Note: Descriptions are shown in the official language in which they were submitted.
MP1271
CONDUCTIVE POLYMER COMPOSITION 1 3 3 4 4 8 0
BACKGROUND OF THE INVENTION
Field of the Invention
This invention relates to conductive polymer
compositions and electrical devices comprising them.
Background of the Invention
Conductive polymer compositions and electrical devices
such as heaters and circuit protection devices comprising
them are well-known. Reference may be made, for example, to
U.S. Patent Nos. 3,793,716, 3,823,217, 3,858,144, 3,861,029,
3,914,363, 4,017,715, 4,177,376, 4,188,276, 4,237,441,
4,242,573, 4,246,468, 4,286,376, 4,304,987, 4,318,881,
4,330,703, 4,334,148, 4,334,351, 4,388,607, 4,400,614,
4,425,497, 4,426,339, 4,435,639, 4,459,473, 4,514,620,
4,520,417, 4,529,866, 4,534,889, 4,543,474, 4,545,926,
4,547,659, 4,560,498, 4,571,481, 4,574,188, 4,582,983,
4,631,392, 4,638,150, 4,654,511, 4,658,121! 4,659,913,
4,661,687, 4,667,194, 4,673,801, 4,698,583, 4,719,335,
4,722,758, and 4,761,541, European Patent Publication Nos.
38,718 (Fouts et al, published October 28, 1981), 158,410
(Batliwalla et al, published October 16, 1985), and 231,068
(Barma et al, published August 5, 1989).
Conductive polymer compositions which exhibit PTC
(positive temperature coefficient of resistance) behavior
are particularly useful for self-regulating strip heaters
and circuit protection devices. These electrical devices
utilize the PTC anomaly, i.e. an anomalous rapid increase
in resistance as a function of temperature, to limit the
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heat output of a heater or the current flowing through a
circuit. Compositions which exhibit PTC anomalies and
comprise carbon black as the conductive filler have been
disclosed in a number of references. U.S. Patent No.
4,237,441 (van Konynenburg et al.) discloses suitable carbon
blacks for use in PTC compositions with resistivities less
than 7 ohm-cm. U.S. Patent No. 4,388,607 (Toy et al)
discloses appropriate carbon blacks for use in compositions
for strip heaters. U.S. Patent No. 4,277,673 (Kelly)
discloses self-regulating articles which comprise highly
resistive carbon blacks. These blacks, either alone or in
combination with a low resistivity carbon black, form PTC
compositions which provide significantly shorter annealing
times.
As indicated in the references, a large number of
carbon blacks are suitable for use in conductive
compositions. The choice of a particular carbon black is
dictated by the physical and electrical properties of the
carbon black and the desired properties, e.g. flexibility or
conductivity, of the resulting composition. The properties
of the carbon blacks are affected by such factors as the
particle size, the surface area, and the structure, as well
as the surface chemistry. This chemistry can be altered by
heat or chemical treatment, either during the production of
the carbon black or in a post-production process, e.g. by
oxidation. Oxidized carbon blacks frequently have a low
surface pH value, i.e. less than 5.0, and may have a
relatively high volatile content. When compared to
nonoxidized carbon blacks of similar particle size and
structure, oxidized carbon blacks have higher resistivities.
It is known that carbon blacks which are oxidized provide
improved flow characteristics in printing inks, improved
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wettability in certain polymers, and improved reinforcement
of rubbers.
SUMMARY OF THE INVE~TION
We have now found that conductive polymer compositions
with improved thermal stability can be made when the
conductive filler comprises carbon black with a low pH. We
have found that the use of such carbon blacks results in an
increased PTC anomaly when compared to similar, nonoxidized
carbon blacks, even when the composition is more highly
reinforced due to an increased filler content required to
compensate for higher resistivity. Therefore, ln one
aspect, this invention provides an electrical device which
comprises
(l) a PTC element comprising a conductive polymer
composition which exhibits PTC behavior, which has
a resistivity at 20C Rcp~ and which comprises
(a) an organic polymer which has a crystallinity
of at least 5% and a melting point Tm~ and
(b) carbon black which has a pH of less than 4.0;
and
(2) two electrodes which can be connected to a source
of electrical power to pass current through the
PTC element,
said electrical device having a resistance Ri at 20C and
being such that if the device is maintained at a temperature
equal to Tm for a period of 50 hours and is then cooled to
20C, its resistance at 20C, Rf50, is from 0.25Ri to 1.75Ri.
