GB1600257A

GB1600257A - Strip heaters comprising conductive polymer compositions

Info

Publication number: GB1600257A
Application number: GB26479/80A
Authority: GB
Original assignee: Raychem Corp
Current assignee: Raychem Corp
Priority date: 1976-12-13
Filing date: 1977-12-07
Publication date: 1981-10-14
Also published as: JPH0562439B2; DE2755077C2; JPS6057192B2; JPH0559557B2; DE2755077A1; NL185545B; FR2392572A1; CA1106890A; NO774258L; SE8505088L; SE447781B; NL7713800A; JPH053120B2; BE861776A; CA1206507B; SE8304042L; AU515034B2; JPH0256886A; SE434587B; SE7714126L

Description

PATENT SPECIFICATION ( 11) 1 600 257

1 ( 21) Application No 26479/80 ( 22) Filed 7 Dec 1977 ( 19) q ( 62) Divided Out of No 1600256, 2 ( 31) Convention Application No 750149 ( 32) Filed 13 Dec 1976 in S X o ( 33) United States of America (US) 'B \ ( 44) Complete Specification Published 14 Oct 1981 " 7 ( 51) INT CL 3 HOSB 3/36 ( 52) Index at Acceptance H 5 H 113 121 125 144 170 176 224 230 232 233 251 AF ( 72) Inventors: HUNDI PANDURANG KAMATH JEFFREY CHRIS LEDER ( 54) STRIP HEATERS COMPRISING CONDUCTIVE POLYMER COMPOSITIONS ( 71) We, RAYCHEM CORPORATION, A Corporation organized according to the laws of the State of California, United States of America, of 300 Constitution Drive, Menlo Park, California 94025, United States of America, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: 5

This invention relates to an electrical device in which an electrode is in contact with a conductive polymer composition.

Conductive polymer compositions are well known They comprise organic polymers having dispersed therein a finely divided conductive filler, for example carbon black or a particulate metal Some such compositions exhibit so-called PTC (Positive Temperature 10 Coefficient) behaviour The terminology which has been used in the past to describe PTC behaviour is variable and often imprecise In this specification, the terms "composition exhibiting PTC behaviour and "PTC composition" are used to denote a composition having at least one temperature range (hereinafter called a "critical range") which is within the limits of -1000 C and-about 250 'C; at the beginning of which the composition has a 15 resistivity below about 105 ohm cm; and in which the composition has an R 14 value of at least 2 5 or an RI( (, value of at least 10 (and preferably both), and preferably has an R 30 value of at least 6, where R 14 is the ratio of the resistivities at the end and the beginning of a 14 'C range R 1 (x) is the ratio of the resistivities at the end and the beginning of a 100 'C range, and R 3 i is the ratio of the resistivities at the end and the beginning of a 30 'C range 20 The term PTC element" is used herein to denote an element composed of a PTC composition as defined above A plot of the log of the resistance of a PTC element, measured between two electrodes in contact with the element, against temperature, will often though by no means invariably, show a sharp change in slope over a part of the critical temperature range, and in such cases, the term "switching temperature" (usually 25 abbreviated to T) is used herein to denote the temperature at the intersection point of extensions of the substantially straight portions of such a plot which lie each side of the portion showing the sharp change in slope The PTC composition in such a PTC element is described herein as having "a useful Tb" The T, is preferably between 00 and 1750 C, e g.

500 C to 1200 C 30 Conductive polymer compositions, especially PTC compositions, are useful in electrical devices in which the composition is in contact with an electrode, usually of metal Devices of this kind are usually manufactured by methods comprising extruding or moulding the molten polymer composition around or against the electrode or electrodes In the known methods, the electrode is not heated prior to contact with the polymer composition or is 35 heated only to a limited extent, for example to a temperature well below the melting point of the composition, for example not more than 650 C as in conventional wire-coating techniques lTemperatures are in 'C throughout this specification l Well known examples of such devices are flexible strip heaters which comprise a generally ribbon-shaped core of the conductive polymer composition a pair of longitudinally extending electrodes, 40 generally of stranded wire, embedded in the core near the edges thereof, and an outer layer of a protective and insulating composition Particularly useful heaters are those in which the composition exhibits PTC behaviour, and which are therefore selfregulating In the preparation of such heaters in which the composition contains less than 15 % of carbon black, the prior art has taught that it is necessary, in order to obtain a sufficiently low 45

2 1 600 257 2 resistivity, to anneal the heater for an extended period such that 2 L + 5 log 10 R 45 5 where L is the percent by weight of carbon and R is the resistivity in ohm cm at room temperature.

