Conductive polymer composition

A conductive polymer composition which has low resistivity and good electrical stability. In one aspect the composition comprises a nonconductive filler which is a dehydrated metal oxide. In another aspect the composition comprises a conductive filler which is metal particles in which the bulk density is less than 0.15 times the true density. Compositions of the invention are particularly useful for circuit protection devices.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to conductive polymer compositions, methods of 
making such compositions, and electrical devices comprising such 
compositions. 
2. Introduction to the Invention 
Conductive polymer compositions and electrical devices comprising them are 
well-known. Such compositions comprise a polymer, and dispersed in the 
polymer, a conductive particulate filler. The type and quantity of the 
conductive particles, as well as the type of the polymer, influence the 
resistivity of the composition. Generally for compositions with 
resistivities greater than about 1 ohm-cm, carbon black is a preferred 
filler. For compositions with lower resistivities, metal particles are 
used. Compositions comprising carbon black are described in U.S. Pat. Nos. 
4,237,441 (van Konynenburg et al), 4,388,607 (Toy et al), 4,534,889 (van 
Konynenburg et al), 4,560,498 (Horsma et al), 4,591,700 (Sopory), 
4,724,417 (Au et al) , 4,774,024 (Deep et al), 4,935,156 (van Konynenburg 
et al), and 5,049,850 (Evans et al) Compositions comprising metal fillers 
are described in U.S. Pat. No. 4,545,926 (Fouts et al) and in U.S. 
application Ser. No. 07/788,655 (Baigrie et al), filed Nov. 6, 1991. The 
disclosure of each of these patents and pending applications is 
incorporated herein by reference. 
In order to improve the electrical stability of conductive polymers it has 
been found that the addition of an inorganic filler such as alumina 
trihydrate is useful. Such compositions comprising carbon black, which are 
particularly useful for high voltage applications, i.e. exposure to 
voltages greater than about 100 volts, are described in U.S. Pat. Nos. 
4,774,024 (Deep et al) and 5,049,850 (Evans et al). In addition, 
metal-filled compositions have been found to be more stable when a second 
filler, either another conductive filler such as a metal or carbon black 
or a nonconductive filler such as alumina trihydrate, is present. Such 
compositions are described in U.S. Pat. No. 4,545,926 (Fouts et al). These 
metal-filled compositions are designed to minimize resistance increase 
after exposure to high temperature conditions. High temperature conditions 
occur either as a result of cycling between an electrically powered and an 
unpowered state, or as a result of passive thermal treatment. Despite the 
objective of maintaining a relatively constant room temperature resistance 
after such exposure, these metal-filled compositions often do increase in 
resistance on cycling. Furthermore, it is difficult to make them 
reproducibly at a given low resistivity value. 
SUMMARY OF THE INVENTION 
We have now discovered, in accordance with a first aspect of the present 
invention, that the electrical stability of conductive polymers can be 
improved by the presence of certain non-conductive fillers. The conductive 
polymer can for example be any of those already known or disclosed in 
copending commonly assigned applications, including in particular those 
described in the patents and applications incorporated by reference 
herein, or any of those novel conductive polymers disclosed in this 
specification. The nonconductive fillers are compounds obtained by partial 
or complete dehydration of the hydrates of metal oxides under conditions 
which do not result in a substantial change in the particle structure of 
the hydrated metal oxide. It is believed that the voids which are present 
in such dehydrated fillers are at least in part responsible for the 
improved stability. It is theorized that these voids, during the 
preparation (including shaping) and/or during the use of the conductive 
polymers, encourage the formation of conductive pathways, and/or 
discourage the disruption of conductive pathways, by one or both of two 
mechanisms. The first mechanism is to scavenge and isolate undesirable 
gases or other moleties within the voids. The second is to provide 
nucleation sites which help such undesirable moieties to produce voids (or 
other imperfections) at locations which do not have an adverse effect on 
electrical properties. 
We have further discovered, in accordance with a second aspect of the 
present invention, that conductive polymers having improved properties can 
be obtained through the use of a conductive filler comprising particles 
which 
(a) comprise metal, and 
(b) have a shape such that particles having the same shape and consisting 
of the same metal have a bulk density, as measured by ASTM B329, DB, which 
is q times the true density, DT, of the metal, where q is less than 0.15, 
preferably less than 0.10, particularly less than 0.075, especially less 
than 0.065. 
Such conductive polymers can, but need not, contain a nonconductive 
dehydrated filler as specified in the first aspect of the invention. 
We have further discovered, in accordance with a third aspect of the 
present invention, that when preparing a conductive polymer composition 
which comprises an organic polymer, a conductive filler which comprises a 
metal, and a non-conductive filler, improved results are obtained if at 
least these three components, and preferably also any additional 
components of the composition, are blended together at a temperature at 
which the polymer is a solid and while the polymer is in the form of a 
powder, and the resulting blend is then processed at a temperature above 
the melting point of the polymer. The conductive filler and/or the 
nonconductive filler can be, but need not be, a nonconductive dehydrated 
filler or a metal filler as specified in the first and second aspects of 
the present invention. 
We have further discovered, in accordance with a fourth aspect of the 
present invention, that when 
(a) an article comprising a laminar conductive polymer element, e.g. a 
laminate comprising two laminar electrodes (e.g. metal foil electrodes) 
and a layer of a conductive polymer sandwiched between the electrodes, is 
produced by a hot-pressing step, e.g. a lamination step in which the 
electrodes are laminated to a sheet of the conductive polymer under heat 
and pressure, followed by a second pressing step in which the article is 
maintained under pressure while it cools (e.g. as described in U.S. Pat. 
