CA2134561A1

CA2134561A1 - Conductive polymer composition

Info

Publication number: CA2134561A1
Application number: CA002134561A
Authority: CA
Inventors: Daniel Chandler; Nelson H. Thein; Edward F. Chu
Original assignee: Individual
Current assignee: Raychem Corp
Priority date: 1992-06-05
Filing date: 1993-06-03
Publication date: 1993-12-23
Also published as: WO1993026014A1; EP0643869A1; ATE208533T1; JP3785415B2; JP3635089B2; DE69331099D1; DE69331099T2; US5378407A; JP2005048191A; EP0643869B1; JPH07507655A

Abstract

2134561 9326014 PCTABScor01 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 an 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 (1).

Description

2 1 ~ 4 S fi 1 PCr/US~3/OS33~ ~

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CONDUCTIYE PO~YMER COMPOSITION

BACKGROUND OF 1~ INVENT~QN ~;

s Field of the Invention This invention relates to conductive polymer composinons, methods of ma~ng -such compositions, and electrical devices comprising ~uch compositions.

Introduction tO the Invention Conducave polymer compositions and electrical devices comprising theIn are well-known. Such composihons comprise a polymer, and dispersed in the polymer, aconductive particulate filler. The type and quantity of the conductive par~icIes, as well s as the type of the polyme~, influence the resistivity of the composition. Generally for compositions with resistivities greater than about 1 ohrn-cm, carbon black is a preferred filler. For composi~ions with lower resistivities, metal par~icles are used. -` -Compositions comprising carbon black are described in U.S. Patent Nos. 4,237,441van 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), 4t724.417 (Au et al), 4,774,024 ~Deep et al),4,935,156 (van Konynenburg et al), and 5,049,850 (Evans et al).
Composi~ons comprising metal fillers are described in U.S. Patent No. 4,545,926 (~;outs et al) and in U.S. Application No. 07n88,6~s (Baigrie et al), filed November 6, 1991. The disclosure of each of these patents and pending applications is incorporated 2s 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. Patent 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 tlihydrate, is presen~ Such compositions are described in U.S. Patent No.
4,545,926 (Fouts et al). These metal-filled compositions are designed to miniIs~ize resistance increase after exposure to high temperature conditions. High temperature conditions occur either as a result of cycling between an electrically powered and an 213~S~; l W~ 93/26014 PCr/US93/0~335 unpowered state, or as a result of passive thermal treatment. DespiR the objective of maintaining a relatively constant room temperature resistance after such exposure, these metal-filled compositions often do increase in resistance on cycling. Furthemlore, it iS
difficult to make them reproducibly at a given low resistivi~y value.
SUMMARY OF l HE TNVENTION

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 10 presence of certain non-conductive fillers. The conductive polymer can for example be any of those already known or disclosed in copending comrnonly assigned applications, including in par~icular those described in the patents and applications incorporated by reference herein, or any of those novel conductive polymers disclosed in this specification. The n~nconductive fillers are compounds obtained by partial or 5 complete dehydration of the hydrates of metal oxides under conditions which do not result in a substantial change in d~e 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 forthe improved stability. It is theorized that these voids, during the - preparation (including shaping) and/or during the use of the conductive pol,~ners~
20 encourage the formation of conductive pathways, and/or discourage the disruption of conductive pathways, by one or both of nYo mechanisms. The first mechanism is toscavenge and isolate undesirable gases or other moieties 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 2s properties.

We have further discovered, in accordance wi~ a second aspect of the present invention, that conductive polymers having improved proper~ies can be obtained t}uough 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 ASTIvI B329, DB.
3s . 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.

wo 93/26014 2 1 3 '~ Pcr/US93/0533~

Such conductive polyrners can, but need not, contain a nonconductive dehydrated filler as specified in the ~ t aspect of the invention.
' s 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 polyrner, a conductive filler which compnses a metal, and a non-conducnve filler, improved results are obtained if at least these three components, and preferably also any additional components of the composition, are blended toge~er 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 conduc~ve 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 presen~ invennon.

