CA1165996A - Conductive polymer compositions and devices - Google Patents

Abstract

ABSTRACT
The invention relates to PTC conductive polymers. The compositions of the invention comprise at least 10% by volume of a first filler which is composed of highly conductive particles and at least 4% by volume of a second filler which is composed of particles which are less conductive than the particles of the first filler and/or which are substantially smaller in size than the particles of the first filler. Figure 1 shows the resistivity/temperature curve of a composi-tion of the invention. The compositions are useful in electrical devices such as circuit protection devices, heaters and EMI shields.

Description

1 3 ~5996 This invention relates to PTC conductive polymer compositions and devices comprising them.
Conductive polymer compositions, and devices comprising -them, are known.
Reference may be made Eor example to United States Patents Nos. 2,978,665, 3,243,753, 3,351,882, 3,571,777, 3,793,716, 3,823,217, 3,861,029, 3,983,075, 4,017,715, 4,177,376, 4,237,441 and 4,246,468; United Kingdom Patent No.
1,534,715i J. Phys. D: Appl. Phys., Vol. II, pages 1457-1462; the article entitled "The PTC Resistor" by R.F. Blaha in Proceedings of -the Electronic Components Conference, 1971; the repor-t entitled "Solid State Bistable Power Switch Study" by H. Shulman and John Bartho (August 1968) under Contract NAS-12-647, published by the National Aeronautics and Space Administration; J. Applied Polymer Science 19, 813-815 (1975), Klason and Kubat; Polymer Engineering and Science 18, 649-653 (1978) Narkis et al; and German Offenlegungsschrift Nos.

2,634,999, 2,755,077, 2,746,602, 2,755,076, 2,821,799, 2,948,281, 2,949,173 and

3,002,721. For details of more recent developments in this Eield, reference may be made to United Sta-tes Patent Nos. 4,272,471, 4,317,027 and 4,352,083, and to Canadian applications Serial Nos. 358,374, 375,856 and 375,886 Eiled August 15, 1980, April 21, 1981 and April 21, 1981 respectively.

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~ 1 65996 21though the prior art often refers to the possibility of using any kind of conductive particle in conduetive polymer compositions, metal particles have been very little used by comparison with earbon black. Gne important rea-son for this is that known metal-filled ccmp~sitions, espeeially PT~ compositions, are liable to internal areing whieh eauses early failure, sometimes with explo-sion or burning, particularly at voltages of lO volts or more.
We have now discovered that the stability of PTC eompositions compris-ing partieles of metal (or other material of similarly high conduetivity) is im-proved if the composition also includes a substantial proportion of another part-iculate filler whieh is substantially less conductive and/or substantially smaller in average particle size.
In one aspeet, the present invention provides a eonductive polymer eom position whieh (i) exhibits PTC behavior with a switehing temperature Ts; (ii) has a minimum resistivity between -40& and Ts of less than 105 ohm-em, prefer~
ably less than 10 ohm~cm, more preferably less than lO ohm-cm, particularly lessthan 1 ohm-cm, more partieulæly less than 0.1 ohm~cm, espeeially less than lO 2 ohn-em, more espeeially less than lO 4 ohm~em; (iii) has a maximum resistivity between Ts and (Ts ~ 100) C whieh is at least: lO00 times, preferably at least 10,000 times, espeeially at least 100,000 times, the minimum resistivity between-40 C and Ts, said maxlmum resistivity being preferably at least 103 ohm-cm, part-icularly at least 104 ohm-cm, espeeially at least 105 ohm~cm; and (iv) ecmprisesa polymerie eomponent, pre~erably a erystalline polymerie eomponent, having dis-persed therein a filler ocmponent which eo~prises (a) a first filler which is presenk in amount at least 10%, preferably lO to 75%, particularly 30 to 60%, byvolume of the eomposition and which eonsists of eonductive partieles whieh have a first average partiele size dl, whieh is from 0.01 to 200 mierons and whieh are eomposed of a metal having a resistivity at 25 & of less than 10 3 ohm~em, pre-.