No. 4,426,633 (Taylor), the disclosure of which is incorporated herein by 
reference) , and 
(b) the conductive filler comprises particles whose shape can be changed by 
the pressures which can be exerted by the equipment used to carry out the 
second pressing step, for example (but not limited to) particles of the 
kind referred to in the second aspect of the invention, and in particular 
filamentary metal particles of the kind described in detail below, 
the pressure exerted on the conductive polymer during the second pressing 
step can have an important effect on the electrical properties of the 
product. In particular, a result of using too high a pressure can be to 
distort the filler particles and thus to increase the room temperature 
resistivity of the final product and/or decrease its switching 
temperature. This is in general disadvantageous. Thus the pressure should 
preferably be chosen to avoid that result (while, of course, maintaining 
it sufficiently high to produce the desired result of that step, e.g. 
adequate bonding between electrodes and the conductive polymer). However, 
there may be cases where the pressure is deliberately chosen to be high 
enough to produce that result. Another practical consequence of this 
discovery is the need to ensure, if the pressure is in the region where 
the electrical properties of the conductive polymer are sensitive to 
pressure, that there is a very uniform pressure over the whole area of the 
article and, if a number of such articles are stacked and pressed 
together, the same uniform pressure on each of the articles. Otherwise 
there will be an undesirable variation in the properties of supposedly 
identical electrical devices which are prepared from different parts of 
the same article or from different articles. In one embodiment of this 
aspect of the present invention, a second pressing step of the type 
described above is carried out using a pressure which is k times 
P.sub.crit, where k is 0.5 to 0.95, preferably at least 0.6, particularly 
at least 0.65, especially at least 0.7, more especially at least 0.75, and 
preferably not more than 0.9, particularly not more than 0.8, and Perit is 
a pressure determined by a series of experiments which are identical to 
the procedure actually employed in the hot-pressing step and the second 
pressing step, except that the pressure in the second pressing step is 
varied and the resistivity of the conductive polymer at an identical 
position near the center of the press is measured after the second 
pressing step. The results of these experiments are recorded in the form 
of a graph of resistivity in ohm-cm at 23.degree. C. (on the vertical 
axis) as a function of average pressure in kg/cm.sup.2 (on the horizontal 
axis). Petit is the lowest pressure at which the resistivity is equal to 
1.1 times the resistivity at a pressure equal to 0.9 times Petit. In 
another embodiment of this aspect of the present invention, a second 
pressing step of the type described above is carried out at an average 
pressure which is x times P.sub.crit, where x is at least 0.8, for example 
at least 0.9, and generally not more than 2, preferably not more than 1.5, 
particularly not more than 1.2, and under conditions such that the maximum 
pressure on the conductive polymer at any point is not more than t times 
the minimum pressure on the conductive polymer at any point, where t is 
1.2, preferably 1.1, particularly 1.05. 
We have further discovered, in accordance with a fifth aspect of the 
present invention, that when an article comprising two metal foils and a 
layer of conductive polymer sandwiched between them, is irradiated, 
particularly to high dosages (as described for example in U.S. Pat. Nos. 
4,845,838, 4,951,382, 4,951,384, and 4,955,267 (all Jacobs et al), the 
disclosures of which are incorporated herein by reference), nonuniformity 
of the radiation dose can result in stresses within the conductive polymer 
which are highly undesirable. Such stresses are particularly likely to 
occur when a stack of such articles, one on top of the other, is 
irradiated. They are also more likely to occur when the conductive polymer 
contains a high loading of the conductive filler, particularly a metal 
filler, for example a filler of the kind described in the second aspect of 
the invention. Such stresses can result in distortion or shrinkage of the 
sheet, and consequent delamination from an electrode or other article 
adjacent to the conductive polymer sheet. In one embodiment of this aspect 
of the invention, a plurality of articles, each comprising a laminar 
conductive polymer element, are stacked one on top of another and are 
irradiated in a plurality of steps. Between at least some of the radiation 
steps, the articles are shuffled (i.e. their order in the stack is 
changed) so that the radiation dose is sufficiently uniform, e.g. the 
maximum dose at any point is not more than 1.5 times, preferably not more 
than 1.4 times, particularly not more than 1.3 times, especially not more 
than 1.2 times, more especially not more than 1.1 times the minimum dose 
at any point.

DETAILED DESCRIPTION OF THE INVENTION 
In the following detailed description of the invention, reference is 
frequently made to conductive polymers which exhibit PTC behavior, which 
comprise a conductive metal filler in accordance with the second aspect of 
the invention, a nonconductive dehydrated filler in accordance with the 
first aspect of the invention, and which are prepared by procedures in 
accordance with the third, fourth, and fifth aspects of the invention. It 
is to be understood, however, that where a feature which relates to only 
one aspect of the invention is disclosed in a particular context or as 
part of a particular combination, this specification should be regarded as 
explicitly disclosing that feature as part of the present invention, 
whether that feature is used on its own, or in another context or another 
combination, including, for example, another combination of two or more 
such features. For example, the dehydrated alumina filler described below 
can be used in conductive polymers which contain carbon black as the sole 
conductive filler, or in conductive polymers which exhibit zero 
temperature coefficient of resistance (ZTC) behavior, or in conductive 
polymers based on amorphous polymers. 
The compositions of this invention preferably exhibit PTC behavior, i.e. 
they show a sharp increase in resistivity with temperature over a 
relatively small temperature range. In this specification, the term "PTC" 
is used to mean a composition or device which has an R.sub.14 value of at 
least 2.5 and/or an R.sub.100 value of at least 10, and it is particularly 
preferred that the composition or device should have an R.sub.30 value of 
at least 6, where R.sub.14 is the ratio of the resistivities at the end 
and the beginning of a 14.degree. C. range, R.sub.100 is the ratio of the 
resistivities at the end and the beginning of a 100.degree. C. range, and 
R.sub.30 is the ratio of the resistivities at the end and the beginning of 
a 30.degree. C. range. Generally the PTC compositions of the invention 
show increases in resistivity which are much greater than those minimum 
values. 