We have filrther disc~vered, in accordance with a fourth aspect of the present inventicn, dlat when t ~ (a? an ar~icle comprising a laminar conductive polymer element, e.g. a -laminate comprising ~vo laminar electrodes (e.g. metal foil electrodes) and a layer of a conduc~ive polyrner 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.
- Patent 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) par~cles 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, ~-3s the pressure exerted on the conductive polymer during the second pressing step can haYe an important effect on the electncal properties of the product. In particular, a result of using too high a pressure can be to distort the filler particles and thus to 213~5~1 WO 93/26~14 Pcr/uss3/os335 increase the room temperature resistivi~y 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 resull (while, of course, maintaining it sufficiently ~-high to produce the desired result of that step, e.g. adequate bonding between electrodes and the conductive polyrner). 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 uniforrn 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 theproperties of supposedly identical electrical devices which are prepared from different parts of the same article or from different articles. ln 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 PCnt~ where k is 0.~ to 0.95, preferably at least 0.6, particularly at least 0.65, especially at least 0.77 more especially at least 0.75, and preferably not more than 0.9, particuLtrly not more than 0.8, and PCnt is a pressure determined by a seAes 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 expeAments are recorded in the forrn of a graph of resistivity in ohm~crn at 23C ~on the vertical axis) as a function of average pressure in kg/crn2 (on the horizontal axis). PCri~ is the lowest pressure at which the resistivity is equal to 1.1 2s tirnes the resistivity at a pressure equal to 0.9 times PCri~. In another embodiment of - this aspect of the present invention, a second pressing step of the type described above is caTried out at an average pressure which is x times PCrit, 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, panicularly not more than 1.2, and under conditions such that the maximum pressure on the conductive ~olymer 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 3 s invention, that when an article comprising two metal foils and a layer of conductive polymer sandwiched between them, is irradiated, particularly to high dosages (asdescribed for example in U.S. Patent Nos. 4,845,838, 4,951,382, 4,951,384, and 4 2 1 3 Ll ~ PCI/US93/05335 4,95~,267 (all Jacobs et al), the disclosures of which are incorporated herein by reference), nonunifo~nity of the radiation dose can result in stresses within the conductive polymer which are highly undesirable. Such stresses are par~cularly likely to occur when a stack of such articles, one on top of the other, is irradiated. They are s 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 kin~ described in the second aspect of the invention. Such stresses can r~sult 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 o plurality of articlest each comprising a laminar conductive polymer element, are stacked one on top of another and are irradiated in a plurality of steps. Bet~veen 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 ~mes, particularly not more than 1.3 times, especially not more than 1.2 times, more especially not more than 1.1 times ~e minimum dose at any point.

BRTEF DESCRIPT~ON OF THE DRAWING
. .
Figure 1 is a plan view of a device of the invention;

Figure 2 is a cross-sectional view of the device of Figure 1 along line 2-2;

Figure 3 is a plan view of another device of the invention; and 2s Figure 4 is a schematic example of a preferred filamentary conductive filler.

DETAILED DESCRT~llON OF THE ~NVENTION

In the following detailed description of the invention, reference is frequently rnade to conductive polymers which exhibit ~C behavior, which comprise a conductive metal filler in accordance with the secon ~spect of t' invention, a nonconductive dehydrated filler in accordance with ne fiIrst aspect of the invention, and which are prepared by procedures in accordance with the third, fourth, and fifth aspects 3s 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 ~3`15~ 1 ~
WO 93/26014 Pcr/US93/0~335 feature as par~ of the present inven~ion, whether that feature is used on itS own, or in another context or anotner combination, inclllding, for example, another combination of tWO or more such features. For example, tne dehydrated alumina filler described below can be used in conductive polymers wnich contain carbon black as the sole conductive s filler, or in conduc~ve polymers which exhibit zero temperature coefficient of resistance (ZTC) behavior, or in conductive po}ymers based on arnorphous polymers.

The compositions of this invention preferably exhibit PI C 'oehavior, i e. they show a shaIp increase in resistivity with temperature over a relatively small tempera~re range. In this specification, the term "PI C" is used to mean a composition or device which has an R14 value of at least 2.5 andlor an Rloo value of at least lO, and it is particularly preferred that the composition or device should have 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 ~e end and the beginning of a 100C
range, and R30 is the ratio of the resis~ivities at the end and the beginning of a 30C -`
range. Generally the PTC compositions of the invention show increases in resistivity which are much greate~ ~an those minirnum values.