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We have found that the physical and electrical
properties of the carbon black may be used to determine
suitable fillers for use in compositions of the invention.
Therefore, in a second aspect the invention provides a
conductive polymer composition which exhibits PTC behavior
and which comprises
(1) an organic polymer which has a crystallinity of at
least 5% and a melting point Tm~ and
(2) carbon black which has a pH of less than 4.0, a
particle size of D nanometers and a dry resis-
tivity RCB such that (RCBjD) is less than or equal
to 0.1.
DETAILED DESCRIPTION OF THE INVENTION
The carbon blacks useful in the conductive polymer
compositions of this invention have pH values of less than
5.0, preferably less than 4.0, particularly less than 3Ø
The pH is a measure of the acidity or alkalinity of the
carbon black surface. A pH of 7.0 indicates a chemically
neutral surface; values less than 7.0 are acidic, those
higher than 7.0 are basic. Low pH carbon blacks generally
have a relatively high volatile content, volatile content
being a measure of the amount of chemisorbed oxygen which is
present on the surface of the carbon black. The amount of
oxygen can be increased by oxidation in a post-production
process. The resulting carbon black will have a higher
surface activity. For purposes of this specification, the
terms "low pH carbon black" and "oxidized carbon black" are
used as equivalent terms. The pH of the carbon black is
that which is measured prior to mixing the carbon black with
the polymer.
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The low pH carbon blacks of this invention are used in
conductive polymer compositions which exhibit PTC (positive
temperature coefficient) behavior in the temperature range
of interest when connected to a source of electrical power.
The terms "PTC behavior" and "composition exhibiting PTC
behavior" are used in this specification to denote a
composition which has an R14 value of at least 2.5 or an
Rloo value of at least 10, and preferably both, and
particularly one which has an R30 value of at least 6, where
R14 is the ratio of the resistivities at the end and the
beginning of a 14C range, Rloo is the ratio of the
resistivities at the end and the beginning of a 100C range,
and R30 is the ratio of the resistivities at the end and the
beginning of a 30C range. In contrast, "ZTC behavior" is
used to denote a composition which increases in resistivity
by less than 6 times, preferably less than 2 times in any
30C temperature range within the operating range of the
heater.
Carbon blacks with suitable size, surface area and
structure for use in PTC compositions are well-known.
Guidelines for selecting such carbon blacks are found in U.S
Patent Nos. 4,237,441 (van Konynenburg et al.) and 4,388,607
(Toy et al.). In general, carbon blacks with a relatively
large particle size, D (measured in nanometers), e.g.
greater than 18 nm, and relatively high structure, e.g.
greater than about 70 cc/100 g, are preferred for PTC
compositions.
Carbon blacks which are particularly preferred for
compositions of the invention are those which meet the
criteria that the ratio of the resistivity of the carbon
black (in powder form) to the particle size (in nanometers)
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is less than or equal to 0.1, preferably less than or equal
to 0.09, particularly less than or equal to 0.08. The
resistivity of the carbon black in ohm-cm is determined by
following the procedure described in Columbian Chemicals
Company bulletin "The Dry Resistivity of Carbon Blacks"
(AD1078). In this test, 3 grams of carbon black are placed
inside a glass tube between two brass plungers. A 5 kg
weight is used to compact the carbon black. Both the height
of the compacted carbon black and the resistance in ohms
between the brass plunger electrodes are noted and the
resistivity is calculated. The ratio is useful for carbons
which are tested in their powder, not pelletized, form.
