CA1104808A

CA1104808A - Conductive polymer compositions

Info

Publication number: CA1104808A
Application number: CA293,136A
Authority: CA
Inventors: Lester T. Toy; Wendell W. Moyer; Bernard J. Lyons; David A. Horsma
Original assignee: Raychem Corp
Current assignee: Raychem Corp
Priority date: 1976-12-16
Filing date: 1977-12-15
Publication date: 1981-07-14
Also published as: FR2374357B1; SE435003B; SE7714127L; AU3139377A; FR2374357A1; DE7737676U1; NO774259L; BE861777A; DE2755076A1; AU520815B2; NL7713801A; GB1597007A; JPS53102356A

Abstract

ABSTRACT

This invention relates to conductive polymer compositions, especially PTC compositions, which comprise a conductive carbon black dispersed in a copolymer of an olefin and a polar comonomer.
It has been discovered that conductive polymer compositions having improved voltage stability are obtained by use of a carbon black having a particle size greater than 18 millimicrons, a d-spacing greater than 360 and a surface area which is less than where S is the DBP absorption of the carbon black, the carbon black being dispersed in a crystalline copolymer of an olefin and at least 10%
of a polar comonomer. The melt index of the copolymer is preferably less than 20. The composition preferably also comprises another crystal-line polymer which serves as a matrix for the copolymer. The composition is preferably cross-linked, e.g., to a gel fraction of at least 0.6. The invention includes the novel compositions, methods of preparing them, and electrical devices containing them.
The novel compositions are particularly useful in electrical devices, especially self-limiting heaters, which comprise at least two electrodes which are connectable to a source of electrical power to cause current to pass through the composition.

Description

This invention relates to conductive polymer composition~, their preparation, and devices comprising them.
It is known that polymers, including crystalline polymers, can be made electrically conductive by dispersing therein suitable amounts of finely divided fillers. Some conductive polymers exhibit what is known as PTC (positive temperature coefficient) behavior. The term "PTC" has been used in various different ways in the past, but in this specification, the terms "composition exhibiting PTC
behavior" and l'PTC composition" are used to denote a composition having at least one temperature range which is within the limits of -100C and about 250C; at the beginning of which the composition has a resistivity below about 105 ohm. cm.; and in which the composition has an R14 value of at least 2.5 or an Rloo value of at least 10 (and preferably both), and preferably has 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 the end and the beginning of a 100C range, and R30 is the ratio of the resistivities at the end and the beginning of a 30C range. The term "PTC element" is used herein to denote an element composed of a PTC composition as defined above. .~ plot of the log of the resistance of a PTC
element, measured between two electrodes in contact with the element, against temperature, will often show a sharp change in slope over a part of the critical temperature range, and in such cases, the term "switching temperature" (usually abbreviated to Ts) is used herein to denote the temperature at the intersection point of extensions of the substantially straight portions of such a plot which lie either side of the portion showing the sharp change in slope. The PTC

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composition in such a PTC element is described herein as having "a useful Ts''. Ts is preferably between 0 and 175C, e.g. 50C to 120C.
PTC compositions and electrical devices, especially heaters, which contain PTC elements, have been described in a number of publications. Reference may be made for example to U.S. Patents Nos. 2,978,665, 3,243,753, 3,351,882, 3,412,358, 3,413,442, 3,591,526, 3,673,121, 3,793,7~6, 3,823,217, 3,858,144, 3,861,029, 3,914,363 and 4,017,715, British Patent No. 1,409,695, Brit. J. Appl. Phys. Series 2, 2 569-576 (1969, Carley Read and Stow), Kautschuk und Gummi II WT, 138-148 (1958, de Meij), Poiymer Engineering and Science, Nov.
1973, 13, No. 6, 462-468 (J. Meyer), U.S. Patent Office - Defensive Publication No. T905,001, German Offenlegungschriften Nos. 2,543,314.1, 2,543,338.9,

2,543,346.9, 2,634,478.5, 2,634,931.5, 2,634,932.6, 2,634,999.5, 2,635,000.5, and 2,655,543.1, and German Gebrauchsmuster 7,527,288.
Particularly useful known PTC compositions comprise ~a thermoplastic crystalline polymer with carbon black ', dispersed therein, and such compositions have been widely - used in self-regula-ting strip heaters. The polymers which have been used include polyolefins, e.g. polyethylene, and copolymexs of olefins and polar comonomers, e.g.

