Note: Descriptions are shown in the official language in which they were submitted.
This invention relates to electrical devices embodying
conductive polymer compositions.
Conductive and semi-conductive compositions comprising
carbon black dispersed in a polymer are known. They may have room
temperature resistivities ranging from less -than 1 ohm.cm to 108
ohm.cm or more, and may exhibit positive tempera-ture coefficient
(PTC) behavior, zero temperature coefficient (ZTC or constant
wattage) behavior or negative temperature coefficient (NTC)
behavior. Reference may be made, for example, to United States
Patent Nos. 2,978,665, 3,351,882/ 3,823,217, 3,861,029, 3,950,604,
4,017,715, 4,177/376 and 4,246,468, and to German OLS Nos.
2,413,475, 2,746,602, 2,755l076 and 2,821,570. Recent advances
in this field are described in German OLS Nos. 2,948,350,
2,948,281, 2,949,173 and 3,002,721, in our Canadian applications
352,413, 352,414, 358,274 and 363,205, in our Canadian applications
filed contemporaneously with this application, namely 375,795,
375,877, 375,879 and 375,886, and in our United States Patent
Nos. A,314,231, 4,317,027 and 4,352,083.
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5~"''h
~ ~ 7~ 4 ~ ~ 157/113 MP0714
The resistivity of a conductive polymeL at a
particular temperature can be changed by subjecting it to
thermal cycling, i.e by exposing it to elevated temp~
erature followed by cooling. The change is often
particularly marked when the composition is first exposed
to elevated temperature after it has been shaped. When
the polymer is an elastomer, although initial exposure to
elevated temperature often produces the greatest change,
the room temperature resistivity remains very unstable,
i.e. it is changed very substantially by further exposure
to heat. When the polymer is crystalline, a composition
having high resistivity can often be annealed above the
polymer melting point to reduce its room temperature
resistivity to a value which is in the range 1~2 to
106 ohm.cm and which is relatively stable, i.e. it is
changed, but only relatively slowly, by Further exposure
to heat. However, 0ven this relatively slow change in
resistivity is undesirable. We have discovered that in
certain compositions, as defined below, these continued
changes in room temperature resistivity on therrnal cycling
(aFter an initial heat treatment) can be reduced or even
substantially eliminated by inclusion oF a suitable amount
of a non-conductive filler.
In one aspect, the inve~tion provides electrical devices
comprising a conductive polymer element and at least two electrodes
which can be connected to a power source so that current flows
through the element, the conductive polymer element being formed
from a conductive polymer composition comprising a crystalline
polymer component and a particulate filler component which has
been dispersed in the polymer component and which comprises carbon
black and a non-conductive particulate filler, said composition
having a resistivity Of 102 to 106 ohm.cm at 23C after having been
subjected to a thermal cycle which consists of heating the
composition from 23C to Tt st and cooling the composition from
TteSt to 23 C, where TteSt is 20C above the crystalline melting
point of the polymer, wherein said particulate filler component has
a total surface area of at least 1800 m~ per 100 cc. of composition
and comprises at least 4% by volume of the composition of carbon
black and at least 4% by volume of the composition of at least
one non-conductive particulate filler.
Where reference is made herein to heating or cooling
a composition, the heating or cooling is effected in such a way
"' `'
~:~7~52
that the temperature of the composition changes at the rate of 2 C
per minute.
The stability of the compositions to thermal cycling
can be expressed as the ratio of the resistivity of the composition
after the thermal cycle defined above to its resistivity after a
second thermal cycle as defined. The higher the resistivity of
the composition, the more the value of this ratio differs from
the ideal value of l. However, we have founcl that the difference
between 1 and the said ratio is consistently less than it is for
comparable compositions which do not contain a non-conductive
filler as defined.
The electrical devices of the invention may, for
instance, take ~he form of heaters, sensors or circuit control
devices.
The polymer component used in this invention preferably
has a crystallinity of at least 1%, ~enerally a-t least 2%,
preferably at least 5%, particularly at least 10~, especially at
least 20%, as measured by x-ray diffraction.