While most nonoxidized carbon blacks fulfill the require-
ments of this ratio, the carbon blacks particularly useful
in this invention are those which both meet the ratio and
have a pH of less than 5Ø
Other conductive fillers may be used in combination
with the designated carbon black. These fillers may
comprise nonoxidized carbon black, graphite, metal, metal
oxide, or any combination of these. When a nonoxidized
carbon black, i.e. a carbon black with a pH of at least 5.0,
is present, it is preferred that the pH of the nonoxidized
carbon black be at least 1.0 pH unit greater than the pH of
the oxidized carbon black. It is preferred that the low pH
carbon black be present at a level of at least 5% by weight,
preferably at least 10% by weight, particularly at least 20%
by weight of the total conductive filler, e.g. 25 to 100%
by weight of the total conductive filler. For most com-
positions of the invention, the low pH carbon black compri-
ses at least 4% by weight, preferably at least 6% by weight,
particularly at least 8% by weight of the total composition.
For compositions which comprise inks, the presence of the
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solvent is neglected and the content of the solid
components, e.g. carbon black and polymer, is considered the
total composition.
Commercially available carbon blacks which have low pH
values may be used. Alternatively, nonoxidized carbon
blacks may be treated, e.g. by heat or appropriate oxidizing
agents, to produce carbon blacks with appropriate surface
chemistry.
The conductive polymer composition comprises an organic
poIymer which has a crystallinity of at least 5%, preferably
at least 10%, particularly at least 15%, e.g. 20 to 30%.
Suitable crystalline polymers include polymers of one or
more olefins, particularly polyethylene; polyalkenamers
such as polyoctenamer; copolymers of at least one olefin
and at least one monomer copolymerisable therewith such
as ethylene/acrylic acid, ethylene/ethyl acrylate, and
ethylene/vinyl acetate copolymers; melt-shapeable fluoro-
polymers such as polyvinylidene fluoride, ethylene/tetra-
fluoroethylene copolymers, and terpolymers of vinylidene
fluoride, hexafluoropropylene, and tetrafluoroethylene;
and blends of two or more such polymers. (The term
"fluoropolymer" is used herein to denote a polymer which
contains at least 10%, preferably at least 25%, by weight of
fluorine, or a mixture of two or more such polymers.) In
order to achieve specific physical or thermal properties for
some applications, it may be desirable to blend one
crystalline polymer with another polymer, either crystalline
or amorphous. When there are two or more polymers in the
composition, the blend must have a crystallinity of at least
5%. The crystallinity, as well as the melting point Tm are
determined from a DSC(differential scanning calorimeter)
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trace on the conductive polymer composition. The Tm is
defined as the temperature at the peak of the melting curve.
If the composition comprises a blend of two or more poly-
mers, Tm is defined as the lowest melting point measured for
the composition (often corresponding to the melting point of
the lowest melting component).
The composition may comprise additional components,
e.g. lnert fillers, antioxidants, flame retardants, prorads,
stabilizers, dispersing agents. Mixing may be conducted by
any suitable method, e.g. melt-processing, sintering, or
solvent-blending. Solvent-blending is particularly
preferred when the conductive polymer composition comprises
a polymer thick film ink. The composition may be cross-
linked by irradiation or chemical means.
The conductive polymer composition of the invention is
used as part of a PTC element in an electrical device, e.g.
a heater, a sensor, or a circuit protection device. The
resistivity of the composition is dependent on the function
of the electrical device, the dimensions of the PTC element,
and the power source to be used. The resistivity may be,
for example, from 0.01 to 100 ohm-cm for circuit protection
devices which are powered at voltages from 15 to 600 volts,
10 to 1000 ohm-cm for heaters powered at 6 to 60 volts, or
1000 to 10,000 ohm-cm or higher for heaters powered at
voltages of at least 110 volts. The PTC element may be of
any shape to meet the requirements of the application.