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ethylene/ethyl acrylate copolymers. ',uch compositions show a rapid increase in resistance over a range which begins at the softening point of the polymer and have a useful Ts at or near the crystalline melting point of the polymer; the greater the crystallinity of the polymer, the smaller the temperature range over which the resistance increase takes place. Generally, the composition is cross-linked, preferably by irradiation at room temperature, to improve its stability at temperatures above Ts.
Carbon blacks vary widely in their ability to impart conductivity to polymers with which they are mixed, and mixtures of`polymers and carbon blacks generally have poor physical properties when the proportion of carbon black becomes too high, e.g~ above 30% to 50%, dependiny on the polymer (percentages are by weight throughout this specification). Not surprisingly, therefore, only a very limited number of carbon blacks have been used or recommended for use in conductive polymer compositions, i~e.
compositions whose utility depends upon their electrical characteristics, especially when the conductive polymer forms part of a circuit through which current must flow.
The carbon blacks in question are, in qeneral,those which have been recognised to have the ability to impart high conductivity, for example acetylene blacks and various ~i furnace blacks, such as Vulcan XC-7~ and Vulcan SC (both sold by Cabot corporation), which are characterised by nigh surface area (as measured by nitrogen absorption) and high structure (as measured by dibutyl phthalate absorption).
The latter two parameters and the particle size are often ; 30 used to characterise carbon blacks, and for details of how they are measured, reference should be made to "Analysis of ~ ~r O ~
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Carbon Black" by Schubert, Ford and Lyon, Vol. 8, Encyclopedia of Industrial Chernical Analysis (1969), 179, published by John Wiley & Son, New York. For details of the nomenclature used in the carbon black industry, reference should be made to ASTM standard D 1765-67. Another characterising property of a carbon black is its d-spacing (the average distance in pico-meters between adjacent graphitic planes in the carbon black); thus acetylene black has a substantially smaller d-spacing (less than 360, typically about 355) than other carbon blacks. The d-spacings given herein are measured by electron microscopy.
For further details reference may be made to "Carbon Black"
by Donnet and Voet, published by Marcel Dekker Inc., New York (1976).

The conductivity of conductive polymers containing carbon black can be increased by annealing, as described in U.S. Patents Nos. 3,861,029 and 3,914,363~ By making use of ; this annealing procedure, it is possible to prepare PTC
compositions which contain less than 15~ of carbon black but which have satisfactory initial conductivity, ror example ~or use in strip heaters.

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A serious problem that arises with conductive polymers, particularly those exhibiting PTC behavior, is lack of voltage stability, i.e. a tendency for the resistivity to rise irreversibly when the composition is subjected to voltages greater than about 110 volts, e.g. 220 or 480 volts AC, generally at a rate which is dependent on - ~
the voltage. This problem is particularly serious with heating devices, because the rise in resistance results in a :
:
~ ~ _a_ corresponding loss in power output. Although voltage instability is a serious problem, it appears not to have been recognized as such in the prior art. German Offenlegungsch~ift No. 2,634,931 is concerned with improving the voltage stability of PTC compositions comprising carbon black dispersed in a polymer containing fluorine, e.g. polyvinylidene fluoride, by cross-linking the composition with an uns~turated monomer. However, this expedient does not yield improved voltage stability with other polymers.