A disadvantage which arises with devices comprising an electrode and a conductive polymer composition in contact with the electrode, and in particular with strip heaters, is 10 that the longer they are in service the higher is their resistance and the lower is their power output, particularly when they are subject to thermal cycling.

It is known that variation, from device to device, of the contact resistance between electrodes and carbon-black-filled rubbers is an obstacle to comparison of the electrical characteristics of such devices and to the accurate measurement of the resistivity of such 15 rubbers, particularly at high resistivities and low voltages; and it has been suggested that the same is true of other conductive polymer compositions Various methods have been suggested for reducing the contact resistance between carbon-black-filled rubbers and test electrodes placed in contact therewith The preferred method is to vulcanise the rubber while it is in contact with a brass electrode Other methods include copper-plating, 20 vacuum-coating with gold, and the use of colloidal solutions of graphite between the electrode and the test piece For details, reference should be made to Chapter 2 of "Conductive Rubbers and Plastics" by R H Norman, published by Applied Science Publishers ( 1970), from which it will be clear that the factors which govern the size of such contact resistance are not well understood 25 We have now discovered that the less is the initial contact resistance between an electrode and a conductive polymer composition, the smaller is the increase in total resistance with time We have also discovered that by placing or maintaining an electrode and a polymer composition in contact with each other while both are at a temperature above the melting point of the composition, the contact resistance between them is 30 reduced The term "melting point of the composition" is used herein to denote the temperature at which the composition begins to melt The time for which the electrode and the composition need be in contact with each other, while each is at a temperature above the melting point of the composition, in order to achieve the desired result, is quite short.

Times in excess of five minutes do not result in any substantial further reduction of contact 35 resistance, and often times less then 1 minute are quite adequate and are therefore preferred Thus the treatment is of a quite different order from that required by the known annealing treatments to decrease the resistivity of the composition, as described for example in U S Patents Nos 3,823,217 and 3,914,363.

In Application No 50916/77 (Serial No 1600256), from which this Application has been 40

Claims

divided, there is claimed a process for the manufacture of a device

comprising an electrode and a conductive polymer composition in contact therewith, which process comprises ( 1) heating a conductive polymer composition to a temperature (Tp) above its melting point (Tm); ( 2) heating an electrode, out of contact with the conductive polymer composition, to a 45 temperature (TJ) above the melting point of the conductive polymer composition; ( 3) contacting the electrode, while it is at a temperature above Tm, with the molten polymer composition; and ( 4) cooling the electrode and conductive polymer composition in contact therewith.

Preferably both TP and Te are at least 20 'C, especially at least 550 C, above Tm It is often 50 preferable that both TP and T, should be above the Ring-and-Ball softening temperature of the polymer composition.

Preferably the conductive polymer composition is subjected to pressure to assist in bringing it into close conformity with the electrode The pressure is generally at least 14 kg/cm-, preferably at least 21 kg/cm 2, for example 21 to 200 kg/cm 2, especially at least 35 55 kg/cm
2 e g 35-70 kg/cm 1 600 257 We have also found that for strip heaters, currently the most widely used devices in which current is passed through conductive polymer compositions, the contact resistance can be correlated with the linearity ratio, a quantity which can readily be measured as described below.

Accordingly, the present invention provides a strip heater comprising: 5 ( 1) an elongate core of a conductive polymer composition which exhibits PTC behaviour, which comprises carbon black, and in which, if the content (L) of carbon black in percent by weight is less than 15, L and the resistivity R of the composition in ohm cm at room temperature are such that 10 2 L + 5 log,( R> 45; ( 2) at least two longitudinally extending electrodes embedded in said composition 15 parallel to each other; and ( 3) an outer layer of a protective and insulating composition; the average linearity ratio (and preferably the linearity ratio at all points) between any pair of electrodes being at most 1 2, preferably at most 1 15, especially at most 1 10 The linearity ratio of a strip heater is defined as 20 Resistance at 30 MV.

Resistance at 100 V 25 the resistance being measured at 21 'C between two electrodes which are contacted by probes pushed through the outer jacket and the conductive polymeric core of the strip heater The contact resistance is negligible at 100 V, so that the closer the linearity ratio is 30 to 1, the lower the contact resistance.

The strip heaters of the present invention are preferably manufactured by the process claimed in Application 50916/77 (Serial No 1600256), described above.