Circuit protection devices and laminar heaters frequently
comprise laminar PTC elements, while strip heaters may be
rectangular, elliptical, or dumbell- ("dogbone-") shaped.
When the conductive polymer composition comprises an ink,
the PTC element may be screen-printed or applied in any
9 1 3 3 ~
suitable configuration. Appropriate electrodes, suitable
for connection to a source of electrical power, are selected
depending on the shape of the PTC element. Electrodes may
comprise metal wires or braid, e.g. for attachment to or
embedment into the PTC element, or they may comprise metal
sheet, metal mesh, conductive (e.g. metal- or carbon-filled)
paint, or any other suitable material.
The electrical devices of the invention show improved
stability under thermal aging and electrical stress. When a
device is maintained at a temperature equal to Tm for a
period of 50 hours, the resistance at 20C measured after
aging, i.e. Rfso, will differ from the initial resistance at
20C, i.e. Ri, by no more than 75%, preferably no more than
60%, particularly no more than 50%, producing an Rfso of
from 0.25 Ri to 1.75 Ri, preferably from 0.40 Ri to 1.60 Ri,
particularly from 0.50 Ri to l.50 Ri. If a similar test is
conducted for 300 hours, the change in resistance will be
less than 50%, preferably less than 40%, particularly less
than 30%, producing a resistance at 20C after 300 hours,
Rf300, of from 0.50 Ri to 1.50 Ri, preferably from 0.60 Ri
to 1.40 Ri, particularly from 0.70 Ri to 1.30 Ri. It is to
be understood that if a device meets the resistance require-
ment when tested at a temperature greater than Tm~ it will
also meet the requirement when tested at Tm. Similar
results will be observed when the device is actively powered
by the application of voltage. The change in resistance may
reflect an increase or decrease in device resistance. In
some cases, the resistance will first decrease and then
increase during the test, possibly reflecting a relaxation
of mechanically-induced stresses followed by oxidation of
the polymer. Particularly preferred compositions comprising
fluoropolymers may exhibit stability which is better than a
30% change in resistance.
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The invention is illustrated by the following examples.
Examples 1 to 10
For each example, an ink was prepared by blending the
designated percent by weight (of solids) of the appropriate
carbon black with dimethyl formamide in a high shear mixer.
The solution was then filtered and powdered Kynar~ 9301 (a
terpolymer of vinylidene fluoride, hexafluoropropylene, and
tetrafluoroethylene with a melting point of about 88C,
available from Pennwalt) in an amount equal to (100 - % car-
bon black) was added to the filtrate and allowed to dissolve
over a period of 24 to 72 hours. (Approximately 60% solvent
and 40% solids was used in making the ink). Silver-based
ink electrodes (Electrodag~ 461SS, available from Acheson
Colloids) were printed onto ethylene-tetrafluoroethylene
substrates and samples of each ink were applied. Samples of
each ink were aged in ovens at temperatures of 65, 85, 107
and 149C. Periodically, the samples were removed from the
oven and the resistance at room temperature (nominally
20C), Rt, was measured. Normalized resistance, Rn~ was
determined by dividing Rt by the initial room temperature
resistance, Ri. The extent of instability was determined by
the difference between Rn and 1.00. Those inks which
comprised carbon blacks with a pH of less than 5 were
generally more stable than the inks comprising higher pH
blacks.
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TABLE I
Stability of Conductive Inks After Aging
at Elevated Temperature for 300 Hours
(Resistance Measured at Room Temperature)
Carbon Wt% Rn @ Rn @ Rn @ Rn @
Example/ Black E~ CB 65C 85C 107C 149C
1 Conductex~ SC7.0 3.0 1.22 1.75 5.61 6.39
2 Raven~ 1500 6.0 3.0 1.01 1.92 11.88 20.0
3 Raven~ 890 6.5 6.0 1.27 1.77 2.92 6.07
4 Raven~ 850 7.0 4.0 1.32 2.05 4.08 8.48
5 Raven~ 1000 6.0 4.0 1.18 1.43 1.94 4.40
6 Raven~ 16 7.0 5.6 1.11 1.89
7 Raven~ 5750 2.1 8.1 0.87 0.92 0.97 0.56
8 Raven~ 1040 2.8 9.1 0.96 1.15 1.47 1.34
9 Raven~ 1255 2.5 6.0 1.04 1.26 1.12 0.65
10 Raven~ 14 3.0 7.0 0.82 1.00
Notes to Table I:
(1) Conductex and Raven are trademarks for carbon blacks
available from Columbian Chemicals.
