DE2345320A1 - PROCESS FOR MANUFACTURING SELF-REGULATING, ELECTRICALLY CONDUCTIVE PRODUCTS - Google Patents

Description

SAYCHEM CORPORATION 300 Constitution Drive, Menlo Park, California 94 025, V.St.A.

Process for the production of self-regulating, electrically conductive products

Electrically conductive thermoplastic compounds were previously produced by adding electrically conductive carbon black to a polymer base compound. In one type of such masses van to sustain itself controlled or current-limiting, semi-conductive products you of the fact that the material in question had a non-linear positive temperature coefficient of resistivity, exercised. US Pat. No. 3,243,753 describes such a compound which contains 25 to 75 liters of carbon black, around which a polymeric embedding compound has been produced by on-site polymerization. If the temperature of such a mass increases as the ambient temperature rises or through resistance heating, the polymeric embedding mass is likely to expand faster than the carbon black particles, which give the mass electrical conductivity in the form of an interconnected array of channels. The resulting reduction in the number of live cables

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287 442 I.

nallen reduces the amount of energy that can be generated by resistance heating. This self-limiting feature can be used, for example, in heat-carrying pipes in chemical plants for anti-freeze protection or to maintain the fluidity of viscous syrups. In such applications Products made from the electrically conductive mass ideally reach and maintain a temperature in which the energy that is lost through heat conduction to the environment is equal to the energy that is lost from the electric current is obtained. If the temperature of the environment then drops, the increased heat conduction to the environment offset by increased energy production due to the decrease in resistance, which in turn is the result the reduced temperature of the product. The equality between heat transfer and energy production then becomes reached at a new temperature level. Conversely, when the ambient temperature rises, the heat transfer from the electrically conductive product decreased, and the increase in resistivity due to the increased temperature decreased or interrupts the resistance heating.

Self-regulating conductive masses can of course also be used in fields other than resistance heating e.g. for heat measurement and power interruption. In in all cases the high carbon black content of most known compositions of this type is a disadvantage. High loads of ruso go with it poor elongation at break, tensile strength and low temperature brittleness Hand in hand. Furthermore, the high soot content also appears to improve the current control capacity of the electrically conductive ones To affect crowds *. When a semiconducting thermoplastic Mass heated from the outside and its specific resistance as a function of the temperature (on the abscissa) is entered in a diagram, shows the thus obtained Resistivity curve shows an increase with temperature from the low room temperature value (Ri) to

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to a "certain point of" peak resistance "(Rp), whereupon a further increase in temperature leads to a steep drop of the specific resistance, which goes hand in hand with the melting of the polymer embedding compound. To this "Passing through" the specific resistance and the associated To avoid irreversible change in resistivity, it has been proposed to crosslink the polymeric embedding compound, in which case the resistivity slowly decreases with the temperature of the tip resistance and then with a further increase in the ambient temperature remains constant. Crosslinked semiconducting products with high carbon black contents, however, have an undesirably low specificity Resistance occurs when the temperature of the is affected by very high or low ambient temperatures Tip resistance are brought. In such baboons, the poor thermal conductivity can cause the heat to be dissipated prevent and thereby lead to burnout.

It would therefore be desirable to be able to produce semiconducting, self-regulating products with a significantly lower carbon black content, which, among other things, have improved flexibility and other physical properties, and if the ratio Rp / Ri could be increased significantly in the process. However, the solution to these tasks was largely prevented by the extremely high specific resistance that polymers with low carbon black contents have at room temperature. In the Technical Report S-8 on Pigm en truss of the Cabot Corporation, entitled "Ca3? Bon B3.acks for Conductive Plastics", curves show the dependence of the specific resistance on the percentage of carbon black for various polymers containing ¹¹ YuIcan XC- 72 ", an oil furnace soot, contain specific resistance values of 100,000 ohm» cm or more, which increase asymptotically at soot contents of about 15 f <>. Similar high specific resistance values have been reported from other sources at low soot contents.

