Note: Descriptions are shown in the official language in which they were submitted.
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This invention relates to the radiation cross-linking of PTC
conductive polymers.
Conductive polymer compositions exhibiting PTC behavior, and elec-
trical devices comprising them, have been described in published documents
and in our earlier specifications. Reference may b0 made, for example, to
United States Patents Nos. 2,952,761, 2,978,665, 3,243,753, 3,351,882, 3,571,777,
3,757,086, 3,793,716, 3,823,217, 3,858,144, 3,861,029, 4,017,715, 4,072,848,
4,085,286, 4,117,312, 4,177,376, 4,177,446, 4,188,276, 4,237,441, 4,242,573,
4,246,468, 4,250,400, 4,255,698, 4,272,471, 4,276,466 and 4,314,230, J. Applied
Polymer Science 19, 813-815 (1975), Kalson and Kubat; Polymer Engineering and
Science 18~ 649-653 (1978), Narkis et al; German OLS 2,634,999, 2,746,602,
2,755,076, 2,755,077, 2,821,799 and 3,030,799; European Published Applications
Nos. 0028142, 0030479, 0038713, 0038714, 0038715 and 0038718; and copending
Canadian Patent Application Nos. 375,780, 375,839, 377,699, 363,205, 405,041
and 410,978.
It is known to cross-link PTC conductive polymers by radiation, and
in practice the dosages employed have been relatively low, e.g. 10-20 Mrads.
Higher dosages have, however, been proposed for some purposes. Thus OLS
2,634,9g9 recommends a dose of 20-45 Mrads; United Kingdom Specification No.
1,071,032 describes irradiated compositions comprising a copolymer of ethylene
and a vinyl ester or an acrylate monomer and 50-400% by weight of a filler,
e.g. carbon black, the radiation
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dose being about 2 to about 100 Mrads, preferably about
2 to about 20 Mrads, and the use of such compositions
as tapes for grading the insulation on cables; and U.5.
Patent No. 3,~51,882 discloses the preparation of
electrical devices by embedding planar electrodes in a PTC
conductive polymer element, and then cross-linking the
conductive polymer by irradiating it to a dosage of 50
to 100 Mrads.
The higher the voltage applied to an electrical
device comprising a PTC conductive polymer, the
more likely it is that intermittent application of the
voltage will cause the device to fail. This has been a
- serious problem, for example, in the use of circuit
protection devices where the voltage dropped over the
device in the "tripped" (i.e. high resistance) condition
is more than about 200 volts. [Voltages given herein
are DC values or RM5 values for AC power sources.] We
have now discovered that the likelihood oF such failure
can be substantially reduced by irradiating the conduc
tive polymer so that it is very highly cross-linked.
In its first aspect, the invention provides a
process for the preparation of an electrical device com-
prising (a) a cross-linked PTC conductive polymer element
and (b) two electrodes which can be connected to a power
source to cause current to flow through the PTC element,
which process comprises cross-linking the PTC element by
irradiating it to a dosage of at least 50 Mrads, subject to
the proviso that if each of the electrodes has a substan-
tially planar configuration, then either (a) the element
is irradiated to a dosage of at least 120 Mrads, or
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(b) the electrodes are metal foil electrodes which are
secured to the PTC element after it has been cross-linked.
Our experiments indicate that the higher the
radiation dose, the greater the number of "t~ips"
(i.e. conversions to the tripped state) a device will
withstand without failure. The radiation dose is,
therefore, preferably at least 60 Mrads, particularly
at least sn Mrads, with yet higher dosages, e.g. at
least 120 Mrads or at least 160 Mrads, being preferred
when satisfactory PTC characteristics are maintained
and the desire for improved performance outweighs the
cost of radiation.
We have further discovered a method of determining
the likelihood that a device will withstand a substantial
number of trips at a voltage of 20û volts. This method
involves the use of a scanning electron microscope (SEM)
to measure the maximum rate at which the voltage changes
in the PTC element when the device is in the tripped
state. This maximum rate occurs in the so-called "hot
zone" of the PTC element. The lower the maximum rate,
the greater the number of trips that the device will
withstand. Accordingly, the present invention provi~es,
in a second aspect, an electrical device which
comprises (a) a radiation cross-linked PTC conductive
polymer element and (b? two electrodes which can be
connected to a power source to cause current to flow
through the PTC element, said device, when subjected to
SEM scanning (as hereinafter defined), showing a
maximum difference in voltage between two points
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separated by lO microns which is less than 4.2 volts,
e.g. less than 4.0 volts, praferably less than 3.0
volts, particularly less than 2.0 volts, especially
less than l.0 volt, subject to the proviso that if each
of the electrodes has a substantially planar configura-
tion, the maximum difference is less than 3 volts.