(2) Wt% CB indicates the percent by weight of carbon black
used in each ink.
(3) Carbon blacks in Examples 1, 2 and 3 produced inks with
ZTC characteristics.
Measurements on two samples at 93C (i.e. Tm + 5C)
showed that after 50 hours Example 6 (pH = 7.0) had an
Rn of 2.53 and Example 10 (pH 3.0) had an Rn of 1.48.
The Rn values for Examples 1 to 6 and Examples 7 to 10
were averaged for each time interval at the test tem-
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1 334480
peratures. The results, shown in Table II, indicate that
the carbon blacks with high pH values were significantly
less stable than those with low pH values.
TABLE II
Average Rn Values
Hours @65C Hours @85C Hours @107C Hours @149C
300 675 1256 300 675 1256 300 675 1256 300 675 1256
1 to 6 1.2 1.2 1.2 1.8 1.8 1.9 5.3 7.9 9.0 9.1 14.2 15.6
(pH>5)
7 to 10 0.9 0.9 0.9 1.1 1.0 1.0 1.2 1.3 1.3 0.9 1.0 1.0
(pH<5)
Additional tests were conducted on samples from
Examples 6 and 10 in order to determine the stability of the
compositions under applied voltage. After measuring the
initial room temperature resistance, the samples were placed
in environmental chambers maintained at either 20 or 65C
and appropriate voltage was applied to each sample in order
to produce comparable watt densities. Periodically, the
voltage was disconnected and the resistance of each sample
measured. Rn was calculated as previously described. It is
apparent from the results in Table III that the samples con-
taining the oxidized carbon black were more stable than
those with nonoxidized carbon black.
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TABLE III
Rn of Samples After Active Testing
(Tlme in Hours)
Applied Power Rn Rn
pH Volts (w/in2) 20C 65C
S~~les at
20C 65C 300 600 1000 4000 300 600 1000 4000
Raven~ 16 7.0 120 2.3 2.8 1.1 1.3 1.5 6.0 1.4 1.5 1.5 2.0
E~ample 10
Raven~ 14 3.0 240 1.9 3.1 0.8 0.8 0.8 0.7 0.9 0.8 0.7 0.8
Examples 11 to 14
Following the procedure of Examples 1 to 10, inks were
prepared using Kynar~ 9301 as a binder and incorporating the
carbon blacks listed in Table IV. The resistance vs.
temperature characteristics were measured by exposing
samples of each ink to a temperature cycle from 20C to
82C. The height of the PTC anomaly was determined by
dividing the resistance at 82C (Rg2) by the resistance at
20C (R20). It was apparent that at comparable resistivity
values the PTC anomaly was higher for the oxidized carbon
blacks than for the nonoxidized carbon blacks.
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TABLE IV
Carbon D S.A. DBP R~B Rho PrC
~ /Black pH (nm) ( ~ ) (cc/lOOg) (ohm-om) ~JD Wt% (~m-om) Height
11 Raven~ 1000 6.0 28 95 63 2.46 0.088 4.0 750 3.1x
12 Raven~ 1040 2.8 28 90 99 19.20 0.695 9.1 720 13.0x
13 Raven~ 450 8.0 62 33 67 1.36 0.021 5.0 150 23x
14 Raven~ 14 3.0 59 45 111 4.36 0.074 12.0 100 42x
Notes to Table IV:
(1) D represents the particle size of the carbon black
in nm.