We have now discovered that improved voltage stability is possessed by a conductive polymer composition which comprises (a) at least one crystalline copolymer which consists essentially of units derived from at least one olefin and at least 10 weight %, based on the copolymer, of-units derived from at least one olefinically unsaturated comonomer containing a polar group, and (b) dispersed in said copolymer, a conductive carbon black having a particle size greater than 18 milllmicrons, a d-spacing greater than 360, and a surface area (A) which is less than S
1.2S + e50 where S is the DBP adsorption of the carbon black.

In one aspect, the present invéntion provides a conductive polymer composition which comprises , .
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(a) at least one crystallir1e copolymer as defined above, and (b) a conductive carbon black as defined above;
subject to the provisos that (l) if the crystalline copolymer (i) has a melt index of more than 20 and (ii) is substantially the only polymeric component in the composition, the composition has a resistivity of more than 80 ohm. cm. at 25C;
and (2) if (i) the crystalline copolymer has a melt index of more than 20, (ii) the composition comprises 65 to 85 % by weight of polyethylene, (iii) the content (L) of carbon lS black is less than 15~ by weight of the composition, and (iv) the resistivity (R) of ; the composition at 25C in ohm~ cm. is such that 2L ~ 5 logl0 R < 451 the composition has a gel fraction of a-t least 0.6.
The Melt Indexes referred to herein are expressed in g/lO min.
The compositions of the invention may contain ~25 other polymers, pr~eferably crystalline polymers, in addition to the crystalline copolymer as defined above. Preferably the carbon-black-containing copolymer is dispersed in a second polymer which serves as a matrix therefor, i.e. which : i forms a continuous phase in the composition. The other ~ polymer is pre~erably substantially free of carbon black but ; may contain a relatively small proportion of carbon black, ~: : :: :
- : :

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temperature characteris-tics of the composition are dominated by the carbon-black-containing copolymer.
The compositions of the invention preferably exhibit useful PTC behavior, and will then generally ha-ve a usefuI Ts, which is preferably from 0 to 120C. The compositions are preferably cross-linked, and it is often preferred that the gel fraction of the polymeric component of the composition is at least 0.6. Generally it is desirable that the composition should have a resistivity at 25C of at least 80 ohm. cm.
The invention is illustrated in the accompanying drawings, in which the Figure shows the relationahip between the surface area and the DBP absorption of the class of carbon blacks defined above, the continuous line corresponding to the relationship A=1.2S ~ e S50, and, more especially, of the specific carbon blacks used in the Examples, lying to the left of the continuous line, and Comparative ~xamples, lying to the right, given below, with the exception of Shawinigan Black, used in comparative Example 12, which has a d-spacing of about 355 and is excluded from the scope of the invention by this feature rather.than the relationship between A and S, and Ketjen black EC, used in Example 23, for which the values of A and S are.too high to be shown.
The copolymer (a) should be a crystalline copo-lymer which consists essentially of units derived from at least one olefin, preferably ethylene and at least 10 ~ by weight, based on the weight of the copolymer, of units derived from at least one olefinically unsaturated comonomer containing a polar group, preferably an acrylate ester, e.g. methyl acrylate or ethyl acrylate, or vinyl acetate, or acrylic : or methacrylic acid. The term "crystalline" is used herein to mean that the polymer has a crystallinity of at least 1%, preferably at least 3%, especially at least 10%. In-creasing polar comonomer content leads to. reduced crystalli-nity,-~nd the polar comonomer conten~ is preferabl~ not more~
than.30%. The ~el* Ind@x of the copolymer is.prefe~ably less tha~ ~0, especially less th`an `10. The hi~her the ~lelt Index, the more desirable it is that the composition should be cross-.. , .. .: .