The term "SEM scanning" is used herein to denote
the following procedure. The device is irlspected to
see whether the PTC element has an exposed clean
surface which is suitable for scanning in an SEM and
which lies between the electrodes. If there is no such
surface, then one is created, keeping the alteration of
the device to a minimum. The device (or a portion of
it if the device is too large, e.g. if it is an elongate
heater) is then mounted in a scanning electron microscDpe
so that the electron beam can be traversed from one
electrode to the other and directed obliquely at the
clean exposed surface. A slowly increasirlg current is
passed through the device, using a DC power source of
200 volts, until the device has been "tripped" and the
whole of the potential dropped across it. The electron
beam is then traversed across the surface and, using
voltage contrast techniques known to those skilled in
the art, there is obtained a photomicrograph in which
the trace is a measure of the brightness (and hence the
potential) of the surface between the electrodes; such
a photomicrograph is often known as a line scan.
A diagrammatic representation of a typical photomicrograph
is shown in Figure l. It w:ill be seen that the trace
has numerous small peaks and valleys and it is believed
that these are due mainly or exclusively to surface
imperfections. A single "best line" is drawn through
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the trace (the broken line in Figure 1) in order to
average out small variations, and from this "best
line", the maximum difference in voltage between two
points separated by 10 microns is determined~
When reference i5 made herein to an electrode "having
a substantially planar configuration''g we mean an electrode
whose shape and position in the device are such that sub-
stantially all the current enters (or leaves) the electrode
through a surface which is substantially planar.
The present invention is particularly useful for
circuit protection devices, but is also applicable
to heaters, particularly laminar heaters. In one class
of devices, each of the electrodes has a columnar
shape. Such a device is shown in isometric view in
Fi9ure 2, in which wire electrodes 2 are embedded in
PTC conductive polymer element 1 having a hole through
its centre portion.
In a second class of devices, usually circuit
protection devices,
(A) the PTC element is in the form of a strip with
substantially planar parallel ends, the length
of the strip beiny greater than the largest
cross-sectional dimension of the strip; and
(B) each of the electrodes is in the form of
a cap having (i) a substantially planar end
which contacts and has substantially the same
cross-section as one end of the PTC element
and (ii) a side wall which contacts the side
of the PTC element.
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Such a device is shown in cross-section in Figure 3, in
which cap electrodes 2 contact either end of cylindrical
PTC conductive polymer element l having a hole ll thorugh
its centre portion.
In a third class of device~s, usually heaters,
(A) the PTC element is laminar; and
(B) the electrodes are displaced from each other
so that current flow between them is along
one of the large dimensions of the element.
In a fourth class of devices, each oF the electrodes
has a substantially planar configuration. Meshed
planar electrodes can be used, but metal foil electrodes
are preferred. If metal foil electrodes are applied to
the PTC element before it is irradiated, there is a
danger that gases evolved during irradiation will be
trapped. It is preferred, therefore, that metal foil
electrodes ~e applied after the radiation cross-linking
step. Thus a preferred process comprises the
(l) irradiating a laminar PTC conductive polymer
element in the absence oF electrodes;
(2) contacting the cross-linked PTC element from
step (l) with metal foil electrodes under
conditions of heat and pressure, and
(3) cooling the PTC element and the metal foil
electrodes while continuing to press them
together.
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PTC conductive polymers suitable for use in this
invention are disclosed in the patents and applications
referenced above. Their resistivity at 23C is prefer-
ably le.ss than 1250 ohm.cm, eg. less than 750 ohm.cm,
particular~ly less than 500 ohm.cm7 with valùes less
than S0 ohm.crrl being preferred for circùit protection
devices. The polymeric component should be one which
is cross-linked and not significantly degraded by
radiation. The polymeric component is preferably Free
of thermosetting polymers and often consists essentially
of one or more crystalline polymers. Suitable polymers
include polyolefins, eg. polyethylene, and copolymers
of at least one olefin and at least one olefinically
unsaturated monomer containing a polar group. The
conductive filler is preferably carbon black. The
composition may also contain a non-conductive filler,
eg. alumina trihydrate. The composition can9 but
preferably does not, contain a radiation cross-linking
aid. The presence of a cross-linking aid can substan-
tially reduce the radiation dose required to produce aparticular degree of cross-linking, but its residue
generally has an adverse effect on electrical charac-
teristics.