(2) S.A. represents the surface area of the carbon black in
m2/g as measured by a BET nitrogen test.
(3) DBP is a measure of the structure of the carbon black
and is determined by measuring the amount in cubic
centimeters of dibutyl phthalate absorbed by 100 g of
carbon black.
(4) Wt% represents the percent by weight of the total
solids content of the ink that is carbon black.
(5) Rho is the resistivity of the ink in ohm-cm.
(6) PTC Height is the height of the PTC anomaly as
determined by R82/R20
(7) RcB is the dry resistivity of the carbon black in
powder form under a 5 kg load.
1 3 3 4 4 8 0 MP1271
-15-
t8) RCg/D is the ratio of the dry resistivity of the carbon
black to the particle size.
Example 15
Using a Brabender~ mixer, 85% by weight of Kynar~ 9301
was melt-processed with 15% by weight of Raven~ 16. (Raven~
16 has a pH of 7.0, a particle size of 61 nm, a surface area
of 25 m2/g, a DBP of 105 cc/100 g and a dry resistivity of
1.92.) The compound was pelletized and then extruded
through a strand die to produce a fiber with a diameter of
approximately 0.070 inch (0.18 cm). Silver paint
(Electrodag~ 504 available from Acheson Colloids) was used
to apply electrodes to pieces of the fiber. The fiber
pieces were then tested at 85C, 107C, and 149C following
the procedure of Examples 1 to 10. The results are shown in
Table V. The test for these samples was discontinued after
743 hours.
Example 16
Following the procedure of Example 15, 20% by weight
of Raven~ 14 was mixed with Kynar~ 9301, extruded into a
fiber, and thermally aged. The results as shown in Table V
indicate that this oxidized carbon black was more stable on
aging than a similar carbon black with a higher pH. When
tested at 93C, i.e. (Tm + 5)C, fibers of Example 15 had an
Rn after 50 hours of~2.76; those of Example 16 had an Rn of
1.73.
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TABLE V
Rn Values for Extruded Fibers
Time in Hours
146 265 743 1058 1687 2566
85C:
Ex.15 (Raven~ 16)2.61 3.13 3.12 - - -
Ex.16 (Raven~ 14)1.40 1.23 1.05 1.15 1.15 1.16
107C:
Ex. 15 (Raven~ 16) 3.954.40 101
Ex. 16 (Raven~ 14) 0.780.98 1.12 0.80 1.16 1.05
149C:
Ex. 15 (Raven~ 16) 27.6137 604
Ex. 16 (Raven~ 14) 0.651.07 1.52 1.43 1.91 2.83
Example 17
Following the procedure of Example 15, fibers were
prepared by blending 55% by weight Elvax~ 250 (ethylene
vinyl acetate copolymer with a melting point of 60C,
available from Dow) and 45~ by weight Raven~ 22 (carbon
black with a pH of 7.0, a particle size of 62 nm, a surface
area of 25 m2/g, and a DBP of 113 cc/100 g, available from
Columbian Chemicals). An ink was prepared by dissolving the
fibers in xylene. After 813 hours at 52C, the Rn value was
0.94.
-- MP1271
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Example 18
Following the procedure of Example 17, fibers were
first prepared with 50% by weight Raven~ 14 in Elvax~ 250
and were then dissolved in xylene. After 813 hours at 52C,
the Rn value of the ink was 0.88.
Example 19
Fibers were prepared from 76% by weight PFA~ 340 (a
copolymer of tetrafluoroethylene and a perfluorovinyl ether
with a Tm of 307C, available from du Pont) and 24% by
weight Raven~ 600 (carbon black with a pH of 8.3, particle
size of 65 nm, DBP of 82 cc/lOOg, and surface area of
34 m2/g, available from Columbian Chemicals) as in Example
15. Samples tested at 311C for 50 hours had an Rn of 0.55.
Example 20
Following the procedure of Example 19, fibers were
prepared with 17% by weight Raven~ i4. After 50 hours at
311C, the Rn value was 0.93.