ferably less than 10 ohm-cm, particularly less than 10 ohm-cm; and (b) a second filler which is present in amount at least 4%, preferably 4 to 50%, part-icularly 6 to 25%, especially 8 to 20%, by vol~e of the composition and which isselected from the group consisting of (1~ particles which are less conductive than the particles of the first filler and are composed of a non-metallic m~ate-rial, and (2) particles which are ccmposed of a metal and which have a second average particle size d2 which is less than 0.5 x dl and is from 0.001 to 50 microns.
In another aspect the invention provides an electrical device compris-ing an element ccmposed of a YTC conductive polymer composition as defined aboveand at least two electrodes for passing current thro~gh the element.
me novel compositions can have resistivities at 23& which are very low, much lower than compositions containing carbon black as the sole conductivefiller, making them particularly useful for circuit protection devices.
me first filler can be composed of virtually any metal, eg. nickel, tungsten or molybdenum, which are preferred, silver, gold, platinum, iron, aluminum, copper, tantalum, zinc, cobalt, chr~nium, lead, titaniumr tin or an alloy such as Nichrome* or brass. It is preferred to use netals having a Brinell hardness of greater thc~n 100. me first filler can also be of graphite.
The particles of the first filler generally have a particle size of 0.01 to 200, preferc~bly 0.02 to 25, particularly 0.1 to 5, especially 0.5 to 2,micro~s. Spherical particles are preferred, but other shapes such as flakes and rods can also be used.
me second filler can comprise conductive particles and/or non-conduc-tive particles, and preferably c~nprises carbon black or metal p æticles. If theaverage particle size of the first filler is designated dl and the average part-icle size of the second filler is designated d2, the ratio d2/dl is preferably 2 *Trademark -5-to lO,000, more preferably lO to 5,000, particularly lO0 to lO00. ~hen the part-icles of the second filler are as conductive as, or more conductive than, the particles of the first filler, (~nd preferably whenever the particles of the second filler are composed of a material whose resistivity at 25C is less than 10 3 ohmrcm, eg. a metal), the ratio d2/dl is at least 2, preferably at least lO.
When the second filler comprises metal particles, the metal can ke one of those mentioned above for the first filler. When both the first filler and the second filler are contposed of metal particles, the metals can be the same or different.
A preferred second filler is a carbon black having an averaye particle size of fran about 0.01 to akout 0.07 mlcrons. Non-conductive particles which can be used as the second filler include alumina trihydrate, silica, glass beads and zinc sulfide. The second filler preferably has an average particle size of 0.001 to 50 microns, particularly 0.01 to 5 microns.
The polymeric component of the novel compositions can be cross-linked or free from cross-linking and can comprise one or more polymers. me component preferably has a crystallinity of at least 5~, particularly at least 10%, especi-ally at least 20%. me component preferably consists essentially of one or more thermoplastics or cross-linked thermoplastics, but can also ccmprise one or m~re ~termoplastic elastomers, elastcmers, therntosetting resins or blends thereof.
Preferred polymers are polyolefins, eg. polyethylene; copolymers comprising units derived frcm (a) one or more olefins, e.g. ethylene and propylene, and (b) one or more olefinically unsaturated monomers containing polar groups, eg. vinyl esters and acids and esters of ~ unsaturated organic acids; halogenated vinyl and vinylidene polymers, eg. polyvinyl chloride, polyvinylidene chloride, polyvinyl fluoride and polyvinylidene fluoride; polyamides; polystyrene; polyacrylonitrile;
thermoplastic silicone resins; thermoplastic polyethers; thermDplastic n~dified celluloses; and polysulphones. Other suitable polymers are disclosed in the patents and applications referred to above.