- - The preferred PI C compositions of the present invendon 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 lO ohm-cm, preferably less than 1 ohm-cm, particularly less than O~l ohm-cm, especially less than 0.05 ohm-cm. Thepolymeric component is preferably a crystalline organic polymer. Suitable c~ystalline 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, ethylenelethyl 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 poly~,ner, 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 cIystalline polyrner 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 lO ohm-cm, the polymer component generally comprises 35 to WO 93/26014 2 1 ~ ~ 5 6 i PCI/US93/05335 7~% by volume of ~he total composition, preferably 40 to 70% by volume, particularly 45 to 65% by volume, e.g. 50 to 60% by volume.

The pa~ac~llate filler component preferably comprises particles which are ~t least s partly composed of metal. l'he tenn "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 par~cles are themselves metal, e.g. tungsten, copper, silver, molybdenum, or nickel, whereas for other applications the par~cles may comprise a - nonconductive material, e.g. glass or ceramic, or a conductiYe material, e.g. carbon o 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 par~icles with improved dispersive tendencies, decreased arcing tendencies, improved hardness, or con~olled resistivity. Thus7 for example, s nickel is commonly coated with a nickel oxide layer which prevents excessive aggregation during compounding. In general, the particulate f1ller compnses particles which have a resistivi~ of less than 10-3 ohm-cm, preferably less ~an 1~4 ohm-cm, particularly less than 1~5 ohm-cm. It is desirable that ~he polymer and the pamculate - filler foIm an interpenetrating network. Because of this, especially when the - -conduc~ve polymer is subjected to a melt-shaping step, the preferred par~cle size and shape of dle 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 fo~nation as the polymer crystallized from dle mel~ Those particles most often used generally have an average particle size of O.l to 50 ~un, preferably 0.5 to 20 ~3, 2s par~icularly l.0 to 10 ~lm, e.g l.0 to 5.0 ,um. When the polymer comprisespolyethylene, it is preferred that the average size of the particle be at least l.0 ,um, preferably at least l.5 ~m, particularly at least 2.0 ~n. The shape of the particle is also important: particles such as spheres tehd to produce devices which exhibit largeresistance increases dunng thermal and electrical tests, whereas par~icles such as flakes 3 o 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" bul nich 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 fused3s together to fo~m a branched chain. Examples of such filamenta~y particles are shown in a product brochure from lnternational Nickel, Inc., "INCO ~iclcel Powders, Properties ' , .. ... . . . .. .. . .

2 13~56 1 and Applica~ons", December, 1983, the disclosure of which is incorporated herein by reference.

Appropriate metal fillers generally have a bulk density Dg of less than 1.3 s g/cm3, preferably less than 1.0 g/cm3, particul~rly less than 0.8 g/cm3. Bulk density, also referred to as apparent density, is the weight of a unit volume of powder in g/cm3.
The values set out herein a.~ç deten~ined by following the pr~cedure of ASTM B329, in which the weight of a known volume of a powder is deteImined under known conditions. Particularly useful compositions contain particulate metal fillers whose I o buLt~ density is q times the true density DT of the metal, where q is less than 0.15, pre~erably less than 0.10, particularly less than 0.075, especially less tban 0.065. The true or elemental density of the metal is the weight per unit volume expressed as g/cm3 of the metal, or when the filler comprises a coated metal or metal-coated nonconductive particle, the density of the composite filler. Particularly prefelTed ~or use as the metal 1 s fille~ is a filarnen~arY nickel available from Novamet Cc)rporation under the tradename `~
IncoTM 255 which has a bulk density of about 0.55 g/cm3 and a true density of 8.9 g/cm3~

- - 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, pa~icularly 30 to 40% by volume, e.g. 3b 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 1~% by volume, paracularly 10 to 15% by volume. In order to avoid producing a matenal which has a viscosi~y 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 cIystalline 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 oxidç, 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 pr~duced by heat-~eating alurnina trihydrate (Al2O3 3H2O) in air at a temperature of wo 93/26014 2 1 3 4 5 ~i 1 PCr/USs3/0~33~

450 to 1000C for a period sufficient to completely dehydrate the alumina tnhydrate and convert the filler in a pseudo-morphic transition from alumina trihydrate to x-alumina.
A treatment at 600C fcr 12 hours in air will produce x-alumina, the total time being dependent on the amount of material and the oven capacity. It is ~lieved that the use of s %-alun~ina 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-folming 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 elirninates moisture which otherwise might form harmful voids during compounding, processing, and use.

The conductive polymer composition may comprise antioxidants, iner~ fillers, radiation crosslinking agents (often referred to as prorads3, stabilizers, dispersing s agents, or other components. To improve the melt-processability of the composition, -`
and to produce greater homogeneity, resistance uniformity, higher yields, and improved electncal 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. 1.2~ 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 ~he 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. lt is particularly preferred that the c~ystalline 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.