The preferred PTC compositions of the present invention are conductive 
polymers which comprise a crystalline polymer component and, dispersed in 
the polymer component, a particulate filler component which comprises 
metal. The compositions generally have a resistivity of less than 10 
ohm-cm, preferably less than 1 ohm-cm, particularly less than 0.1 ohm-cm, 
especially less than 0.05 ohm-cm. The polymeric component is preferably a 
crystalline organic polymer. Suitable crystalline polymers include 
polymers of one or more olefins, particularly polyethylene; 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 fluoropolymers such as polyvinylidene 
fluoride and ethylene/tetrafluoroethylene copolymers (including 
terpolymers); and blends of two or more such polymers. For some 
applications it may be desirable to blend one crystalline polymer with 
another polymer, e.g. an elastomer, an amorphous thermoplastic polymer, or 
another crystalline polymer, in order to achieve specific physical or 
thermal properties, e.g. flexibility or maximum exposure temperature. For 
applications in which the composition is used in a circuit protection 
device, it is preferred that the crystalline polymer comprise 
polyethylene, particularly high density polyethylene. In compositions 
suitable for use in circuit protection devices in which the resistivity of 
the composition is less than 10 ohm-cm, the polymer component generally 
comprises 35 to 75% by volume of the total composition, preferably 40 to 
70% by volume, particularly 45 to 65% by volume, e.g. 50 to 60% by volume. 
The particulate filler component preferably comprises particles which are 
at least partly composed of metal. The term "metal" is used herein to 
include an alloy, though a single metal or a mixture of single metals is 
preferred. Therefore, for some applications, the particles are themselves 
metal, e.g. tungsten, copper, silver, molybdenum, or nickel, whereas for 
other applications the particles may comprise a nonconductive material, 
e.g. glass or ceramic, or a conductive material, e.g. carbon black, which 
has been at least partially coated with a metal to produce a filler with 
an appropriate resistivity. Alternatively, the particle may comprise metal 
which has been coated with another material of a different conductivity, 
e.g. a metal, a metal oxide, or carbon, in order to provide particles with 
improved dispersive tendencies, decreased arcing tendencies, improved 
hardness, or controlled resistivity. Thus, for example, nickel is commonly 
coated with a nickel oxide layer which prevents excessive aggregation 
during compounding. In general, the particulate filler comprises particles 
which have a resistivity of less than 10.sup.-3 ohm-cm, preferably less 
than 10.sup.-4 ohm-cm, particularly less than 10.sup.-5 ohm-cm. It is 
desirable that the polymer and the particulate filler form an 
interpenetrating network. Because of this, especially when the conductive 
polymer is subjected to a melt-shaping step, the preferred particle size 
and shape of the particulate filler are partially dependent on the nature 
of the crystalline polymer and the ability of the polymer to force the 
particles into a particular orientation or formation as the polymer 
crystallized from the melt. Those particles most often used generally have 
an average particle size of 0.1 to 50 .mu.m, preferably 0.5 to 20 .mu.m, 
particularly 1.0 to 10 .mu.m, e.g. 1.0 to 5.0 .mu.m. When the polymer 
comprises polyethylene, it is preferred that the average size of the 
particle be at least 1.0 .mu.m, preferably at least 1.5 .mu.m, 
particularly at least 2.0 .mu.m. The shape of the particle is also 
important: particles such as spheres tend to produce devices which exhibit 
large resistance increases during thermal and electrical tests, whereas 
particles such as flakes or fibers tend to produce devices which exhibit 
electrical instability. In order to achieve optimum electrical and 
physical characteristics, it is preferred that the metal particles have a 
structure of the kind which is often referred to as "filamentary" but 
which is not a simple filament of constant cross-section but is, rather, 
dendritic in form. Such filamentary particles comprise generally spherical 
metal "beads" which are fused together to form a branched chain. Examples 
of such filamentary particles are shown in a product brochure from 
International Nickel, Inc., "INCO Nickel Powders, Properties and 
Applications", December, 1983, the disclosure of which is incorporated 
herein by reference. 
Appropriate metal fillers generally have a bulk density D.sub.B of less 
than 1.3 g/cm.sup.3, preferably less than 1.0 g/cm.sup.3, particularly 
less than 0.8 g/cm.sup.3. Bulk density, also referred to as apparent 
density, is the weight of a unit volume of powder in g/cm.sup.3. The 
values set out herein are determined by following the procedure of ASTM 
B329, in which the weight of a known volume of a powder is determined 
under known conditions. Particularly useful compositions contain 
particulate metal fillers whose bulk density is q times the true density 
D.sub.T of the metal, where q is less than 0.15, preferably less than 
0.10, particularly less than 0.075, especially less than 0.065. The true 
or elemental density of the metal is the weight per unit volume expressed 
as g/cm.sup.3 of the metal, or when the filler comprises a coated metal or 
metal-coated nonconductive particle, the density of the composite filler. 
Particularly preferred for use as the metal filler is a filamentary nickel 
available from Novamet Corporation under the tradename Inco.TM. 255 which 
has a bulk density of about 0.55 g/cm.sup.3 and a true density of 8.9 
g/cm.sup.3. 
The metal filler is generally present in the composition at a loading of 20 
to 50% by volume of the total composition, preferably 25 to 45% by volume, 
particularly 30 to 40% by volume, e.g. 30 to 35% by volume. The conductive 
filler component may also contain a second conductive filler, e.g. carbon 
black, graphite, a second metal or a metal oxide. 