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157/113 MP0714
In one preferred class of compositions, the crystalline
polymer component is cross-linked, the gel fraction oF the
polymer (as calculated from the measured gel Fraction of the
composition) preFerably being at lzast 0.6, especially at
least 0.75.
When the polymer component is crystalline, it
generally comprises at least 40~, preferably at least 60,~o~
especially substantially 100~, by weight of une or more
polymers having a carbon-containing backbone. When the
polymer component comprises more than one polymer, it is
often preferred that each of the polymers should be crystal-
line. Suitable crystalline polymers include polyolefins,
especially polyethylene (high density and low density) and
polypropylene; halogenated polyolefins such as chlorinated
polyethylene; copolymers which consist essentially of units
derived from at least on~ olefin, preferably ethylene, ano
units derived From at least one olefinically unsaturated
comonomer containing a polar group, preferably vinyl ace-
tate, an acrylate ester, eg. methyl or ethyl acrylate, or
acrylic or methacrylic acid, said comonomer-derived units
preferably constituting at least 10~ and generally not more
than 30O by weight of the copolymer; and crystalline poly-
mers which comprise 50 to 100~, preferably 8û to 100$, by
by weight of -CH2CHCI- or -CH2CF2- units, eg. poly-
vinylidene fluoride or a copolymer of vinylidene fluoride
eg. with tetrafluorethylene. Other suitable crystalline
~7~452 157/113 MP0714
polymers are disclosed in the patents and apDlications
referred to aboYe.
The polymer component, when crystalline,
generally provides up to 60o~ preFerably 40 to 60~o ~ by
weight of the composition, the remainder being provided by
the filler component and other non-particulate ingredients
such as antioxidants.
The polymer component can also be non-crystal-
line, but in this case, the filler component must be such
that the composition has an elongation (as measured by
ASTM D638) oF at most 1C~ot preferably at most 2~, at 23C
and preFerably of at most 50Z at lOO~C. The non-crystalline
polymer component preferably has an elongation of at least
25~ at 23~C. Typically such polymers are elastomers or
thermoplastic elastomers prod~ced by cross-linking a
composition obtained by dispersing the filler component in
an elastomeric gum, ie a non-crystalline polymer which
exhibits elastomeric properties when cross-linked and
which preferably has a glass transition temperature below
23C. The non-crystalline polymers can have a carbon-
containing bacl<bone, e.g. non-crystalline chlorinated
polyethylene and fluorinated elastomer~ such as Vitnn.
The compositions generally contain a single
carbon black, but mixtures of carbon blacks can be used.
The carbon black will preFerably be the sole conductive
~ -r~rJ~rJ~ J~ -8-
~7~
157/113 MP0714
material in the composition. We have surprisingly
found that, for a composition containing a particular
polymer and a given volume percent of a particular
carbon black based on the polyme., the presence of the
non-conductive filler does not cause any great chanye in
the reslstivity of the composition at 23aC or the basic
nature of its resistivity/temperature relationship (ie.
PTC, ZTC or NTC). It is, therefore, possible to produoe
compositions having desired properties by making use of
the information given in the patents and applications
referred to above in order to select suitable combinations
of carbon black and polymer. Thus in one preferred class
of compositions, which generally exhibit consta~t wattage
behavior? the carbon black has a particle size (D) in
millimicrons and a surface area (5) in m2/g such that
5/D is at least 10, preferably at least 12, especially at
least 18, with D preferably being less than 27, especially
less than 18, particularly less than 15, millimicrons. In
another preferred class of compositions, which generally
exhibit PTC behavior, the carbon black has a particle size
(D) of 20 to 150 millimicrons, eg. 20 to 75 millimicrons,
and a surface area in m2/gram (5) such that 5/D is not
more than 10; preferably the value of the quantity
S x volume of filler component
D ~ p~onent
i9 less than 1.
7~
157/113 MP0714
The volume of carbon black is at least 4
eg. 4 to 20~o~ by volume of the composition, with
10 to 45~ generally being used for PTC compositions, 4
to 20,o for ZTC compositions and 4 to 45u for NTC compo-
sitions.