2~ 34i36:L
wo 93/26014 Pcr/uss3/os335 ' ~ ' 10 The compound can be melt-shaped by any sui~ble method to produce devices.
Thus, the compound may be melt-extruded, injection-molded, or sintered. For manyapplications, it is necessary ~hat the compound be ex~uded into sheet. To avoid melt-fracture which creates cracks and voids which are potential sites for arcing in a device, s a very low shear rate die is preferably used. If melt-fracture does occur, the extruded sheet car~ be trea2ed, e.g. by hot-pressing, tO remove the fractures. For most materials, an extrusion ~mperature of 15 to l 1~C higher than the melting point of the crys~lline organic polymer ~as dete~nined by the peak of melting on a differential scanningcalonmeter trace) is needed. At temperatures below this range, the melt viscosity of the o composition tends to be too high; at temperatures above this range, surging tends to occur in the die. Thus for composi~ions in which the polymer is high density polyethylene, a temperature range of 150 to 240C is generally appropliate. 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 15 for a period of 2 to 48 hours.

The compositions of the invention can be used to prepare slec~rical devices, e.g.
circuit pr~tection devices, heaters, or resistors. Although the circuit protection devices can have any shape, e.g. planar or dogbone~ particularly useful circuit protection 20 devices of the invention comprise two laminar elec~rodes, preferably metal foil electrodes, and a conductive polymer element sandwiched between them. Particularly suitable foil electrodes are disclosed in U.S. Patents Nos. 4,689,475 (Matthiesen) and 4,800,2~3 (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 2s conditions during the lamina~on 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 lbsfiln2 (7 kg/cm2), 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 30 pressure (e.g. at least 100 lbsrln2 (7 kg/cm2), 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 dunng the hot-press step. For many compositions of the invention, the 35 maximum pressure to which the composition is exposed during the cold-press step is at most 10,000 lbsfin2 (700 kg/cm2), preferably at most 1000 lbs/in2 (70 kg/cm2), particularly at most 200 lbs/in2 (14 kg/cm2). If the conducti- e polymer composition is WO 93/26014 2 1 3 r~ 5 ~ 1 Pcr/US93/05335 exposed tO a relatively high pressure during the cold-press step, we have found that the switching temperature, Ts~ i.e. the temperature at which the device switches from 2 IOW
to a high resistance state, will decrease by 5 to 20C, and the resistivity at room temperan~re will increase.
s 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 conseructed so that they do not inhibit expansion of the device, as disclosed for example in U.S. Patent No. 4,685,025 (Carlomagno), the disclosure of which is ` 10 inco~porated 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. Patent No. 4,255,698 (Simon), the disclosure of which is incorporated herein by re~erence, in which the leads are in the form of ribbons or straps which are electrically connected to a 15 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 dle 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 ~ips 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 in~lude epoxics, silicone resins, glass, or insulating tapes.

2s For many applications, dle elec~ical 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~ ~ irradiation source. Due to the high density of the metal-filled compound compared to conYentional 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 3s temperature be maintained. Thus the temperature preferably should remain below the melting point of the polymer by a margin which is generally at least 10C, preferably at least 1 5C, particularly at least 20C, e.g. 25 to 30C. lf the temperature is allowed tO

.

21 3 ~ ~ S ~
W O 93/26014 ~ PC~r/US93/05335 increase, for example, due to a high beam current (e.g. >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 ~C anomaly at a lower temperature than expected. During irradiation, stresses may be induced in the composition as a result of a nonuniform s iIradiation profile across the composition. Such stresses can produce a nonunifo~n 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 nonunifo~n ~radiation 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 1~0 Mrads. Thus irradiation levels of 10 to iso 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 sheets electrodes, irradiation may be conducted either before or after the lamination.
~.
The low resistivity (<10-~ ohm-cm) and high PTC anomaly (in some instances, more than 10 decades of resistance change) of compositions of the invention make them t - suitable for use in a number of applications in which conventional carbon black-filled compositions are inadequate. For example, when used in a thermal pr~tector, the high PI C 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 25 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 tor~ue 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 30 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 hign temperatures. Devices prepared from the 35 composi~ons of the invention can be thermally coupled with a conventional conductive polymer device to produce an interlock device for circuit protection. When an over-c~rent event causes the conventional device-to switch into its high resistance state and, - WO 93/26014 2 1 ~ 1 P(:~T/US93/0533 as a result. to heat, the metal-filled dev~ce; s driven to a high resistance stat~ which configures a second independent circuit il. Lrle open sta ~. The devices also can be thermally and/or eleclrically coupled to other electr.cai components, e.g. varistors, to form a composite device in a manner disclosed in U.S. Patent No. 4,780,598 (Fahey et s al), the disclosure of which is incorporated herein by reference. Devices of the invention have a sufficien~y low resistance that the device does not degrade the voltage clamp performance of the varistor dunng nonnal 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 o overheating and self-destructing.