The composition preferably comprises a nonconductive filler in an amount 0 
to 20% by volume of the total composition, preferably 5 to 15% by volume, 
particularly 10 to 15% by volume. In order to avoid producing a material 
which has a viscosity too high to be melt-processed in standard 
compounding equipment such as an extruder, the total amount by volume of 
the metal filler and the nonorganic filler generally should be at most 45% 
by volume of the total composition. This upper limit is subject to the 
viscosity of the crystalline organic polymer and the presence of other 
fillers, and may be different depending on the type of compounding 
equipment used. Suitable nonconductive fillers include alumina trihydrate, 
magnesium oxide, zeolites, quartz, and calcium hydride. Such a filler 
imparts resistance stability and flame retardancy to the composition. When 
the nonconductive filler is alumina trihydrate, it is preferred that it be 
in the form of X-alumina. X-alumina, also known as activated alumina, can 
be produced by heat-treating alumina trihydrate (Al.sub.2 O.sub.3 
.multidot.3H.sub.2 O) in air at a temperature of 450.degree. to 
1000.degree. C. for a period sufficient to completely dehydrate the 
alumina trihydrate and convert the filler in a pseudo-morphic transition 
from alumina trihydrate to X-alumina. A treatment at 600.degree. C. for 12 
hours in air will produce X-alumina, the total time being dependent on the 
amount of material and the oven capacity. It is believed that the use of 
X-alumina improves the electrical performance over similar compositions 
which comprise alumina trihydrate for two reasons. First, X-alumina 
controls void formation better because it scavenges void-forming gases 
generated during arcing and because new voids are nucleated in positions, 
e.g. adjacent a nonconducting particle, where they are least detrimental. 
Second, unlike alumina trihydrate, X-alumina eliminates moisture which 
otherwise might form harmful voids during compounding, processing, and 
use. 
The conductive polymer composition may comprise antioxidants, inert 
fillers, radiation crosslinking agents (often referred to as prorads), 
stabilizers, dispersing agents, or other components. To improve the 
melt-processability of the composition, and to produce greater 
homogeneity, resistance uniformity, higher yields, and improved electrical 
life, it is preferred that a coupling agent, particularly a titanate 
coupling agent, be used. Substituted titanates, e.g. zirconium titanate, 
are particularly preferred. The coupling agent is present at 0 to 5% by 
volume, preferably 1 to 3% by volume, particularly 1 to 2% by volume of 
the total composition, e.g. 1.25 to 1.75% by volume. 
Dispersion of the conductive filler and other components may be achieved by 
melt-processing, solvent-mixing, or any other suitable means. In order to 
achieve low resistivity at a low metal filler loading, it is preferred 
that mixing equipment which provides low shear mixing be used. Increased 
shear results in high resistivity and destruction of the structure of the 
metal filler, requiring more metal filler for a given resistivity level, 
increasing the cost and damaging the physical properties of the compound. 
In order to avoid mechanical fusion of the metal particles into aggregates 
during compounding, it is desirable that the metal be "diluted" or mixed 
with the other ingredients prior to melt-processing. Thus the metal can be 
preblended, e.g. by means of a V-mixer or a conical blender, with the 
nonconductive filler and/or the polymer. It is particularly preferred that 
the crystalline polymer be in the form of a powder and that all of the 
components be premixed. Such preblending minimizes the formation of 
aggregates which can act as sites for physical splitting of extruded sheet 
or sites for electrical failure during testing of devices prepared from 
the compound. 
The compound can be melt-shaped by any suitable method to produce devices. 
Thus, the compound may be melt-extruded, injection-molded, or sintered. 
For many applications, it is necessary that the compound be extruded into 
sheet. To avoid melt-fracture which creates cracks and voids which are 
potential sites for arcing in a device, a very low shear rate die is 
preferably used. If melt-fracture does occur, the extruded sheet can be 
treated, e.g. by hot-pressing, to remove the fractures. For most 
materials, an extrusion temperature of 15.degree. to 115.degree. C. higher 
than the melting point of the crystalline organic polymer (as determined 
by the peak of melting on a differential scanning calorimeter trace) is 
needed. At temperatures below this range, the melt viscosity of the 
composition tends to be too high; at temperatures above this range, 
surging tends to occur in the die. Thus for compositions in which the 
polymer is high density polyethylene, a temperature range of 150.degree. 
to 240.degree. C. is generally appropriate. Mechanical stresses inherent 
in the melt-shaped compound can be relieved by heat-treatment, e.g. by 
heating at a temperature slightly above the melting point of the polymer 
in vacuum for a period of 2 to 48 hours. 
The compositions of the invention can be used to prepare electrical 
devices, e.g. circuit protection devices, heaters, or resistors. Although 
the circuit protection devices can have any shape, e.g. planar or dogbone, 
particularly useful circuit protection devices of the invention comprise 
two laminar electrodes, preferably metal foil electrodes, and a conductive 
polymer element sandwiched between them. Particularly suitable foil 
electrodes are disclosed in U.S. Pat. Nos. 4,689,475 (Matthiesen) and 
4,800,253 (Kleiner et al), the disclosure of each of which is incorporated 
herein by reference. We have found that it is important to control the 
temperature and pressure conditions during the lamination of the metal 
foils onto the conductive polymer element. In a conventional lamination 
procedure, the conductive polymer material is positioned between two metal 
foil electrodes, and the laminate is exposed first to high pressure (e.g. 
at least 100 lbs/in.sup.2 (7 kg/cm.sup.2), and generally higher) at a 
temperature above the melting point of the polymer (i.e. the "hot-press 
step"), and then to a similar high pressure (e.g. at least 100 
lbs/in.sup.2 (7 kg/cm.sup.2) , and generally higher) at a temperature well 
below the melting point of the polymer, in particular at room temperature 
or below (i.e. the "cold-press step"). For the compositions of the 
invention, devices with improved stability have been produced when a lower 
pressure is used during the cold-press step than during the hot-press 
step. For many compositions of the invention, the maximum pressure to 
which the composition is exposed during the cold-press step is at most 
10,000 lbs/in.sup.2 (700 kg/cm.sup.2), preferably at most 1000 
lbs/in.sup.2 (70 kg/cm.sup.2) , particularly at most 200 lbs/in.sup.2 (14 
kg/cm.sup.2). If the conductive polymer composition is exposed to a 
relatively high pressure during the cold-press step, we have found that 
the switching temperature, T.sub.s, i.e. the temperature at which the 
device switches from a low to a high resistance state, will decrease by 
5.degree. to 20.degree. C., and the resistivity at room temperature will 
increase. 