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Other additives can also be present in -the ccmposition. Such additives include antioxidants, fire retardants and cross-li~cing agents.
The compositions of this invention can be prepared by conventional techniques, preferably by melt blending the polymeric component and the fillers.Extended mixing times may be required for highly loaded compositions.
m e invention is illustrated by the following Examples in which Examples 1 and 19 are Comparative Examples.
Examples _ Conductive compositions of the invention were prepared using the ingre-dients and amounts thereof listed in the Table below.
In Examples 1-4, 10, 12, 1~ and 15-19, the follcwing prooe dure was followed. A 7.6 cm electric roll mill was heated to 25-40& above the polymer melting point. The polymer was added and allowed to melt and band. Antioxidant was added and allowed to disperse. rme first filler and the second filler were slowly added, by portions, and allowed to mix in a manner such that the metal particles did not come into contact with the rolls and thereby cause the polymerto disband. The composition was w~rked unti~L uniform and then was milled for about three more minutes. The final composil:ion was removed from the mill in sheets and allowed to oool before being compression ~olded into slabs.
In Examples 5 to 9 and 11, the follcwing procedure was used. The cavity of a Brabender* mixer was hea~ed to about 20-40 & abova the polymer melt-ing point~ With the rotor speed at 20 rpmt the polymer, in pellet forn, was added and mixed until melted. The antioxidant was added and allowed to disperse.In small increments the first and second fillers were added. When all ingredi-ents had been mixed, the rotor speed was increased to 60 rpm and the compositionwas mixed for about 2 minutes. The Brabender* was turned off, the material scraped from the blades and walls, and allowed to cool. The composition was thencompression molded into slabs.

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1 J ti~99~i In Example 14, the following procedure was followed. A Banbury* mixer was preheated with steam to 150-180C. With the speed at about 500 rpm, the polymer and antioxidant were added. When the pol~er began to flux, the first and second fillers were added by portions, m~intalning a constant temperature.
With the ram down, the composition was mixed for 5 minutes, then dumped, cooled,and granulated. me granules were then compression molded into slabs or extru~ed into tape.
In each Example, the resistivity of the composition was measured as the temperature was raised, and the Table gives the "resistivity ratio" for each com~
position, i.e. the ratio of the peak measured resistivity to the resistivity at 25&. The r sistivity/temperature curves for the ccmpositions of Examples 1-8 and Comparative Example 19 are shown in Figures 1-9 respectively (a flat line atthe top of a curve merely reflects the inability of ~he equipment to measure a higher resistivity). The compositions of Examples 1-7 and 14-19 we~e also sub-jected to an electrical stability test in which transient currents in the composi-tion were observed using an oscilloscope. These transient currents are believed to be evidence of internal arcing and sparking which can lead to tracking and short circuiting. A 0.64 cm wide StL^ip of a conductive silver paLnt was appliedalong each short edge of a 3.8 x 0.64 om rectangle of the cQmposition to providea test area 2.5 x 0.64 cm. The sa~ple was inserted into a circuit which also oon-tamed a 1 ohm resistor and a completely distortion-free 60 Hertz power source (derived from an audio signal) whose voltage could be varied by means of a Variac* from 0 to 120 volts. The voltage across the resistor, which is a measureof the current thl-ough the conductive polymer element, was ~onitored on an oscil-- loscope over 5 minute periods during which the voltage was maLntained constant at 10, 20, 60 or 120 volts. Current transients in the conductive polymer, observed as sharp random spikes on the osd lloscrope, are indications of electrical *Trademark -8-. . .=, 1 3 6~996 instability of the sample. me samples produced in Co~parative Examples 1 and 19 were unstable ln this test. The samples produced in Examples 2 to 7 were stable.
The various ingredients referred to in the Table are further identified belcw.
HDPE - high density polyethylene (Phillips Marlex* 6003) LDPE - low density polyethylene (Union Caride DYNH-l) MDPE - medium density polyethylene (Gulf 260*M~
EEA - ethylene-ethyl acrylate copolymer (Union Cæbide DPD 6169) EEA - ethylene-acrylic acid copolymer (Dow Chemical Co. EEA 455) FEP - hexafluoroethylene-tetrafluoroethylene copolymer (Du Pont FEP100) Epon* 828 - epoxy resin available from Shell Chemical Co.
Versamid* 140 - polyamide curing agent available from General Mills AO - an-tioxidant, an oligomer of 4,4'-thiobis (3-methyl-6-tert. butyl phenol) with an average degree of polymerization of 3-4, as described in U.S. Patent No.
3,986,981.