linked to a relatively high level, especially when the composition is prepared by a process in which annealing is used to decrease the resistivity of the composition. Thus the composition should preferably have a gel fraction of at least 0O6 when the copolymer has a melt index of more than 20 and the composition has been annealed so that gl0 R < 45 where L is the content of carbon black in percent by weight, based on the weight of the composition; and R is the resistivity of the composition at 25C in ohm. cm.
When the composition comprises a polymer which serves as a matrix for the carbon-black-containing copolymer, i.e. for the dispersion of the carbon black in the copolymer, then the matrix polymer preferably has a higher softening point than the copolymer. Preferably the matrix polymer has limited compatibility for the copolymer, so that migration of the carbon black into the matrix polymer is minimised.
Particularly suitable matrix polymers are crystalline polymers which consist essentially of units derived from one or more olefins, e.g. high, medium or low density polyethylene. Other polymers which can be used are crystalline polymers which comprise 50 to 100%, preferably 80 to 100%, by weight of -CN2CF2- or -CH2C~Cl- units, and in compositions wnich are n~t annealed, polymers which contain ~ 25 at least 50~, preferably at least 80%, by weight of units `~ derived~from one or more olefins, together with suitable comonomers.

~Suitable blacks for use in the invention include blacks selected from furnace blacks, thermal blacks and channel blacks. The content of carbon black is preferably 5 -8- ~

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to 25 % by weight of the composition. The content may be relatively low, e.g. not more than 12 or 15~, in which case it is preferred that the composition should be annealed, prior to any cross-linking, at a temperature above the melting point of the copolymer, and preferably above the melting point of the highest-melting polymer in the composition, so as to decrease its resistivity. Typically the composition will be annealed so that 2L ~ 5 log10 R < 45, where L and R are as defined above.
Alternatively, the content of carbon black may be relatively high, e.g. above 15%, in which case annealing prior to cross-linking may be unnecessary, or may be for a limited time such that, at the end of the annealing, 2L + 5 log10 R > 45.
In such compositions the particle size of the carbon black is preferably greater than 30 millimicrons~ It is of~en advantageous, whether or not the composi-tion has been annealed before cross-linking, to heat the cross-linked composition for a short period at a temperature ahove its melting point.
Cross-linklng of the compositions is carried out after they have shaped, e~g. by melt extrusion, and can be effected by any of the methods well known in the art, preferably with the aid of ionising radiation or an organic peroxide. Preferably the composition is cross-linked at least to an extent equal to that induced by exposure to ionising radiation to a dosage of at least 0.75 M megarads, where M is the Melt Index of the copolymer, e.g. to a gel fraction of at least 0.6.

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The compositions of the invention may contain other ingredierlts which are conventional in the art, e.g.
antioxidants, flame retardants, inorganic fillers, thermal stabilisers, processing aids and cross-linking agents or the residues of such ingredients after processing. The addition of a prorad (an unsaturated compound which assists radiation cross~linking) is often useful in improving stability, especially in unannealed products; suitable amounts of pro-rad are less than 10%, preferably 3 to 6%.
The compositions of the invention in which the only polymeric component is the copolymer (a) can be made by blending the ingredients in conventional mixing equipmen4 at a temperature above the melting point of the copolymer, followed by annealing and cross-linking as desired.
Alternatively, a master batch containing the carbon black and part of the copolymer can first be prepared, and the master batch then blended with the remainder of the copolymer. Similarly, when the composition contains a matrix polymer in which the carbon-black-containing copolymer is distributed, such compositions are made by blending the matrix polymer and a master batch of the carbon black in the copolymer, followed by annealing and cross-linking as desired~ The master batch preferably contains 20 to 50%, e.g. 30 to 50~ of the carbon black.
~ ~ 25 The invention includes electrical devices ; comprising an element composed of a composition of the invention ancl at least two electrodes adapted to be connected to a source of electrical power so as to cause current to flow through the elementO One class of such devices comprise a pair of laminar electrodes having a said element in the form of a lamina therebetween. Another class of such devices comprise an elongate element of a composition of the invention; at least two longitudinally ~ ~/c~ /
~; extending electrodes embedded in said element ~r~l~e to each other; and an outer layer of a protective and insulating composition.