The devices usually comprise leads which are secured, e.g. soldered or 
welded, to the electrodes. These leads can be suitable for insertion into 
a printed circuit board and may be constructed so that they do not inhibit 
expansion of the device, as disclosed for example in U.S. Pat. No. 
4,685,025 (Carlomagno), the disclosure of which is incorporated herein by 
reference. Leads may also be prepared so that devices can be 
surface-mounted onto a printed circuit board. However, devices of the 
invention are particularly suitable for applications, e.g. battery 
protection as described in U.S. Pat. No. 4,255,698 (Simon), the disclosure 
of which is incorporated herein by reference, in which the leads are in 
the form of ribbons or straps which are electrically connected to a 
substrate, such as a battery terminal. Because the resistance of the 
devices is so low, e.g. generally 0.0005 to 0.015 ohms, the resistance of 
the leads, even if composed of a low-resistance metal, can comprise a 
substantial proportion of the total device resistance. Thus the leads can 
be selected to influence or control the thermal properties of the device, 
including the rate at the which the device trips into a high resistance 
state. 
The device can be encapsulated to provide electrical insulation and 
environmental protection, e.g. from moisture and/or oxygen. Suitable 
encapsulants include epoxies, silicone resins, glass, or insulating tapes. 
For many applications, the electrical stability of the device (as defined 
by one or more of improved resistance stability when powered, decreased 
failure rate, increased voltage withstand capability, and lower surface 
temperature) is enhanced if the composition is crosslinked. Crosslinking 
can be accomplished by chemical means or by irradiation, e.g. using an 
electron beam or a Co.sup.60 .gamma. irradiation source. Due to the high 
density of the metal-filled compound compared to conventional carbon 
black-filled conductive polymer compositions, electrons from an electron 
beam are readily reflected and deflected by the metal, tending to generate 
high temperatures which can be detrimental to the polymer. Therefore, for 
most applications, it is preferred that a low beam current (e.g. 5.5 mA 
with a 3.0 MeV electron beam) be used and a low temperature be maintained. 
Thus the temperature preferably should remain below the melting point of 
the polymer by a margin which is generally at least 10.degree. C., 
preferably at least 15.degree. C., particularly at least 20.degree. C., 
e.g. 25.degree. to 30.degree. C. If the temperature is allowed to 
increase, for example, due to a high beam current (e.g. &gt;7 mA with a 3.0 
MeV electron beam), some crosslinking will tend to occur in the melt, 
resulting in a composition which exhibits a PTC anomaly at a lower 
temperature than expected. During irradiation, stresses may be induced in 
the composition as a result of a nonuniform irradiation profile across the 
composition. Such stresses can produce a nonuniform crosslinking density, 
resulting in shrinkage and distortion of the sheet and delamination of 
foil electrodes. Particularly is this so when irradiating a stack of 
individual sheets or laminates (each comprising two metal foils and a 
sheet of conductive polymer between the foils). In order to minimize the 
effects of the nonuniform irradiation profile, it is useful to irradiate 
the stack in several steps, interchanging the sheets or laminates between 
the steps to achieve uniform irradiation. For most compositions, the total 
dose is preferably at least 10 Mrads, but no more than 150 Mrads. Thus 
irradiation levels of 10 to 150 Mrads, preferably 25 to 125 Mrads, 
particularly 50 to 100 Mrads, e.g. 60 to 80 Mrads, are useful. If the 
conductive polymer is to be laminated between sheet electrodes, 
irradiation may be conducted either before or after the lamination. 
The low resistivity (&lt;10.sup.-1 ohm-cm) and high PTC anomaly (in some 
instances, more than 10 decades of resistance change) of compositions of 
the invention make them suitable for use in a number of applications in 
which conventional carbon black-filled compositions are inadequate. For 
example, when used in a thermal protector, the high PTC anomaly of the 
composition permits less leakage current at elevated ambient temperature 
than a typical carbon loaded device. The low resistivity allows very small 
devices to be prepared, thus minimizing space requirements. Such small 
devices are particularly useful on printed circuit boards, e.g. to protect 
computer mother boards and disk drives; in compact battery packs for 
hand-held devices, e.g. video cameras and power tools; for thermal 
protection of compact electrical components, e.g. tantalum capacitors; and 
for protection of small devices which require large operating currents, 
e.g. high torque motors. If the metal filler which is used is nickel, the 
device will be magnetic and will heat efficiently in the presence of an 
inductive field. Such devices can be used as induction switches. Because 
the materials are very thermally conductive, they can act as 
self-regulating heat-sinks. The thermal conductivity, like the electrical 
resistivity, undergoes a discontinuity near the melting point of the 
polymer. As a result, the composition adjusts its ability to conduct heat 
in response to temperature, restricting heat transfer at high 
temperatures. Devices prepared from the compositions of the invention can 
be thermally coupled with a conventional conductive polymer device to 
produce an interlock device for circuit protection. When an over-current 
event causes the conventional device to switch into its high resistance 
state and, as a result, to heat, the metal-filled device is driven to a 
high resistance state which configures a second independent circuit in the 
open state. The devices also can be thermally and/or electrically coupled 
to other electrical components, e.g. varistors, to form a composite device 
in a manner disclosed in U.S. Pat. No. 4,780,598 (Fahey et al), the 
disclosure of which is incorporated herein by reference. Devices of the 
invention have a sufficiently low resistance that the device does not 
degrade the voltage clamp performance of the varistor during normal 
operating conditions. In the event of an overvoltage condition of long 
duration (i.e. more than a few seconds), however, the PTC device switches 
into a high resistance state and protects the varistor from overheating 
and self-destructing. 