The invention is illustrated by the drawing in which Figure l is a plan view of a -circuit protection device 1 and Figure 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 l l, l 3.
1 s The PTC element 3 comprises a conductive polymer element S which is sandwiched between two metal electrodes 7,9. Figure 3 shows an alternative configura~on for the leads l 1 ,l 3 to give a device suitable for attachment to the terrninals of a batte~.

Figure 4 is a schematic drawing of a filamentary nickel particle which is suitable 20 for use in compositions of the invention.

The inven~ion is illustrated by the following examples~

Example 1 Alumina trihydrate (MicralTM 916, available from J.M. Huber Chemicals) was heated for 16 hours at 600C 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'M 255, available from Novamet) in a Patterson-Kelly V-blender until the 30 color was uniform. The nickeValumina trihydrate mixture was then dry-blended in a ratio of 6.03:l with ground high density polyethylene (PetrotheneTM LB832 G, available from Quantum Chemicals) using a conical mixer. The ingredients were mixed with a zirconate coupling agent (NZIM 33, available from Kenrich) in a preheatedMoriyama mixer for a total of 20 minutes to give the final composition listed in Table I.
3s The mixture was granulated and dried at 80C for 16 hours before being extruded through a l.j inch (38 mm) extruder to produce a sheet (0.030 x 9 inch/0.76 mm x0.23 m). ~e sheet was . ~t into 12 inch (0.030 m) lengths and dried at 140C in WO 93/26014 2 1 3 ~ ~ 6 1 ` PCr/US93/OS33~ -vacuum for 16 hours. Electrodes were attached to the extruded sheet by laminating 0.001 inch ~0.025 mrn) electrodeposited nickel foil (available from Fukuda) by aprocess which required first that the extruded sheet be positioned between two sheets of nickel foil, two TeflonlM-coated release sheets, two silicone rubber pads, two TeflonTM-coated release sheets, and two metal plates, and then be exposed to contact `
pressure (about 37 lbs~ln2; 2.6 kg/cm2) at 200C for three minutes, 200 to 400 Ibs/in2 (14 to 28 kg/cm2) at 200C for three minutes, and 200 to 400 lbsfin2 (14 to 28 kg/cm2) at room temperature for three minutes. The laminated sheet was dried at 70C for 16 ~
hours in vacuum before irradiation. Four laminated sheets were posit;oned in a stack -l o and irradiated to a total dose of 80 Mrad using a 3.0 MeV electron beam at a bearn - 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 70C for 16 hours in vacuum before solder dipping and dicing into individual chips. The chips were 0.20 x 0.43 s inch (5 x 11 mm~ and had a resistance of 0.015 to 0.018 ohm. I~Ietal leads (1.38 x 0.12 inch/35 x 3 mm) were attached to the surfaces of each chip tO give a device as shown in Figures 1 and 2.

TABLE I
Components Vo!~me ~ht Component ~ PLi~e~r % %
_ Polyeth lene (PetrotheneTM LB832 G) Quantum Cherr~icals55.7 14.2 Y .
Nickel (IncoTM 255) _ Novamet 33.7 79.6 Alurnina trihydrate (MicralTM 916) J.M. Huber 9;1 5.8 Coupling agene (NZ~M 33) Kenrich _ 1.5 0.4 Each device was temperature cycled from -40 to +80C six times, holding the device at each temperature for 30 minutes. Devices were tested for cycle life by using a 2s 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 ~ 30 23 C had increased to twice its initial resistance at 23 C. Other tests were conducted ,, . ~ . ............. . ... . . ... . .. . . . . . . ...

- wo 93/26014 2 1 3 ~ ~ 6 ~ P~/USg3/0~33s using a similar C31CUit in which the power source ~ aried from 12 to 48 volts DCand the current was limited to 40 or lOOA. The re .s are shown in Table II.