The invention is illustrated by the following Examples.
EXA

In the examples which follow, the test samples were prepared in accordance with the procedure described below unless otherwise stated. The ingredients for the master batches were milled together on a 2 roll mill, 10 to 30C above the melting point of the polymer. When used, additives were added before the carbon black. The preferred range of carbon black concentration in the master batch is 30 to 50~ and most of the mixes prepared were in this range, although for some compositions loadings as low as 20 or as high as 70% were used. The carbon black master batch was milled for five minutes, then removed from the mill and either cooled to room temperature for subsequent use, or immediately mixed with the matrix polymer to form the final blend. For t:he~preparation of the final blend, the desired amount of master batch was fluxed on a 2 roll mill at a temperature 10-30C higher than the melting temperature of the highest melting polymer in the final blend. The remaining constituents, including the other polymer(s), were immediately added to the master batch and the mixture blended for five minutes. The amount of master batch was chosen to yield a resistance of about 10 kilo ohm. in the ~Q~
test samples. The final blends were hydraulically pressed into 15 x 15 x 0.06 cm. sheets at 2,800 kg/cm2 and a temperature of at least 175C. Samples 2.5 x 3.75 cm. were cut from the slabs and 0.6 cm. strips~of conductive silver paint were coated on each end of the longest dimension to define a test area 2.5 x 2.5 cm.

Where indicated, prior to crosslinking, the above samples were annealed at 150 to 160 (200 for polypropylene) cyclically for up to two hours periods followed by cooling to room temperature until a minim~m resistance level was reached. (Usually, two or three annealing cycles sufficed). Usually the samples were crosslinked by radiation; the doses used ranged from 6 to 50 Mrads, with most samples receiving 12 Mrads.

Voltage stability was assessed by measuring the room temperature resistance of the sample before (Ri) and after (Rf) the sample had been subjected to a period of operation at high voltage stress. In most instances this involved operating the heater for 72 hours at 480 volts in ambiant air, then disconnecting from the source of electrical power and cooling to room temperature before . .
remeasurement. The voltage stability is expressed as the ratio of initial resistance to final resistance (Ri/Rf).

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EXAMPLE l It should be noted that the proportion of master batch (and hence of carbon) required to achieve the desired resistance level of lO
~! kilo ohms is somewhat dependent on the processing conditions and on thetype of carbon black. To illustrate this, blends containing Sterling S0, Vulcan XC-7~ and Black Pearls 880 were prepared as described above 0.~5 and also using a/kg Banbury mixer in place of the two roll mill, the temperatures and times being the same in each experiment. The master batch polymer was an ethylene (18%) ethyl acrylate copolymer (DPD6169) and the matrix polymer was a low density polyethylene (Alathon 34). The concentration of carbon in the master batch in each case was 36%. Table I shows the percentage of master batch in the final blend (% MB) and the percentage of carbon black in the final blend (% ce ) .

TABLE I
Carbon Black Two Roll Mill Banbury Mixer %MB %CB %MB %CB
; Sterling S0 50 18 60 22 Vulcan XC-72 40 l4.4 50 l8 " ~ Black Pearls 880 40 l4.4 40 l4.4 ; A variet.y of carbon blacks were blended with DPD 6l69 to pro-vide master batches which were then mixed with Alathon 34 as the matrix to achieve a resistance level polymer in the amount needed/in the final product of lO kilo ohms. All the samples were irradiated to a dosage of 12 Mrads, and most were an-nealed prior to irradiation.
The carbon blacks employed are identified in Table II below,giving the~trade ia~e, the ASTM code, the particle size in miliimicrons (D), the surface anea as measured by ~irrosen absorption in M 2/9 (A) .

and the dibutylphthalate absorption in CC/lOOg(s). Table II
also shows the percentage of carbon black in the different samples, and the results of stability tests on these sarnples.
In Table II, the samples marked C are comparative Examples.