The invention is illustrated by the drawing in which FIG. 1 is a plan view 
of a circuit protection device 1 and FIG. 2 is a cross-sectional view 
along line 2--2. The device consists of a PTC element or chip 3 to which 
are attached metal leads 11,13. The PTC element 3 comprises a conductive 
polymer element 5 which is sandwiched between two metal electrodes 7,9. 
FIG. 3 shows an alternative configuration for the leads 11,13 to give a 
device suitable for attachment to the terminals of a battery. 
FIG. 4 is a schematic drawing of a filamentary nickel particle which is 
suitable for use in compositions of the invention. 
The invention is illustrated by the following examples. 
EXAMPLE 1 
Alumina trihydrate (Micral.TM. 916, available from J. M. Huber Chemicals) 
was heated for 16 hours at 600.degree. C. to achieve a weight loss of at 
least 30%. Approximately 6.8% by weight dried alumina trihydrate was 
dry-blended with 93.2% by weight nickel powder (Inco.TM. 255, available 
from Novamet) in a Patterson-Kelly V-blender until the color was uniform. 
The nickel/alumina trihydrate mixture was then dry-blended in a ratio of 
6.03:1 with ground high density polyethylene (Petrothene.TM. LB832 G, 
available from Quantum Chemicals) using a conical mixer. The ingredients 
were mixed with a zirconate coupling agent (NZ.TM. 33, available from 
Kenrich) in a preheated Moriyama mixer for a total of 20 minutes to give 
the final composition listed in Table I. The mixture was granulated and 
dried at 80.degree. C. for 16 hours before being extruded through a 1.5 
inch (38 mm) extruder to produce a sheet (0.030.times.9 inch/0.76 
mm.times.0.23 m) . The sheet was cut into 12 inch (0.030 m) lengths and 
dried at 140.degree. C. in vacuum for 16 hours. Electrodes were attached 
to the extruded sheet by laminating 0.001 inch (0.025 mm) electrodeposited 
nickel foil (available from Fukuda) by a process which required first that 
the extruded sheet be positioned between two sheets of nickel foil, two 
Teflon.TM.-coated release sheets, two silicone rubber pads, two 
Teflon.TM.-coated release sheets, and two metal plates, and then be 
exposed to contact pressure (about 37 lbs/in.sup.2 ; 2.6 kg/cm.sup.2) at 
200.degree. C. for three minutes, 200 to 400 lbs/in.sup.2 (14 to 28 
kg/cm.sup.2) at 200.degree. C. for three minutes, and 200 to 400 
lbs/in.sup.2 (14 to 28 kg/cm.sup.2) at room temperature for three minutes. 
The laminated sheet was dried at 70.degree. C. for 16 hours in vacuum 
before irradiation. Four laminated sheets were positioned in a stack and 
irradiated to a total dose of 80 Mrad using a 3.0 MeV electron beam at a 
beam current of 5 mA. The 80 Mrad total dose was accumulated in four 20 
Mrad steps, rotating the laminated sheet from the bottom to the top of the 
stack following each 20 Mrad increment. The crosslinked sheet was dried at 
70.degree. C. for 16 hours in vacuum before solder dipping and dicing 
into individual chips. The chips were 0.20.times.0.43 inch (5.times.11 mm) 
and had a resistance of 0.015 to 0.018 ohm. Metal leads (1.38.times.0.12 
inch/35.times.3 ram) were attached to the surfaces of each chip to give a 
device as shown in FIGS. 1 and 2. 
TABLE I 
__________________________________________________________________________ 
Components 
Volume 
Weight 
Component Supplier % % 
__________________________________________________________________________ 
Polyethylene (Petrothene .TM. LB832 G) 
Quantum Chemicals 
55.7 14.2 
Nickel (Inco .TM. 255) 
Novamet 33.7 79.6 
Alumina trihydrate (Micral .TM. 916) 
J. M. Huber 
9.1 5.8 
Coupling agent (NZ .TM. 33) 
Kenrich 1.5 0.4 
__________________________________________________________________________ 
Each device was temperature cycled from -40.degree. to +80.degree. C. six 
times, holding the device at each temperature for 30 minutes. Devices were 
tested for cycle life by using a circuit consisting of the device in 
series with a switch, a 6 volt DC power source, and a fixed resistor which 
limited the initial current to 15A. The test consisted of a series of test 
cycles. Each cycle consisted of closing the switch for 3 seconds, thus 
tripping the device, and then allowing the device to cool for 60 seconds. 
A device was deemed to have failed when it overheated, causing the leads 
to detach, or when its resistance at 23.degree. C. had increased to twice 
its initial resistance at 23.degree. C. Other tests were conducted using a 
similar circuit in which the power source was varied from 12 to 48 volts 
DC and the current was limited to 40 or 100A. The results are shown in 
Table II. 
TABLE II 
______________________________________ 
Cycle Life Performance 
Number of First Cycle to 
Voltage Current Cycles Failure 
______________________________________ 
6 VDC 15A 6000 No failures 
6 VDC 40A 1000 No failures 
6 VDC 100A 1000 No failures 
12 VDC 40A 1000 No failures 
12 VDC 100A 1000 No failures 
24 VDC 40A 1000 21 
48 VDC 40A 1000 3 
______________________________________ 
EXAMPLE 2 
Devices were prepared as in Example 1 except that the size was 
0.20.times.0.55 inch (5.times.14 mm). Thirty devices were tested for cycle 
life by using a circuit as in Example 1 in which the power source was 12 
volts DC and the fixed resistor limited the initial current to 40A. Each 
test cycle consisted of closing the switch for 10 seconds to trip the 
device, and then allowing the device to cool for 180 seconds. As shown in 
Table III, all devices survived 1000 cycles without failure. 