- TABI~ ~I
s Cvcle Life PerfQrmance .~
Voltap,e ---- First C:vcl~ to Failure _ _ . , . .
6 VDC lSA 6000 No ~ailures _ _ ~_ . _ 6 VDC 40A 1000 No failures . . . _ _ 6 VDC lOOA 1000 No failures . . . . _ _ _, .
12 VDC 40A 1000 No f~ulures . _ .
12 VDC lOOA 1000 No failures . . . _. . ... .

_~ . . . , _ .

Example 2 o Devices were prepared as in Example 1 excep~ that the size was 0.20 x O.SS
inch (S x 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 S Table III, all devices suIvived 1000 cycles without failure.

Additional devices were also tested for trip endurance. In this test, the device, in series with a lS volt DC power supply, was tripped, and then was maintained in its tripped state until failure, as indicated by buming, occurred. Of the devices in which the fillers and the polymer were preblended, 100% survived more than 3000 hours.
Exar,nple 3 Devices were prepared as in Exarnple 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 .

... . . . . . . .

Wo 93t260l4 2 1 ~ ~ S 6 3L PCr/US93/0~33~ ;~

conducted as in Exarnple 2. In the cycle life test, 63% of the devices failed before 500 cycles. In the ~p endurance test, the time of survival was only 400 hours.

TABL~ m s ~ ~ . ~:
Components ~1~1~1~!l Tnp Endurance Exam~le emixçd ~00 cvcles l O()0 cvcles (hours) ~ . . - -- ....... ~ .
2 Yes 100% 100% >3000 . _ ~ _ . .. .
3 No 63% 400 Exarnples 4 to 7 Different types of nickel were tested using the following procedure. Using a Brabender mixer heated to 200C, 40% by volume nickel, as shown in Table IV, wasmixed wi~ 53.5% by volume polyethylene (PetrotheneTM LB832 G), 5% by volume alumina trihydrate as prepared in Example 1, and 1.5% by volume coupling agent. The compound was compr~ession molded into plaque (0.020 inch/0.51 mm thick) and eachplaque was laminated with metal foil electrodes as in Example 1. Each plaque wasirradiated to 20 Mrad using a 3 MeV electron beam, and was cut into devices withdimensions of 0.5 x 0.5 x 0.02 inch (12.7 x 12.7 x 0.51 mm). Copper ~,vire leads (18 AWG; 0.040 inch/1.0 mm diameter) were attached to each of the metal foil surfaces.
The initial device resistance Rj was measured for each device. Resistance stability was measured by testing each device for trip endurance. Devices were powered at 1~ volts 20 DC and were maintained in the ~ipped state at 1~ volts DC ~or 100 hours before ~e power was removed and the devices were cooled. The final device resistance Rf was measured and ~he ratio Rf/~i was calculated. The device resistance was considered unstable if the ratio Rf~Rj was more than 10; a ra~o RÇlRi of S to 10 indicated that dle resistance was metastable. Devices were determined to have stable resistance if the 2s ratio Rf/Ri was less than S during the test. Devices with stable resistance generally had a ratio RftR; of less than 2.

.

~.,, . . , . ~ . .

- WO 93/26014 ~ I 3 1 5 ~1 PCr/US~3/05335 - ~

:.

TABLE IV

E ample 4 6 7 ~:
. , Nickel Type ICD ONF lnco 123 Inco 255 _ _. .
Su lier SumitomoShen~tt Novamet Novamet PP _ _ , _.
Particle Sha~e S~hencalSpherical Spiked Sphere Filamentary _ . . _ Bulk Densi2~Icm3) 1.5 1.45 1.8 0.55 ,., . .. . . , .
Average Particle Size (llm) 0.3 1.0 2.8 2.5 , . _ _ .
Surface Area (m2/g) N/A N/A 0.39 0.68 Resistivity 1 x 107 2 x lo6 0.05 0.003 (ohm-cm) ~ . _ Resistance Stability _ unstable unstable metastable _ stable Examples 8 an~
s - --- - Following the procedure of ~xamples 4 to 7, compositions were prepared using the 35% by volume nickel as shown in Table V, 53.5% by volume polyethylene (PetrotheneTM 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, o and then were tested by detennining the resistivity versus temperature characteristics of the devices over a temperature range from 0C to 160C. 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 ~at of the nickel of Example 9, exhibited less than one decade of PI C anomaly, compared to more than 10 decades for Example 9.