TABLE I I

Carbon Bl ack Annealed Unannealed Samples Sarnples ASTM D A S % %
Trade Mark Code Carbon Ri/ Carbon Ri/
Black Rf Black Rf 1. Sterling NS N7747527 70 15.1 0.76 2. Philblack N765N76560 30 10211.1 0.56

3. Furnex N765 N765~030 107 9.70.4

4. Sterling N765N76560 30 1169.4 0.58 16.2 0.76

5 . Sterling V N6605035 91 10.8 0.7

6. Sterling VH N6506036 122 9.40.49

7. Statex N550 N5504240 122 7.90.83

8. Sterling So-lN5394242 109 10.8 0.55

9. Sterling SO N5504242 120 9.70.6 18 0.63

10. Philblack N550N55042 44 1189 4 0.65

11. Regal 99N440 364660 19.1 0.35 C 12. Shawnigan Black --42 64 - 15.1 0.004 13. Vulcan KN351 2870124 10.~ C.47 14. Vulcan 3N330 2780103 10.1 0.48 15. Vulcan 3H N3472690 124 7.90.38 ' C 16. Regal 330 N3272594 70 16.2 0.19 17. Vulcan 6H N24221124 128 10.1 0.38 C 18. Vulcan CN293 23145100 11.9 0.29 16.2 *
C 19. V~lcan SC N29422203 106 10.1 0.24 C 20. Black Pearls 880 --16 220 110 14.4 *
~ .
;C.21. Vulcan XC-72N472 35 254178 10.8 0.23 C 22. Black Pearls 74 --17 320 10910.8 23. Ket jen black EC --30 1000 340 5.3 0.52 ' ~ Sample has such poor voltage stability that it burns.

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Tests similar to thGse described in Example 2 were carried out using different polymers in place of the DPD 6169 and/or the Alathon 34. The tests are summarized in Table III below.

Copolymer Trade Mark Polymer in Trade Mark in master batch and Melt Final i31end Melt Index Remarks Index (M.I.) (M.I.) _ Ethylene (18%) ethyl DPDA 61 81 Polyethylene Alathon 3~ Very similar acrylate M.I. 2.2 0.93 density M.I.-3 results to those of Table II
Ethylene-(18%) ethyl DPDA 916g as above as above Similar acrylate M.I. 20 results to those of - Table~
Ethylene-(6.6%) DPD 7365 as above as above Voltage stab-ethyl acrylate M.I. 8 ility very poor with most carbon blacks Ethylene-(5.5%1 DPD 7870 as above as above VoltagQ stab-ethyl acrylate M.I. ility very poor with most carbon blacXa i Ethylene-(18%) vinyl Alathon 3175 as above as above Very similar acetate M.I. 8 results to those of - Table II
Ethylene-(28%) vinyl Alathon 3172 as above as above Very similar acetate M.I. 6 results to Table II
Ethylene-(30%) Vistalon 702 as above as above Voltage stab-propylene Mooney Visc. 30 with most carbon blacks Polyethylene DYNH Polyethylene Alathon 7030 0.93 density M.I. 2 O.g6 density M.I. 3 Ethylene-~18%) ethyl DPD 6169 Ethylene-(6.6%) DPD 7365 Results similar - acrylate M.I. 6 ethyl acrylate M.I. 8 to Table II
s~ightly diff-erent preferrad . range E Polyethylene DYNH Polypropylene Profax 8263 Voltage stab-0.93 density M.I. 2 (High Impace) ility very poor with most carbon blacks Ethylene-(18%) ethyl DPD 6169 Vinylidene di Kynar 7201 ~esults similar acrylate M.I. 6 Fluoride copolymer M.I.33 to Table II
as above as above none - Results very similar to - lS -' '~ :
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Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A conductive polymer composition which comprises (a) at least one crystalline copolymer which consists essentially of units derived from at least one olefin and at least 10 weight %, based on the copolymer, of units derived from at least one olefinically unsaturated comonomer containing a polar group, and (b) dispersed in said copolymer, a conductive carbon black having a particle size greater than 18 millimicrons, a d-spacing greater than 360, and a sur-face area (A) which is less than where S is the DBP adsorption of the carbon black;
subject to the provisos that (1) if the crystalline copolymer is substan-tially the only polymeric component in the composition, then the composition is in the form of an element which is in electrical contact with at least two electrodes which are adapted to be connected to a source of electri-cal power and which when so connected cause current to pass through the element; and (2) if the composition comprises 65 to 85% by weight of polyethylene and the content (L) of carbon black is less than 15% by weight of the composition, then the resistivity (R) of the composition at 25°C
in ohm.cm is such that 2L + 5 log10R ? 45.