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More recently, one has resistivity values low enough for anti-freeze protection "with low soot levels by using "special separation methods, such as coating with solvents, achieved. Extruded, self-limiting masses are described, for example, in U.S. Patent 3,435,401; if you do tried to produce these masses with low soot contents, they have specific resistance values at room temperatures

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temperature of 10 ear-cm and more is shown, which are practically as high as that of the polymeric embedding compounds themselves. In British Patent 1 201 166, the avoidance of hot-melt methods is urgently advised if there are significant electrical conductivities with soot contents of less than et v / a 20 fi are to be achieved, but it has recently been found that extruded thermoplastic compositions which contain no more than about 15 percent by weight of electrically conductive carbon black can be improved by reducing their specific resistance to technically acceptable values by leaving the extrudate for a longer time, often longer than 15 hours, at temperatures above about 120 ° C., preferably of at least about 150 ° C., and above the crystalline melting point of the polymeric embedding compound. These methods form the subject of the patent ...... (patent application filed at the same time - internal reference 287 444 - entitled "Process for the production of electrical resistance bodies with positive, non-linear resistance coefficient") such a long-lasting heat treatment does not "thermo-structure" the carbon it contains. On the other hand, the action of temperatures above the crystalline melting point of the embedding compound leads to the extrudate being poured over. Depending on the degree of change in shape, the effect of such a flow ranges from strong fluctuations in the electrical wattage over the length of the heat-treated extrudate to complete destruction. One has

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tries to change the shape of the extrudate obtained first "preserve that one has extruded a surrounding insulating layer over the extrudate made of a material that has the heat treatment temperature maintains its shape, i.e. off a material that does not have its softening point or its crystalline melting point or melting range when subjected to heat treatment achieved. However, buy caused the for this coating The selected material increases the specific resistance of the heat-treated extrudate far beyond that practically permissible ranges. There was therefore still a need to find guidelines according to which suitable sheathing materials can be selected.

The invention is based on the finding that suitable sheath materials, which have the required resistivity properties in the heat-treated condition, those are, in addition to their shape retention in the Heat treatment temperature of the electrically conductive interior material exhibit a significant degree of residual strain when even from room temperature heated to the heat treatment temperature.

The invention therefore relates to a process for the production of self-regulating, electrically conductive products, in which a mass of a crystalline thermoplastic polymer, which, based on the total amount by weight, contains up to 15 ° carbon black, on a pair of spaced apart , elongated, parallel electrodes are extruded to form a web connecting the two electrodes and the extrudate is then heat-treated at or above the temperature of the crystalline melting point of the polymer and above 120 C, the carbon black content and the heat treatment conditions being coordinated so that the specific resistance is at room temperature in the range from 5 to 100,000 ohms <> cm, characterized in that before the heat treatment

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treatment on the extrudate is a surrounding, electrically insulating Layer of a thermoplastic material extruded, which is incompatible with the polymer and "at the heat treatment temperature the mass is dimensionally stable, and when it moves from room temperature to the heat treatment temperature is heated, a residual strain having.

The state of residual stress is a property that many polymers have exhibit at elevated temperature. Such a state of tension (strain) can be perceived quantitatively, by warming a rubber band that is held taut between the fingers. A noticeable increase is observed the force required to keep the belt taut. The increased compressive force exerted by the band is apparently similar to the stress that the According to the invention, coated, soot-containing extrudates in the Exposed to heat treatment. The presence and strength of the residual stress state of a given polymer at the heat treatment temperature of the electrically conductive material can quantitatively with a suitable device, such as the Instron tester. An extruded length of the material under test is given at Room temperature pinched between the jaws of the Instron, whereupon there is enough tension to act brings to a given state of tension on the reading device to be recorded. Then the material is heated to the heat treatment temperature and the state of stress recorded at the heat treatment temperature. If this fourth is higher than that at room temperature, it is suitable the material to this extent for use within the meaning of the invention. On the other hand, materials with or soften or theirs below the heat treatment temperature Reach melting point, a significant relaxation before this temperature is reached, and therefore show at the heat