The non-conductive particulate fillers have
resistivities of greater than 106, preferably greater
than 10a, and often greater than 101, ohm.cm.
The particles may be solid or, in suitable cases, hollow.
Excellent results have been Gbtained using glass beads.
Other inorganic fillers which can be used include titanium
dioxide, silica and antimony trioxide. Organic particulate
fillers can also be used1 for sxample particles which are
composed oF an organic polymer having a softening point
such that the particles remain as a discrete particulate
phase during use of the composition. The non-conductive
filler may have a particle size which is smaller or
greater than the carbon black, for example From 0.1 to
100 microns, preferably 1 to 70 microns.
It is usually convenient to use a single non-
conductive filler1 but mixtures of fillers can be used.
Thc filler may have a Rurface coating of a wetting or
coupling agent to render it more readily dispersible in
the polymer component.
--10--
~L~'7~
The volume of non~conductive filler is at least 4%,
eg. 4 to 80%, preferably 6 to 30%, by volume of khe composition.
In PTC compositions, the volume of non-conductive filler is
preferably less than, eg. 0.3 to 0.8 times, the volume of carbon
black. In ZTC compositions the volume of non-conductive filler
is preferably greater than, eg. 2 to 12 times, preferably 3 to 10
times, the volume of the carbon black.
The quantities and types of carbon black and non-
conductive filler used should be such that they have a total
surface area of at least 1800 m2 per 100 cc. of composit:Lon,
preferably at least 2000 m2/100 cc., especially at least 3000
m2/100 cc., particularly at least 4000 m2/100 cc., with higher
values, eg. at least 8,000 m2/100 cc., at least 10,000 m2/100 cc.
and at least 12,000 m2/100 cc. being particularly preferred. The
particulate filler component generally provides at least 10%,
preferably at least 20%, particularly at least 25%, by volume of
the composition.
Certain NTC compositions containing carbon black and a
non-conductive filler are among those described and claimed in our
Canadian application filed contemporaneously herewith, No. 375l886.
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Certain compositions containing carbon black and an
arc-controlling additive such as alumina trihydrate are among
those described and claimed in our Canadian application filed
contemporaneously herewith, No. 375,879.
The filler component can be dispersed in the polymer
component in any suitable way. It is often convenient to use a
master batch technique, ie. to disperse the carbon black in a part
of the polymer and the non-conductive filler in another part of the
polymer, and then to mix the two master batches and the remainder
of the polymer. The dispersion can be shaped by molding or
extrusion of another melt-shaping technique into an element of the
desired shape, any cross-linking thereof being carxied out after
such shaping.
The invention is illustrated by the following Examples,
in which Examples 1, 6 and 7 are comparative examples. The
ingredients used in the Examples and the amounts thereof are set
out in Table I below. The resistivities of the compositions as
- prepared, 20 ~ ~ after a first thermal cycle as defined, 21 ~,
and after a second thermal cycle as defined, 22 ~ ~ are shown in
Table 2.
In the accompanying drawings, both Figures 1 and 2 are
graphs showing the resistivity of a composition according to the
invention plotted against the temperature at which the resistance
measurement was made.
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157/113 MP0714
EXAMPLE_I (Comparative)
The ingredients were introduced into a Banoury
mixer with water-cooled rotors turning at high g~ar and
were mixed at high gear For 4.5 minutes and at low gear
for 1.5 minutes. The mixture was dumped, coo1ed and
granulated. The granules were dried under vacuum at 70C
for 16 hours. A portion oF the dried granules was compres-
sion molded into a slab 0.05 cm. thick. Rectangular
samples 2.5 x 3.8 cm. were cut from the slab and electrodes
were placed on the samples by painting ~.6 x 2.5 cm.