Hydral* - alumina trihydrate, with most of the particles being in the range of 0.0005-2, available frcm Alcoa.

Cab-o-Sil* particulate silica with most of the particles being in the range of 0.007-0.016, available from Cabot CorporationO

Gla5s beads - particle size in the range of .004-44, available from Potters Industries.

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Claims (20)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A conductive polymer composition which (i) exhibits PTC behavior with a switching temperature Ts; (ii) has a minimum resistivity between -40°C and T
of less than 105 ohm-cm; (iii) has a maximum resistivity between T and (T +
loo)& which is at least 1000 times the minimum resistivity between -40°C and T ;
and (iv) comprises a polymeric component having dispersed therein a filler com-ponent which comprises:
(a) a first filler which is present in amount at least 10% by volume of the composition and which consists of conductive particles which have a first average particle size d1, which is from 0.01 to 200 microns and which are com-posed of a metal having a resistivity at 25 & of less than 10 3 ohm-cm; and (b) a second filler which is present in amount at least 4% by volume of the composition and which is selected from the group consisting of (1) part-icles which are less conductive than the particles of the first filler and are composed of a non-metallic material, and (2) particles which are composed of a metal and which have a second average particle size d2 which is less than 0.5 x dl and is from 0.001 to 50 microns.

2. A composition according to claim 1 wherein the first filler is present in amount 30 to 60% by volume of the composition.

3. A composition according to claim 1 wherein the second filler is present in amount 6 to 25% by volume of the composition.

4. A composition according to claim 1 wherein said polymeric component is a thermoplastic or cross-linked thermoplastic material having a crystallinity of at least 10%.

5. A composition according -to claims 1, 2 or 4 wherein the first filler consists of particles having an average particle size of 0.02 to 25 microns.

6. A composition according to claims 1, 2 or 4 wherein the second filler consists of metal particles having an average particle size d2, where dl/d2 is from 10 to 5,000.

: ' :

7. A composition according to Claims 1, 2 or 4 wherein the second filler consists of carbon black or a non-conductive filler having an average particle size of û.001 to 50 microns.

8. A composition according to Claim 1 wherein the first filler consists of particles having an average particle size, d1 of 0.02 to 25 microns.

9. A composition according to Claim 8 wherein the second filler consists of particles having an average particle size d2 and the ratio d1/d2 is from 2 to 10,000.
from

10. A composition according to Claim 9 wherein the ratio d1/d2 is from 10 to 5,000.

11. A composition according to Claim 9 wherein the ratio d1/d2 is from 100 to 1000.

12. A composition according to Claim 9, 10 or 11 wherein the first filler consists of metal particles having an average particle size of 0.1 to 5 microns.

13. A composition according to Claim 9, 10 or 11 wherein the first filler and the second filler consists of metal particles,

14. A composition according to Claim 9, 10 or 11 wherein the first filler and the second filler consist of particles of the same metal.

15. A composition according to Claim 8, 9 or 11 wherein the first filler consists of metal particles and the second filler consists of carbon black particles.

16. A composition according to Claims 8, 9 or 11 wherein the second filler consists of` non-conductive particles.

17. A composition according to Claim 1 wherein the polymeric component is a thermoplastic or cross-linked thermoplastic material having a crystallinity of at least 10%, and the filler component comprises a first filler which consists essentially of metal particles having an average particle size, d1, of 0.1 to 5 microns and which is present in amount 10 to 60% by volume of the composition and a second filler which consists essentially of carbon black particles having an average particle size, d2, of 0.01 to 0.07 microns and which is present in amount 4 to 50% by volume of the composition.

18. A composition according to Claim 17 wherein the ratio d1/d2 is at least 10.

19. A composition according to Claim 17 wherein the ratio d1/d2 is at least 100.

20. An electrical device which comprises an element composed of a PTC conductive polymer composition and two electrodes which can be connected to a source of electrical power and which when so connected cause current to pass through the element, wherein the conductive polymer composition is as claimed in Claims 1, 3 or 17.