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treatment temperature either no stress state or a lower state of stress than at room temperature. in the Case of materials that when heated to the heat treatment temperature thermal, hardening, there may be an initial relaxation before hardening begins; in the In the case of suitable materials, such as thermosetting silicone rubbers, according to one embodiment of the invention are used, but the material recovers from this relaxation, and tempera doors during the heat treatment a residual stress state occurs to an extent that is valuable within the meaning of the invention. Particularly preferred sheath materials are polyether polyurethanes, e.g. that derived from Mobay Chemical Company under the designation "Texin 591-A" in is brought on the market, which is a polymer which has no reactive end groups, a Adhesion temperature of 152 C, a Shore A hardness of 91-3, one Has a tensile strength of 455 kg / cm and an elongation at break greater than about $ 500. Another suitable polymer is a thermoplastic polyester polyurethane available from B.F. Goodrich Company under the. Designation "Estane 58305" in is brought on the market and has a sticking temperature of 149 C having. In the case of soot-containing extrudates, their polymeric embedding compounds have significantly higher crystal melting temperatures have than the polyethylene, for the sheathing of which the latter compounds are preferably used copolymers of vinylidene fluoride and hexafluoropropene, e.g. those under the name "Viton" from the company EGG. du Pont de Nemours and Company commercialized copolymers have been found to be suitable. Other suitable substances are familiar to the person skilled in the art on the basis of the conditions given above and below.

In order to obtain self-limiting masses, the polymer must Embedding compound in which the electrically conductive bus is distributed, a suitably non-linear thermal

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Have expansion coefficients, for which reason a certain degree of crystalline content is considered essential. In general, polymers with a crystalline content of at least about 20 i * ("determined by X-ray diffraction) are suitable for the purposes of the invention. Polyolefins, such as polyethylenes of lower, medium and high density, as well as polypropylene, polybutene (i), poly (dodecamethylene pyromellitic acid imide), copolymers of ethylene and propylene, copolymers of dienes with non-conjugated double bonds and two other monomers, polyvinylidene fluoride, copolymers of polyvinylidene fluoride, etc. which are dimensioned according to the desired application purpose (e.g. for anti-freeze, for thermostatic application, etc.) can be determined by a suitable choice of the polymeric embedding mass. For example, products with self-limiting temperatures of around 38 ° C, 54 ° C, 66 ° C, 82 ° C or 121 ° C with mixtures of wax and copoly merisates obtained from ethylene and vinyl acetate, with high-pressure polyethylene, low-pressure polyethylene, polypropylene and with polyvinylidene fluoride. Other important factors in the selection of the polymer are, under certain circumstances, the desired elongation at break, the desired environmental resistance and ease of extrudability.

Particularly preferred materials for the embedding race are Mixtures of several ingredients, in which case the rust? - is processed with a Geinisch constituent to a premix, whereupon one then the premix with the main constituent of the polymer mixture mixes. The first and the second Components of the Polyraerisatgemisches are selected so, that when mixed with one another they deliver a positive mixing free energy. Your resulting intolerance apparently the effect that the soot contained therein is

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limited areas of the polymer embedding compound accumulates, and such mixtures have proven to be extremely resistant to the action of alternately high and low temperatures in practical use. The component of the polymer mixture, which is present in a smaller proportion, is selected so that it is more compatible with carbon black than the main component, while the latter is selected on the basis of the physical properties that the entire extrudate should have. The weight ratio of the main component to the component present in a smaller amount, with which the carbon black is initially mixed, is preferably at least 3: 1. The particularly preferred mixtures contain polyethylene as the main component, while the other component is a copolymer of ethylene and a vinyl ester, such as a copolymer of ethylene and vinyl acetate or a copolymer of ethylene and ethyl acrylate. A particularly preferred extrudate core contains about 70 parts by weight of polyethylene and 20 parts by weight of a copolymer of ethylene and ethyl acrylate. It is considered essential that the jacket material is incompatible (ie not miscible) with the polymer embedding compound of the core extrudate. Whenever the same material was used for the extruded jacket as for the core of the embedding compound or for the main polymer component thereof, the specific resistance of the heat-treated product was undesirably high; Materials for additional coat, v as yet applied to the immediately adjacent to the carbon black-containing embedding ground shell / ground Koen but NEN be selected without regard to the compatibility with other ingredients of the product.