Additional devices were also tested for trip endurance. In this test, the 
device, in series with a 15 volt DC power supply, was tripped, and then 
was maintained in its tripped state until failure, as indicated by 
burning, occurred. Of the devices in which the fillers and the polymer 
were preblended, 100% survived more than 3000 hours. 
EXAMPLE 3 
Devices were prepared as in Example 1 except that the fillers and the 
polymer were not preblended. During compounding, the nickel powder and 
alumina trihydrate were slowly added to the molten polymer until mixing 
was complete. Testing was conducted as in Example 2. In the cycle life 
test, 63% of the devices failed before 500 cycles In the trip endurance 
test, the time of survival was only 400 hours. 
TABLE III 
______________________________________ 
Ex- Components Cycle Life (% Survival) 
Trip Endurance 
ample Premixed 500 cycles 
1000 cycles 
(hours) 
______________________________________ 
2 Yes 100% 100% &gt;3000 
3 No 63% 400 
______________________________________ 
EXAMPLES 4 TO 7 
Different types of nickel were tested using the following procedure. Using 
a Brabender mixer heated to 200.degree. C., 40% by volume nickel, as shown 
in Table IV, was mixed with 53.5% by volume polyethylene (Petrothene.TM. 
LB832 G), 5% by volume alumina trihydrate as prepared in Example 1, and 
1.5% by volume coupling agent. The compound was compression molded into 
plaque (0.020 inch/0.51 mm thick) and each plaque was laminated with metal 
foil electrodes as in Example 1. Each plaque was irradiated to 20 Mrad 
using a 3 MeV electron beam, and was cut into devices with dimensions of 
0.5.times.0.5.times.0.02 inch (12.7.times.12.7.times.0.51 mm). Copper wire 
leads (18 AWG; 0.040 inch/1.0 mm diameter) were attached to each of the 
metal foil surfaces. The initial device resistance R.sub.i was measured 
for each device. Resistance stability was measured by testing each device 
for trip endurance. Devices were powered at 15 volts DC and were 
maintained in the tripped state at 15 volts DC for 100 hours before the 
power was removed and the devices were cooled. The final device resistance 
R.sub.f was measured and the ratio R.sub.f /R.sub.i was calculated. The 
device resistance was considered unstable if the ratio R.sub.f /R.sub.i 
was more than 10; a ratio R.sub.f /R.sub.i of 5 to 10 indicated that the 
resistance was metastable. Devices were determined to have stable 
resistance if the ratio R.sub.f /R.sub.i was less than 5 during the test. 
Devices with stable resistance generally had a ratio R.sub.f /R.sub.i of 
less than 2. 
TABLE IV 
__________________________________________________________________________ 
Example 4 5 6 7 
__________________________________________________________________________ 
Nickel Type ICD ONF Inco 123 
Inco 255 
SNP-030 
Supplier Sumitomo 
Sherritt 
Novamet 
Novamet 
Particle Shape 
Spherical 
Spherical 
Spiked Filamentary 
Sphere 
Bulk Density 
1.5 1.45 1.8 0.55 
(g/cm.sup.3) 
Average Particle 
0.3 1.0 2.8 2.5 
Size (.mu.m) 
Surface Area (m.sup.2 /g) 
N/A N/A 0.39 0.68 
Resistivity 1 .times. 10.sup.7 
2 .times. 10.sup.6 
0.05 0.003 
(ohm-cm) 
Resistance unstable 
unstable 
metastable 
stable 
Stability 
__________________________________________________________________________ 
EXAMPLES 8 AND 9 
Following the procedure of Examples 4 to 7, compositions were prepared 
using the 35% by volume nickel as shown in Table V, 53.5% by volume 
polyethylene (Petrothene.TM. LB832 G), 10% by volume alumina trihydrate 
prepared as in Example 1, and 1.5% by volume coupling agent. Devices were 
prepared as in Examples 4 to 7, and then were tested by determining the 
resistivity versus temperature characteristics of the devices over a 
temperature range from 0.degree. C. to 160.degree. C. The devices prepared 
from Example 8, in which the nickel had a comparable bulk density but a 
smaller particle size and larger surface area than that of the nickel of 
Example 9, exhibited less than one decade of PTC anomaly, compared to more 
than 10 decades for Example 9. 
TABLE V 
______________________________________ 
Example 8 9 
______________________________________ 
Nickel Inco 210 Inco 255 
Supplier Novamet Novamet 
Particle shape Filamentary Filamentary 
Bulk Density (g/cm.sup.3) 
0.50 0.55 
Average Particle Size (.mu.m) 
0.94 2.5 
Surface Area (m.sup.2 /g) 
1.86 0.68 
Resistivity (ohm-cm) 
0.09 0.003 
PTC Anomaly (decades) 
&lt;1 &gt;10 
______________________________________ 
EXAMPLES 10 TO 12 
Using a Brabender mixer heated to 200.degree. C., the ingredients listed in 
Table VI were mixed. For Example 12, the alumina trihydrate had been 
heated as in Example 1. Devices with dimensions of 
0.5.times.0.5.times.0.020 inch (12.7.times.12.7.times.0.51 mm) were 
prepared and irradiated following the procedure of Examples 4 to 7. Copper 
wire leads (18 AWG; 0.040 inch/1.0 mm diameter) were attached to each of 
the metal foil surfaces. Devices were tested for cycle life by using a 
circuit consisting of the device in series with a switch, a 15 volt power 
supply, and a fixed resistor which limited the initial current to 100A. 