Shaping of the conductive polymer will generally
be effected by a melt-shaping technique, eg. by melt-
extrusion or molding.
The invention is illustrated by the following
Example
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MPO762 & MPO764 COM.
EXAMPLE
The ingredients and amounts thereof given in
the Table below were used in the Example.
TABLE
Masterbatch Final_ ~ix
g wt% vol% g ~t~vol%
Carbon black 144046.8 32.01141.5 33.7 26.7
(Statex G~
Polyethy~ene 158451.5 66.01256.2 37.1 55.2
(Marlex 6003)
Filler * 948.3 28.0 16.5
(Hydral 705)
Antioxidant 52.5 1.7 2.0 41.5 1.2 1.6
Notes-
Statex Gl available from Columbian Chemicals, has adensity of 1.8 g/cc, a surface area (S) of 35
m /g, and an average particle size (D) of 60
millimicrons.
*
Marlex 6003 is a high density polyethylene with
a melt index of 0.3 which is available from
Phillips Petroleum.
~ydral 705 is alumina trihydrate available from
Aluminum Co. of America.
The antioxidant used was an oligomer of 4,4-thio
bis (3-methyl-6-5-butyl phenol) with an
average degree of polymerization of 3 4, as
described in U.S. Patent Number 3,986,981.
*
Trade Mark
_ g _
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After drying the polymer at 70C and the carbon
black at 150C for 16 hours in a vacuum oven, the
ingredients for the masterbatch were dry blended and
then mixed for 12 minutes in a Banbury mixer turning at
high gear. The mixture was dumped, cooled, and granulated.
The final mix was prepared by dry blending 948.3 9. of
Hydral 705 with 2439.2 9. of the masterbatch, and then
mixing the dry blend for 7 minutes in a Banbury mixer
turning at high gear. The mixture was dumped, cooled,
granulated, and then dried at 70C and 1 torr for 16
hours.
Using a cross-head die, the granulated final
mix was melt extruded as a strip 1 cm. wide and 0.25 cm.
thick, around three wires. Two of the wires were pre-
heated 20 AWG (0.095 cm. diameter) 19/32 stranded
nickel-plated copper wires whose centers were 0.76 cm~
apart, and the third wire, a 24 AWG (O.Oh4 cm. diameter)
solid nickel-plated copper wire, was centered between
the other two. Portions 1 cm. long were cut From the
extruded product and from each portion the polymeric
composition was removed from about half the length, and
the whole of the center 24 AWG wire was removed,
leaving a hole running through the polymeric element.
The products wers heat treated in nitrogen at 150C for
30 minutes and then in air at 110C for 60 minutes,
and were then irradiated. Samples were irradiated to
dosages of 20 Mrads, 80 Mrads or 160 Mrads. These
samples, when subjected to SEM scanning, were found to
haye a maximum difference in voltage between two points
separated by 10 microns of about 5.2, about 4.0 and
about 2.0 respectively. Some of these samples were
then sealed inside a metal can, with a polypropylene
envelope between the conductive element and the can.
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The resulting circuit protection devices we-re tested to
determine how many test cycles they would withstand
when tested in a circuit consisting essentially of a
240 volt AC power supply~ a switch, a fixed resistor
and the device. The devices had a resistance of 20-30
ohms at 23C and the fixed resistor had a resistance
of 33 ohms 5 SO that when the power supply was first
switched on, the initial current in the circuit was
4-5 amps. Each test cycle consis-ted of closing the
switch, thus tripping the device, and after a period of
about 10 seconds, opening the switch and allowing the
devics to cool for 1 minute before the next test cycle.
The resistance oF the device at 23C was measured
initially and after every fifth cycle. The Table below
shows the number of cycles needed to increase the
resistance to 1-1/2 times its original value.
Device irradiated to Resistance increased to
a dose of 1-1/2 times after
20 Mrads 40-45 cycles
80 Mrads 80-85 cycles
160 Mrads 90-95 cycles