The same types of soot are used as those conventionally used in electrically conductive plastic compounds e.g. highly structured varieties such as Ftirnace and Ohannel soot. Also other conventional additives,

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ORIGINAL

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287 442 ^ ^υ

such as oxidation retardants, can be added to the polymeric embedding compound be added. Particularly suitable additives are substances such as waxes, although they are the main constituent of the mixture are compatible, but have a lower melting point. This leads to the result that a given wattage is reached even at a lower temperature, possibly because the melting point. Wax in first Caused a maximum in the resistance-temperature curve. That Mixing is done in the usual way and generally consists of mixing in the Banbury mixer, grinding and crumbling, before the self-limiting product from e'er melt is extruded.

According to the preferred embodiment, the carbon black-containing embedding mass is extrudier.t a pair of stationary spaced apart elongated electrodes into a rod or, preferably, a product of dumbbell cross-section, en that the extruded thermoplastic mass, the electrodes of both encapsulates and interconnects.

The extrusion of the insulating jacket can according to the known Tube extrusion processes take place, ie. by using the Core extruded a tubular section, which was then a few inches from the extrusion head as long as when it is still molten, it is drawn onto the core by maintaining reduced pressure between the core and the cladding. Each other method can be used to extrude the jacket by direct extrusion using a suitable Apply the mold to the core. However, the tubular extrusion process is preferred, especially when the product to be manufactured has a complicated (e.g., dumbbell-shaped) cross-section.

The heat treatment takes place at temperatures of more than about 120 G, preferably of at least about 150 C, and every

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ORIGINAL INSPECTED

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287 442 AA "

if at or above the melting point or melting range of the polymeric embedding compound in which the carbon black is distributed. The duration of the heat treatment depends on the type of embedding compound and the amount of ruso contained therein. In any case, the heat treatment must be carried out long enough to reduce the specific resistance of the heat-treated product so far that the equation 2 L + 5 log-in ^ - 45, preferably <40, is fulfilled (in which L is the percentage by weight of the carbon black, based on the amount by weight of the electrically conductive material, and R denotes the specific resistance), and the time required for this in each particular case can easily be determined empirically. Typically, the heat treatment takes more than 15 hours, usually about 24 hours. If the product is kept at the heat treatment temperature all this time, it is advisable to control the cooling after the end of the heat flow so that at least about 1 1/2 hours are required until the product has reached room temperature . The control of the cooling is less important, however, if the required total heat build-up time is divided into three roughly equal stages, with the product being cooled to room temperature each time between the individual heat treatment stages.

After the end of the heat treatment and, if necessary, after the addition of another insulating wreath, e.g. »made of polyethylene, the self-limiting element becomes expedient subjected to the action of ionizing radiation, to network the soot-containing core. The radiation dose that is required is preferably used is to wet the polymer so far that it is the for degree of heat resistance required for the respective application obtained without the crystalline portion of the polymeric embedding material dropping too much, i.e. a total

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ORIGINAL INSPECTED

The degree of crystallinity of the soot-containing embedding compound of less than about 20 % is preferably avoided. If these guidelines are observed, radiation doses in the general range between 2 and 15 megarads and preferably of about 12 megarads can be used.

Unless otherwise indicated in the following examples, parts and percentages are by weight and all resistivity values are ^{measured at room temperature using a Wheatstone 1} bridge.

Beis]) ie l 1

A Banbury mixer is filled with 34.5 kg of polyethylene (density 0.929), 14.5 kg of a mixture of 34 $> "Vulcan XC-72" and a copolymer of ethylene and acrylic acid ethyl ester (density 0.930 g / cm, acrylic acid ethyl ester content 18 $> ) loaded together with 0.45 kg of oxidation retarder. The flask is closed and mixing begins. If tie temperature reaches 115-120 ° C, the mixture is poured out, placed on a two-roll mill and cut into strips, which are fed to an extruder crumbs. The crumbled mixture is then extruded onto two parallel, tinned copper electrodes (20 AWG 19/32) to form an extrudate with a dumbbell-shaped cross-section. The electrodes are at a center-to-center distance of 6.9 mm from one another, and the web connecting them is 0.38 mm thick, with at least 0.2 mm of the thickness of the semiconducting mixture on the parts surrounding the electrodes. The extrusion takes place in a plasticizing extruder with a cross head attachment (Davis standard 51 mm extruder, 24/1 L / D, with PE screw). The same extruder is then arranged to extrude a 0.2 minute thick insulating jacket of polyurethane ("Texin 591-A" from Mobay Corporation). A conventional pipe extrusion method is used for optimal geometric adaptation, in which the molten pipe is subjected to a yakuum (from, for example