strips of a silvzr-Viton~composition (Electrodag~504) on
both surFaces at each end of the sample. The ~amples wsre
thermally conditioned by maintaining them at 160C For 15
minutes by external heating and then cooling to room
temperature. The resistivity of the composition was
calculated from sample resistance measurements taken at
3C intervals as the samples were subjected to two thermal
cycles, in the first of which the sample was externally
heated from 23 to 160C and then cooled at 23C and in
the second of which the sample was externally heated from
23 to 180 and then cooled to 23C
Figure 1 shQws the resi~tivity of the compo-
sition a~ a function of temperature during these cycles,
the first cycle being shown as a broken line and the
second as a solid line.
fY/~ Pt ~~ K ~ 13 -
5;~
157/113 MP0714
EXAMPLE 2
The procedure of Example I was Followed except
that the ingredients were as shown in Table I; the rotor
was steam-heated until the torque increased considerably,
when the steam was turned off and water was turned on; and
the ingredients were mixed at high gear for 4 minutes.
Figure 2 shows the resistivity of the composition during
the heating and cooling cycles, the first cycle being
shown as broken line and the second as a solid line.
EXAMPLE 3
This example shows the production of a planar
heater comprising planar mesh electrodes having between
them a PTC layer and a contig~ous zrc layer composed of a
composition according to the invention.
Preparation of_ZTC Sheet Material
Master Batch 1 was prepared From the ingredients
shown in Table I. The ingredients were introduced into a
Banbury mixer whose rotor had been preheated by steam and
wa~ turning at Fourth gear. When the torque had increased
considerably, the iteam to the rotor was turned oFF and
water was passed through the rotor to cool it. Mixing was
continued at Fourth gear For 2.5 mins. after the water had
:~7~5;~
157/117 MP0714
been turned on and for a further 2 mins. at third gear.
The mixture was dumped, held on a steam-heated mill,
extruded into a water bath through a a.s cm. extruder
fitted with a pelletizing die, and chopped into pellets.
The pellets were dried under vacuum at 60C for at least
18 hours.
Master Batch 2 was prepared From the ingredients
shown in Table I. The ingredients were introduced into a
Banbury mixer whose rotor was water-cooled and was
turning at fourth gear; mixing was carried out at Fourth
gear for 2 mins. and at third gear for 1.75 mins. The
mixture was dumped, cooled and granulatedO The granules
were dried under vacuum at 60C for at least 18 hours.
The Final mix, containing the ingredients shown
in Table I, was prepared by introducing 11,523 9. of Master
Batch 1, 3,127 9~ of Master Batch 2, 3,48û 9. of high
density polyethylene (Marlex 6003) and 77.7 g. of anti-
oxidant into a Banbury ~ixer whose rotor was water-cooled
and was turning at high gear; mixing was carried out at
high gear for 4 mins. and at low gear for 1 min. The
mixture was dumped, held on a steam-heated miLl, extruded
into a water bath through a 8.9 cm. extruder fitted with a
pelletizing die, and chopped into pellets. The pellets
were dried under vacuum at 70C for 24 hours, and then
r /7~ ~ D /~: ~r~
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s~
157/113 MP0714
extruded into sheet 30 cm. wide and 0.053 cm~ thick,
, ~ using a Davis-Standard~Extruder fitted with a 15 inch
sheet die and operating at 20 RPM with a throushput of 122
cm/minute, The sheet was stored under argon.
Preparation of PTC Sheet Material
The ingredients shown in Table I for the PTC
material were introduced into a Banbury mixer. The
mixture was dumped from the Banbury and converted into
sheet by the same procedure as the Final Mix. The sheet
was stored under argon.