2. A composition according to Claim 1 which also comprises at least one crystalline polymer (c) which has a softening point higher than copolymer (a) and which consists essentially of units derived from one or more olefins or which contains at least 50% by weight of -CH2CF2- units or -CH2CHC1- units.

3. A composition according to Claim 1 wherein the copolymer (a) has a Melt Index less than 20.

4. A composition according to Claim 3 wherein the copolymer (a) has a Melt Index less than 10.

5. A composition according to Claim 1 wherein the polymeric component has a gel fraction of at least 0.6.

6. A composition according to any one of Claims 1, 2 or 5 which contains less than 15% by weight of carbon black and in which 2L + 5 log10 R ? 45.

7. A composition according to any one of Claims 1, 2 or 5, which contains at least 15% by weight of carbon black and wherein the carbon black has a particle size of at least 30 millimicrons.

8. A composition according to any one of Claims 1, 2 or 5 which has a resistivity at 25%C of 80 to 105 ohm.cm.

9. A composition according to any one of Claims 1, 2 or 5, which exhibits PTC behavior and which has a use-ful Ts of 0° to 175°C.

10. A process for the preparation of a conductive polymer composition as claimed in Claim 2 which process comprises mixing the polymer (c) with a dispersion of carbon black in the copolymer (a).

11. A process according to claim 10 wherein the composition resulting from the mixing process contains less than 15 % by weight of carbon black and it subse-quently is annealed at a temperature above the melting point of the highest melting polymer in the composition for a time such that 2L + 5 log10 R ? 45 where L is the content of carbon black in percent by weight of the composition, and R is the resistivity of the annealed composition at 25°C in ohm.cm.

12. A process according to Claim 10 or Claim 11 wherein the composition, after any annealing, is cross-linked by exposure to ionising radiation to a dosage of at least 0.75M megarads, where M is the Melt Index of the copolymer (a).

13. An electrical device comprising an element composed of a conductive polymer composition as claimed in any one of Claims 1, 2 or 5 and at least two electrodes adapted to be connected to a source of electrical power so as to cause current to pass through the element.

14. An electrical device which comprises (1) an elongate element of a composition as claimed in any one of Claims 1, 2 or 5, having a useful Ts between 0 and 120°C, (2) at least two longitudinally extending electrodes embedded in said element parallel to each other; and (3) an outer layer of a protective and insulating composition.

15. A self-regulating strip heater which comprises (1) an elongate element of a conductive polymer composition which exhibits PTC behavior;
(2) at least two longitudinally extending elec-trodes which are embedded in said element parallel to each other and which are adapted to be connected to a source of electrical power and which when so connected cause current to pass through the element; and (3) an outer layer of a protective and insulating composition, the conductive polymer composition comprising (a) a crystalline copolymer of ethylene and at least 10%, based on the weight of the copolymer of one or more of methyl acrylate, ethyl acrylate and vinyl acetate; and (b) dispersed in said copolymer, at least 15%, by weight of the composition, of a conductive carbon black having a particle size of 18 to 75 millimicrons, a d-spacing greater than 360, and a surface area (A) which is less than where S is the DBP absorption of the carbon black.

16. A heater according to Claim 15 wherein the composition is cross-linked.

17. A heater according to Claim 17 wherein the polymer in the composition has a gel fraction of at least 0.6.

18. A heater according to any one of Claims 15 to 17 wherein the composition also comprises polyethylene.