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125-500 mm water column) is exposed to the pipe in. A Distance of 75 mm from the extrusion head around the semiconducting Core around to collapse. The coated product is then wound onto aluminum disks (660 mm diameter) and exposed to a temperature of 150 ° C for 24 hours in a circulating air oven. According to this heat structure process and after cooling to room temperature over 1 1/2 hours, the specific resistance of the sample becomes determined at different temperatures. The results are shown in the table below

Table I.

Change in specific resistance with temperature T, ⁰ CR, Ohm «cm

4th 800 5 910 9 600 20th 950 69 900 481 500 6 150 000

16 27 38

49 60 71 82

93

Examples 2 to

There are further extrudates with different polymers and carbon black contents, unless otherwise indicated, after Example 1 produced. The polymeric embedding compounds in the various examples are as follows: Example 2: A mixture of 3 parts by weight of high-pressure polyethylene and 1 part by weight of ethylene and copolymer Acrylic acid ethyl ester; Example 3: a mixture of 5 parts by weight of high pressure polyethylene and 1 part by weight of a copolymer of ethylene and vinyl acetate;

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* ²³ * ⁵³²⁰

Example 4: polyvinylidene fluoride;

Example 5: a mixture of 3 parts by weight of medium-sized polyethylene Density and 1 part by weight copolymer of ethylene and ethyl acrylate;

Example 6: a mixture of 3 parts by weight of medium pressure polyethylene and 1 part by weight of a copolymer of ethylene and ethyl acrylate;

Example 7: a copolymer of ethylene and propylene ("Polyallomer" from Eastman Chemical Company); Example 8: Polybutene- (1);

Example 9 ' a copolymer of polyvinylidene fluoride and tetrafluoroethylene ("Kynar 5200" from Pennwalt Chemical Comp.)

In all mixtures, the 'soot with the lower Proportion "forming component processed into a premix which -then mixed with the other polymer component will. The jacketed extrudates of each of these examples show a non-linear positive temperature coefficient of the specific resistance. The results are as follows:

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4098U / 0872 - 15 -

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Example 10

One adjusts. Example 1 a coated with polyurethane Extrudate. This extrudate is made nine times in a row each Heat-treated for 3 hours at 149 ° C, whereby the product between the individual heat treatment periods Let room temperature cool down. Then the warm one is handled Provide the product with a last insulating jacket made of polyethylene (0.3 mm thick) using the tubular extrusion method and put it through Irradiate with a total of 12 megarads of electron beams networked through and through by 1 kev. The strip thus obtained shows at the temperatures given below the following values for the resistivity.

Table III

T, ⁰ C R, Ohm · cm T, ⁰ O R, Ohm • cm 16 4,800 60 69 900 27 5 910 71 481 500 38 9 600 82 6th 150 000 49 20 950 93 > 2 χ 10 ^ B e i s ρ i e 1 e 11 to 15

Unless otherwise stated, the method of Example 1 is used to produce further polymers as sheath materials used and checked for suitability. The results are as follows:

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345320

Tabel

IY

ie-
play Mant elwerks tο ff Heat re
action s-
time at
149 ⁰ G, h More specific
Resistance
at room temp.
after the heat
treatment,
Ohm.cm __ none (control) 24 3 185 11 Polyvinylidene fluoride from
high molecular weight
("Kureha .1000") 30th 4,990 12th Polyether polyurethane
("Texin 591-A") - 24 3 185 13th Polyvinylidene fluoride from
low molecular weight
weight ("Kureha 850") 30th 72,000 14th Silicone rubber
(General Electric Co.) 20th 5 260 15th "Silastic S-2288" 24 5 550

For comparison purposes, the same extrudates are otherwise provided with jackets made of two different polyamides, which are known to have no residual stress state at 149 ° C., namely polylauryl lactarn or poly-11-aminoundecanoic acid, which are sold by the Aguitane-Organico Company under ¹ the designation "Nylon 12" or "Nylon 11" are available. To
of the heat treatment are the Werv. for the specific
Resistance 278,000 or 2,220,000 ohm-era.

