymers and resistivities available in CW compositions.
his in turn means that in devices comprising a conductive
polymer PTC element and an adjacent conductive polymer CW
element, the polymers in the two elements can be selected
so that the contact resistance between the elements does
not change excessively in use, eg. due to temperature
variations which take place during operation of the device.
We have found that for this purpose it is desirable that
the polymers in the PTC and CW elements should be selected
so that, if the elements are bonded directly to each other
and are then separated from each other at room temperature,
the bond fails by cohesive failure. One of the factors
influencing changes in contact resistance is the relative
melting points of the polymers, and in preferred devices of
the invention the melting points of the first and second
organic polymers differ by at most 25C. Another factor is
the type of polymer. Thus it is preferred that both
polymers should be addition polymers, for example that both
should comprise at least 50 molar percent of units derived
from an olefin, especially ethylene or another d-olefin,
e.g. low or high density polyethylene, or that both should
comprise units derived from vinylidene fluoride.
Alternatively both can be polyesters or polyamides etc.
The polymers are preferably crystalline, i.e. have a
crystallinity of at least 1%, preferably at least 3%,
especially at least 10%.
One class of polymers preferably used in the CW
compositions are crystalline copolymers which consist
~lff~f3~3f8s
essentially of units derived from at least one olefin,
preferably ethylene, and at least 10% preferably not more
than 30% by weight, based on the weight of the copolymer,
of units derived from at least olefinically unsaturated
comonomer containing a polar group, preferably vinyl
acetate, an acrylate ester, e.g. methyl or ethyl acrylate,
or acrylic or methacrylic acid. Another preferred class of
polymers are crystalline polymers which comprise 50 to
100%, preferably 80 to 100%, by weight of -CH2CF2 or
-CH2CHCl- units, for example polyvinylidene fluoride or a
copolymer of vinylidene fluoride, e.g. with
tetrafluoroethylene.
The CW compositions used in this invention can
contain one or more thermoplastic polymers, and can also
contain one or more elastomers, usually in amount less than
20~ by weight. When more than one thermoplastic polymer is
present, the continuous phase can be provided by a single
thermoplastic polymer or a mixture of two compatible
thermoplastic polymers. 'I'he carbon black can be dispersed
in the continuous phase only or, when the composition
contains a discontinuous polymeric phase, in the
discontinuous phase only or in both the continuous and
discontinuous phases.
In preparing the CW compositions, any method
which provides a satisfactory dispersion of the carbon
black in the thermoplastic polymer can be used, but it
should be noted that the electrical characteristics of the
composition do depend on the method used. Preferably the
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1~330~5~
carbon black is mixed with the molten polymer. The C~
compositions preferably contain a srnall q~antity of
antioxidant, and this and any other desired ingredients can
be added at the same time. The composition is shaped to
the desired shape, e.g. by molding or extrusion. The
shaped composition is preferably annealed, e.q. by heating
to 150-200C for a period of 10 to 20 minutes, followed by
cooling, two or more times until the resistivity reaches 2
stable value. ~f the composition is to be cross-linked, as
is preferred, it is then cross-linked e.g. by irradiation
or by heating to a temperature which activates a chemical
cross-linking agent. Especially after cross-linking by
irradiation, the shaped composition is preferably again
annealed as described above.
The accompanying Figures 1-3 show the resistance-
temperature characteristics of samples prepared from a number
of CW compositions, the samples being 1-1/2 s 1 x 0.03 inch
(3.8 x 2.5 x 0.075 cm.), with silver paint electrodes on
both sides at two ends, and having been cut from slabs
pressed from compositions obtained by mixing a carbon black
with a molten polymer. The polymers and carbon blac~s used
t-' and the amounts of carbon black (in % by weight of the
composition) are given in the Table below. In each case the
composition also contained a small amount of an appropriate
radiation cross-linking agent and/or antioxidant and/or-other
stabilising agent. The ~ytrel 4055 referred to in the Table
is a block copolymer of polytetramethylene terephthalate and
polytetramethylene oxide having about 50% crystallinity. The
compositions were cross-linked by irradiation to the dosage
..... .
11;~30~35
-- 10 --
given in the Table and were then given a heat treatment
involving heating at 180C - 200C for 15 to 20 minutes
followed by cooling for 20 minutes, and repeating this
sequence until a stable resistance was obtained. In some
cases, as noted int he Table, the compositions were qiveD a
similar heat treatment before being cross-linked~
Figure 3 shows the resistance/temperature curves of
the samples used for Figure 2 after they had been cooled back
to room temperature; it will be seen that the compositions
are very stable.
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o o o o o o o o o o o o o o o o o
~ O O ~ ~ ~ O Q ~
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r ~ ~ ' o~
1133Q~
EXAMPLE
A CW composition having a resistivity at 25C of
about 115 ohm. cm. was prepared by blending 79 g. of high
density polyethylene (Marlex 6003), 20 g. of Raven 8000
carbon black and 1 g. of an antioxidant on a 3 inch (7.5 cm.)
electric roll mill at about 175C. The resulting CW
composition was granulated and a portion of it pressed into a
slab 1 inch t2.5 cm) by 1 inch (2.5 cm.) by 0.061 inch (0.15
cm.), using a pressure of 10,000 psi (700 kg/cm2) and a
temperature of 205C. One face of the slab was covered by a
nickel mesh electrode (Delker 3 Ni 5-077) 1.1 inch (2.8 cm.)
by 1 inch (2.5 cm.) by 0.003 inch (0.0075 cm.) and the
electrode was impressed into the slab under the same pressing
conditions.
A PTC composition was prepared by blending 54 g. of
high density polyethylene, 44 9. of Furnex N 765 carbon black
and 2 9. of an antioxidant in a Banbury mixer. The resulting
PTC composition was granulated and a portion of it pressed
into a slab 1 inch (2.5 cm.) by 1 inch (2.5 cm.) by 0.015
inch (0.04 cm.), using a pressure of 10,000 psi l700 kg/cm2)
and a temperature of 205C. One face of the slab was covered
by a nickel mesh electrode as described above and the
electrode was impressed into the slab under the same pressing
conditions.
The CW slab and the PTC slab were then pressed
together, with the electrodes on the outside, using a
pressure of 10,000 psi (700 kg/cm2) and a temperature of
205C. The composite structure thus formed was irradiated to
a dosage of 20 megarad to cross-link the compositions, thus
forming a heater which is suitable, for example, for
maintaining a printed circuit or other electronic component
at a desired elevated temperature.
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