The test consisted of a series of test cycles, each cycle consisting of 
closing the switch for 10 seconds, thus tripping the device, and then 
allowing the device to cool for 180 seconds. A device was deemed to have 
failed when it overheated or when its resistance at 23.degree. C. had 
increased to 15 times its initial resistance at 23.degree. C. Sixteen 
devices of each type were tested. The composition which contained the 
dehydrated alumina trihydrate did not show a failure until more than 6000 
cycles, compared to the composition without alumina trihydrate which 
showed a failure at 300 cycles, and the composition with hydrated alumina 
trihydrate which showed a failure at about 1000 cycles. 
TABLE VI 
__________________________________________________________________________ 
Example 10 
Example 11 
Example 12 
Component Vol % 
Wt % 
Vol % 
Wt % 
Vol % 
Wt % 
__________________________________________________________________________ 
High density polyethylene 
68.5 
20.3 
53.5 
15.0 
53.5 
15.5 
(Petrothene .TM. LB832 G) 
Nickel (Inco .TM. 255) 
30.0 
79.3 
30.0 
74.5 
30.0 
77.5 
Al.sub.2 O.sub.3.3H.sub.2 O 
15.0 
10.1 
(Micral .TM. 916) 
Al.sub.2 O.sub.3.3H.sub.2 O dehydrated 
15.0 
6.5 
(Micral .TM. 916) 
Coupling agent (NZ .TM. 33) 
1.5 
0.4 
1.5 
0.4 
1.5 
0.5 
Cycles to failure 
300 1000 6000 
__________________________________________________________________________ 
EXAMPLE 13 
Following the procedure of Examples 10 to 12, devices were prepared from a 
composition containing 55% by volume high density polyethylene 
(Petrothene.TM. LB832 G) , 30% by volume nickel (Inco.TM. 255), and 15% by 
volume alumina trihydrate (ATH) (Micral.TM. 916) and were irradiated 20 
Mrad. Devices were tested for cycle life at 15 volts DC/100A. As shown in 
Table VII, the resistance increased rapidly during the first 50 cycles. 
EXAMPLE 14 
Devices were prepared and tested as in Example 13, but instead of hydrated 
alumina trihydrate, 15% by volume of dehydrated alumina trihydrate 
prepared as in Example 1, was used. When tested for cycle life, as shown 
in Table VII, the devices showed greater stability than those of Example 
13. 
TABLE VII 
______________________________________ 
Resistance in milliohms 
Cycle 
Example 0 10 20 30 40 50 60 70 80 90 100 
______________________________________ 
13: hydrated 
2 8 18 31 37 38 36 34 32 31 -- 
ATH 
14: .chi.-alumina 
1 4 8 11 11 10 10 9 8 7 7 
______________________________________ 
EXAMPLE 15 
Devices with dimensions of 0.5.times.0.5.times.0.030 inch 
(12.7.times.12.7.times.0.76 mm) were prepared using the composition of 
Example 1 and following the procedure of Examples 4 to 7 except that the 
devices were not irradiated. The devices were tested for cycle life as in 
Examples 10 to 12. The devices showed a dramatic increase in resistance 
during the first 30 cycles, followed by a decrease to 430 cycles. The 
results are shown in Table VIII. 
EXAMPLE 16 
Devices were prepared and tested as in Example 15 except that the 
electrode-laminated sheet had been irradiated 10 Mrads prior to cutting 
the devices. The device resistance showed a slow increase over 500 cycles 
of the test, as indicated in Table VIII. 
TABLE VIII 
______________________________________ 
Resistance in milliohms 
Cycle 
Example 0 15 33 66 100 200 300 400 500 
______________________________________ 
15: 0 Mrad 
3 14 30 25 20 15 12 11 10 
16: 10 Mrad 
2 3 4 6 9 15 16 17 17 
______________________________________ 
EXAMPLE 17 
Devices were prepared as in Example 1 in which the chip dimensions were 
0.20.times.0.43 inch (5.times.11 mm). Nickel leads 
(0.12.times.1.38.times.0.045 inch/3.0.times.35.times.0.12 mmz) were 
attached to the surfaces of each chip to give a device as shown in FIGS. 1 
and 2. The average power output of the device was 0.5 watt. When tested 
for cycle life (15 VDC/100A inrush current; 10 seconds on/200 seconds 
off), 50% of the devices failed by 100 cycles. 
EXAMPLE 18 
Devices were prepared and tested as in Example 17 except that instead of 
nickel leads, copper leads with dimensions of 0.43.times.0.55.times.0.045 
inch (11.times.14.times.0.12 mm) were attached to the surface to give a 
device as shown in FIG. 3. When tested for cycle life at 15 VDC/100A, 100% 
of the devices survived 100 cycles. In addition, the average power output, 
2.5 watt, was 5 times greater than for the devices of Example 17. 
EXAMPLE 19 
Using a Brabender mixer heated to 235.degree. C., 55% by volume 
polyvinylidene fluoride (Kynar.TM. 460, available from Pennwalt), 35% by 
volume nickel (Inco.TM. 255, available from Novamet), and 10% by volume 
alumina trihydrate (Micral.TM. 916, available from J. M. Huber Chemicals 
and prepared as in Example 1) were mixed. The compound was laminated 
between two sheets of 0.001 inch (0.025 mm) electrodeposited nickel foil 
(available from Fukuda), and the laminated sheet was irradiated in two 40 
Mrad steps to a total of 80 Mrad. Devices with a resistivity of 0.015 
ohm-cm were obtained. These devices, which had a resistance of about 0.003 
ohms, exhibited 5 decades of resistance change (i.e. PTC anomaly) between 
110.degree. and 160.degree. C.