Preparation of Heater
Rectangles 22.2 x 22.9 cm. were cut from the ZTC
sheet material and from the PTC sheet material, and dried
under vacuum at 60C For 9 hours. Two rectangles 20.3 x
lS Z2.9 cm, were cut from a sheet of fully annealed nickel
mesh that had been thoroughly cleaned. The rectangles
were sprayed until the nickel was completely covered, but
the mesh apertures were not Filled, with a composition
containing 60 parts by wcight of methylethyl ketone and 40
parts of Electrodag 502. The coated mesh rectangles were
dried under vacuum for 2 hours at 100C.
r~ ~, b~ f r~ , 5
157/113 MP0714
The PTC, ZTC and mesh rectangles were laminated
to each other by layering a fluoroglass sheet (a release
shest of a glass-fiber reinforced Fluorinated polymer), a
mesh electrode, a PTC layer, a ZTC layer, another mesh
electrode, and another Fluoroglass sheet in a mold and
pressing with a 30.5 cm. press with plate temperatures of
(224C) (top) and 215C (bottom) for 3.5 minutes at l2.5
tonnes ram pressure. The mold was then cooled in an 45
cm. cold press with air cooling at ram pressure for 5
minutes.
The resulting heater blank was masked, leaving
3.8 cm. at each end unmasked. A razor was used to scrape
away PTC or ZTC material (which had been pressed through
the coated mèsh) from the mesh on opposite sides of the
heatar in the unmasked area. The scraped area on each
side of the heater blank was then further abraded with a
grit blaster.
Strips 2.5 x 25.4 cm. were cut from flat and
fully annealed Cu mesh which had besn thoroughly cleaned.
One aide of the strips was coated with a silver/silicone
contact elastomer and strips were then dried in vacuum at
room temperature for a minimum of 4 hours. One end of
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1~764S2 157/113 MP0714
each of the strips were then bent back at a 45 angle and
0.02 x 0.5 x 15.2 cm. flat copper wire was soldered onto
the bent end. One of these strips was applied to each oF
the abraded areas of the heater blank with the silver side
down and then each side of the heater blank was covered
with a 22.2 x 3.~ x 0.03 cm. polyethylene sheet. The
assembly was placed between two 1.3 cm. aluminum plates
and compression molded at 200~C for 3 min. at 2,300 kg.
pressure, and then placed in the cold press for 10 minutes
at 2,30U kg. pressure.
EXAMPLE 4
2191 9. of Master Batch 1 as used in Example 3,
535 9. of Master Batch 2 as used in Example 3, 709 9. of
Alathon 7D50 and 16 9. of the antioxidant were mixed in a
Banbury mixer t3.3 minutes at high gear, 1 minute at low
gear), to give a final mix having the composition shown in
Table 1. The mixture was dumped, cooled and granulated.
The granules were dried under vacuum at 60C For at least
18 hours and then extruded into a tape using a 3/4 inch
Brabender~extruder and a 7.6, 0.05 cm. tap0 die at 40 RPM.
The extruded tape was taken off on a roll stack with the
top roll at 115C and the bottom roll at 95C.
f~O~~f~/'S -lB-
~1764~2 157/113 MP0714
EXAMPLE 5
The ingredients listed in Table I were introduced
into a steam preheated ~anbury mixer turning at high
gear and were mixed for approximately one minute in high
gear. At this point the torque increased considerably, the
cooling water was turned on, and the mixing continued at
high gear for one minute, then low gear for one minute.
The material was dumped, cooled, and granulated, and the
granules were dried at 60C in vacuum for at least 18
hours. The granules were extruded into tape as in
Example 4.
EXAMPLE 6 (Com arative)
642 9. of Master Batch 2 as used in Example 3,
~ 1836 9. of Marlex~6003 and 41 9. of antioxidant were added
to a 5 lb. Banbury mixer turning at high gear. After 4.75
mins., the torque increased considerably and cooling water
was turned on; mixing was continued for 1.5 minutes, and
the mixture was then dumped, cooled and granulated. The
granules, whose composition is shown in Table I, wece
dried under vacuum for at least 18 hours at 60C. The
granules were extruded into tape a9 in Example 4.
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1~L76452
157/113 MP0714
EXAMPLE_7 (Comparative)
The ingredients listed in Table I were introduced
into a Banbury mixer turning at high gear. After mixlng
for 5.6 minut:es at high gear and 1.4 minutes in low
S gear, the mixture was dumpedl cooled and granulated. The
granules were dried~at 60C in vacuum at le~st 1a hours.
:~ The granules were ~extruded into tape as in Example 4.
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