WO 93/26014 21~ 1 PCr/lJS93/0533 TABLE V
, Nickel Inco 210 Inco 255 . _ . , ~ . . ,_ .
Su~ier Novamet Novamet ~ . . . _ , .. . . , Particle sha~e hlamentary Filamentar~
. ~ . . . , . ., _ BulkDensity tg/cm3) U :~0 0.55 Average Particle Siæ (~lm) 0.94 2.5 .. . . - . . . , _ ,, Surface Area (m2/g) _ 1.86 0.68 Resistivity (ohm-cm) _0.09 _003 Pl C Anomaly (decades) <1 ~-10 Examples 10 to 12 s Using a Brabender mixer heated to 200~C, the ingredients listed in Table VI
were mixed. For Example 12, the alurnina tnhydrate had been heated as in Example 1.
Devices with dimensions of 0.5 x 0.5 x 0.020 inch (12.7 x 12.7 x 0.51 mm) were prepared and ~rradiated following the procedure of Examples 4 to 7. Copper wire leads o (18 AWG; 0.040 inchf l.0 mm diarneter) 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 c~ent to lOOA. The test consisted of a series of test cycles, each cycleconsisting of closing the switch for 10 seconds, thus ~ipping the device, and then s allowing the device to cool for 180 seconds. A device was deemed to have failed when it overheated or when its resistance at 23C had increased to 15 times its initial resistance at 23C. 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.

wo g3/260l4 2 1 t~ p~r/uss3~o~33~ ~

- TABLE VI
.
Component Example 10 Example 11 Example 12 Vol% Wt% Vol% Wt% Vol% Wt%
_ _ _ _ . _ _ _ High density polyethylene 68.5 20.353.5 15.0 ~3.S 15.5 (Petrothene~M LB832 G) -- _ Nickel (IncolM 255) 30.0 79.330.0 74.5 30.0 77.5 , . _ . . ., A12O3-3~2O 15.0 10.1 ~MicralrM 916) l . .
A1203-3H20dehydrated 15.0 ~` ~; ..
(MicrallM 916) _ _ _ Coupling agent (NZsM 33) 1.5 0.4 1.5 0.4 1.5 0 5 Cvcles to failure 300 1000 6000 Exam~le l 3 s Following the procedure of Examples 10 to 12, devices were prepared from a composition containing 55% by volume high density polyethylene (PetrotheneTM
LB832 G), 30% by volume nickel (IncolM 255), and 15% by volume alumina trihydrate (Al~) (MicralTh 916) and were irradiated 20 Mrad. Devices were tested for -~
o cycle life at 15 volts DC/lOOA. As shown in Table VII, the resistance increased rapidly du~ing the first 50 cycles.
Exam~le 14 lS 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 vn, the devices showed greater stability than those of Example 13.

wo 93/26014 213 k ~ G 1 ~ PCr/US93/0~335 TABLE VII
Resistance in milliohms xample ____ Cvcle ___ _ _ ~ lû 20 30 40 S0 60 70 80 90 1 100 _ _ _ _ _ 13: hydrated ATH 2 8 18 31 37 38 36 34 32 31 14: x-alumina 1 4 8 11 11 _ lû -- 8 7 7 5 Example 15 Devices with dimensions of 0.5 x 0.5 x 0.030 inch (12.7 x 12.7 x 0.76 mm) were prepared using the composition of Example 1 ~d following the procedure of Examples 4 to 7 except that the devices were not irradiated. The devices were tested for o 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 Cvcle _ Example 0 15 33 66 100 200 300 400 500 15: 0 Mrad 3 14 30 25 20 15 12 11 10 16: lO~ad 2 3 4 6 9 15 16 17 17 . .

wo 93/26~14 2 1 3 1 ~ 5 1 PCr~USs3/05335 Example l?

Devices were prepared as in Example 1 in which the chip dimensions were 0.20 x 0.43 inch (S x 11 mm). Mckel leads (0.12 x 1.38 x 0.045 inchJ3.0 x 35 x 0.12 mm) were attached to the surfaces of each chip to give a device as shown in Figures 1 and 2.
The average power OUtput of the device was 0.5 watt. When tested for cycle life (15 ;~
VDC/lOOA inrush current; 10 seconds on/200 seconds off~, 50~o of the devices failed by 100 cycles.

o Example 18 Devices were prepared and tested as in Example 17 except that instead of nickel leads, copper leads with dimensions of 0.43 x 0.55 x 0.045 inch (11 x 14 x 0.12 mm) were attached to the surface to give a device as shown in Figure 3. When tested for cycle life at 15 VDC/lOOA, 100% of the devices survived 100 cycles. In addition, the average power output, 2.5 watt, was 5 ~imes greater than for dle devices of Example 17.