B ice ρ i e 1

16

A Banbury mixer is charged with 3780 grams of polyvinylidene fluoride and 306 grams of electrically conductive furnace black. The batch is mixed for 7 minutes and, as soon as the temperature has reached 171 ° C., poured out. Then the material, as in
Example 1, ground, cut, crumbled, extruded and surrounded with a jacket, for which one ^ in copolymer from
Hexafluoropropylene and vinylidene fluoride ("Viton" from Du

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iCTED

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Pont) is used. Then the sample is 2 hours at 204 ° C v / heat-treated, indicating its specific resistance at room temperature 1050 Ohm.cm. Sin comparative extrudate without Dumbbell shows a specific resistance after heat treatment at room temperature of 3,740,000 ohm-cm, from which the importance of the residual stress state of the sheath material at particularly low soot contents.

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Claims (9)

2 3 A b 3 2 G Raychem Corporation 287 442 Claims /1. Process for the production of self-regulating, electrically conductive products in which a mass is made a crystalline thermoplastic polymer which, based on the total amount by weight, contains up to 15% carbon black, on a pair of elongated, parallel electrodes to form a web connecting the two electrodes and the extrudate then at or above the temperature of the crystalline melting point of the polymer and above 120 ° C., wherein the soot content and the heat treatment conditions must be coordinated with one another so that the specific resistance at room temperature in the range of 5 to 100,000. 0hm «cm, characterized in that Dian is in front of the Heat treatment of a surrounding, electrically insulating layer made of a thermoplastic material on the extrudate Extruded material that is incompatible with the polymer and dimensionally stable at the heat treatment ^ temperature of the mass and that, when heated from room temperature to the heat treatment temperature, has a residual stress g s state is up. 2. Verf £ their according to claim 1, characterized in that the heat treatment is carried out at at least 150 ° C. 3. The method according to claim 1 or ?., Characterized in that <Ls insulating layer is applied which consists essentially of a polyether polyurethane. - 19 A098U / 0872 ORIGINAL INSPECTED 442 Iq 2 3 A b 3 2 O. 4. The method according to claim 1 or 2, characterized in that such an insulating layer is applied as the insulating layer, which essentially consist of a copolymer of hexafluoropropene and vinylidene fluoride. 5. The method according to claim 1 or 2, characterized in that an insulating layer is applied as the insulating layer, consisting essentially of a silicone rubber which is thermoset at or below the heat treatment temperature is. 6. Method of making self-regulating, electrically Conductive products, characterized in that (1) a pair of spaced-apart, elongated, parallel electrodes extruded a web connecting the electrodes from a mass, consisting essentially of (a) a crystalline thermoplastic polymer and (b) electrically conductive carbon black exists, whereby the percentage by weight of soot, based on the total weight of the mass, is a maximum of 10, (2) A surrounding, electrically insulating layer is extruded onto the extrudate produced in step (1) iind (3) the product thus obtained at or above the The crystal melting point of the polymer is at least about 1 ^ 0 C heat treated until the specific electrical Resistance of the product at room temperature is in the range of about 5 to 100,000 ohm.cm, the insulating layer consists essentially of a thermoplastic material with the said polymer is incompatible with the treatment temperature is dimensionally stable and when heated to λ ^ οη room temperature the heat-curing temperature a certain degree of RBstspannungüstensaxLd has. - 20 - ORIGINAL INSPECTED 4098U / 0872 7. The method according to claim 6, characterized in that an insulating layer is applied as the insulating layer which consists “essentially of a polyether polyurethane”. 8. The method according to claim 6, characterized in that one applies as an insulating layer that is im consists essentially of a copolymer of hexafluoropropene and vinylidene fluoride. 9. The method according to claim 6 to 8, characterized in that the polymer is a mixture of a larger one Proportion of polyethylene and a smaller proportion of a copolymer made from ethylene and a vinyl ester. - 21 409814/0872