~le l9 - ~

Using a Brabender mixer heated to 235C, 55% by volume polyvinylidene fluoride (Kynar~M 460, available from Pennwalt), 35% by volume nickel (IncoTM 255, -available from Novamet), and 10% by volume alumina trihydrate (Micral~ 916, available from J.M. Huber Chemical$ and prepared as in Example l) were mixed. ~he compolmd was laminated between two sheets of 0.001 inch (0.025 rnm) electrodeposited nickel foil (available from Fukuda), and the laminated sheet was ~radiated 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 0.003ohms, exhibited 5 decades of resistance change (i.e. PTC anomaly) between 110 and 160C.

., ' '

Claims

What is claimed is:

1. A conductive polymer composition which comprises (1) 40 to 75% by volume crystalline organic polymer, and (2) 25 to 40% by volume conductive particulate filler which is dispersed in the organic polymer, wherein at least one of the following conditions is met:
(A) the conductive particles (i) comprise metal; and (ii) have a shape such that particles having the same shape and consisting of the same metal have a bulk density DB as measured by ASTM B329 which is q times the true density DT of the metal, where q is less than 0.15, and (B) the conductive particles comprise (i) nickel, and (ii) have a filamentary structure, and the composition further includes (i) 1 to 20% by volume ?-alumina, and (ii) 0 to 5% by volume coupling agent.

2. A composition according to claim 1 wherein condition A is met and the composition further includes a nonconductive filler.

3. A composition according to claim 2 wherein the non-conductive filler comprises ?-alumina or alumina trihydrate.

4. A composition according to claim 1 wherein the conductive particles have a particle size of at least 1.0 µm.

5. A composition according to claim 1 wherein the conductive particles have a bulk density of less than 1.0 g/cm3.

6. A composition according to claim 1 wherein condition B is met and the coupling agent comprises a zirconate coupling agent.

7. A method of making a conductive polymer composition according to claim 1, said method comprising (a) preblending the conductive filler and the polymer, and the nonconductive filler if present, to form a uniform mixture;
(b) mixing the uniform mixture to melt the polymer and disperse the conductive filler and any nonconductive filler in the molten polymer; and (c) cooling the molten mixture.

8. A method of making a conductive polymer article, said article comprising (a) a PTC element which comprises a composition according to claim 1, and (b) two laminar electrodes which are attached to the PTC element, said method comprising (1) a first lamination step in which the electrodes are laminated to a conductive polymer sheet comprising the composition of claim 1 under heat and and a first pressure to form the article, and (2) a second lamination seep in which the article is maintained under a second pressure while it cools, the second pressure being 0.5 to 0.95 Pcrit where Pcrit is a pressure determined by (i) conducting a series of experiments which are identical to the procedure actually employed in the first step and the second step, except that the pressure in the second step is varied, (ii) measuring the resistivity of the conductive polymer sheet at. an identical position near the center of the press after the second step, (iii) plotting a graph of resistivity in ohm-cm at 23°C as a function of average pressure in kg/cm2, and (iv) identifying Pcrit as the lowest pressure at which the resistivity is equal to 1.1 times the resistivity at a pressure equal to 0.9 times Pcrit.

9. A method of making a conductive polymer article, said article comprising (a) a PTC element which comprises a composition according to claim 1, and (b) two laminar electrodes which are attached to the PTC element, said method comprising (1) laminating the electrodes to the PTC element to form an article;

(2) stacking a plurality of the articles on top of one another to form a stack;
(3) irradiating the stack to a specified dose in a first step;
(4) changing the order of the articles in the stack; and (5) irradiating the reordered stack to a specified dose in a second step so thatthe maximum radiation dose at any point of an article is at most 1.5 times the minimum dose at any point of an article.

10. A circuit protection device which comprises (a) a PTC element which comprises a composition according to claim 1, and (b) two laminar electrodes which are attached to the PTC element, said composition (i) having a resistivity of less than 0.010 ohm-cm, (ii) being irradiated to a dose of at least 10 Mrad.