The present invention is useful with any type of electrode, for example metal plates, strips or wires, but particularly so with electrodes having an irregular surface, e g stranded 35 wire electrodes as conventionally used in strip heaters, braided wire electrodes (for example as described in German Offenlegungschrift No 2,635,000 5) and expansible electrodes as described in German Offenlegungschrift No 2,655,543 1 Preferred stranded wires are silver-coated and nickel-coated copper wires, which are less susceptible to difficulties, such as melting or oxidation, than tin-coated or uncoated copper wires, though the latter can be 40 used without difficulty providing the temperatures employed are not too high.

The conductive polymer compositions used in this invention generally contain carbon black as the conductive filler, e g in amount greater or less than 15 % by weight, for example greater than 17 % or 20 % by weight In many cases, it is preferred that the compositions should exhibit PTC behaviour The resistivity of the composition is generally 45 less than 50,000 ohm cm at 21 'C, for example 100 to 50,000 ohm cm For strip heaters designed to be powered by A C of 115 volts or more, the composition generally has a resistivity of 2 000 to 50,000 ohm cm, e g 2,000 to 40,000 ohm cm The composition is preferably thermoplastic when applied to the electrode However, it may be lightly cross-linked, or be in the process of being cross-linked, provided that it is sufficiently fluid 50 under the contacting conditions to conform closely to the electrode surface The polymer is preferably a crystalline polymer.

The strip heaters with which the present invention is concerned generally have two electrodes separated by a distance of 0 15 to 1 cm, but greater separations, e g up to 2 5 cm or even more, can be used The core of conductive polymer can be of conventional 55 shape, but preferably its major transverse dimension is not more than 3 times, especially not more than 1 5 times, e g not more than 1 1 times, its smallest dimension A core of round cross-section is especially preferred.

As indicated above, in manufacture, the electrode and the polymer composition are preferably heated separately before being contacted It is preferred that the composition be 60 melt-extruded over the electrode e g by extrusion around a pair of spacedapart wire electrodes using a cross-head die The electrode is pre-heated to a temperature (Te) which may be greater or less than the temperature of the molten polymer composition (Tp) but is generally more than (TP-550 C) and preferably more than (TP-300 C) TP will normally be substantially above the melting point of the composition, for example 30 to 80 'C above Of 65 1 600 257 course, neither the electrode nor the composition should be heated to a temperature at which it undergoes substantial oxidation or other degradation.

Particularly when the conductive polymer composition exhibits PTC behaviour, it is often desirable that in the final product the composition should be crosslinked Cross-linking can be carried out as a separate step after the treatment to reduce contact resistance; in this 5 case, cross-linking with aid of radiation is preferred Alternatively cross-linking can be carried out simultaneously with the said tratment, in which case chemical cross-linking with the aid of cross-linking initiators such as peroxides is preferred.

The invention is illustrated by the following Examples, some of which are comparative Examples 10 In each of the Examples a strip heater was prepared as described below The conductive polymer composition was obtained by blending a medium density polyethylene containing an antioxidant with a carbon black master batch comprising an ethylene/ethyl acrylate copolymer to give a composition containing the indicated percent by weight of carbon black The melting point of the composition was about 115 'C The composition was 15 melt-extruded at a melt temperature of about 180 WC through a cross-head die having a circular orifice 0 36 cm in diameter over a pair of stranded silvercoated copper wires, each wire having a diameter of 0 08 cm and comprising 19 strands, and the axes of the wires being on a diameter of the orifice and 0 2 cm apart Before reaching the cross-head die, the wires were pre-heated by passing them through an oven 60 cm long at 8)00 C The 20 temperature of the wires entering the die was 820 C in the comparative Examples 1, 4 and 6, in which the speed of the wires through the oven and the die was 21 m/min, 1650 C in Examples 2 and 7 and 1930 C in Examples 3 and 5.

The extrudate was then given an insulating jacket by melt-extruding around it a layer O 051 cm thick of chlorinated polyethylene or an ethylene/tetrafluoroethylene copolymer 25 The coated extrudate was then irradiated in order to cross-link the conductive polymer composition.

Examples 1-3 These Examples in which Example 1 is a comparative Example, demonstrate the 30 influence of Linearity Ratio (LR) on Power Output when the heater is subjected to temperature changes In each Example, the Linearity Ratio of the heater was measured and the heater was then connected to a 120 volt AC supply and the ambient temperature was changed continuously over a 3 minute cycle, being raised from -370 C to 65 WC over a period of 90 seconds and then reduced to -370 C again over the next 90 seconds 35 The peak power output of the heater during each cycle was measured initially and at intervals and expressed as a proportion (PN) of the initial peak power output.

The polymer composition used in Example 1 contained about 26 % carbon black The polymer composition used in Examples 2 and 3 contained about 22 % carbon black.

The results obtained are shown in Table 1 below 40 TABLE 1 No of Cycles Example 1 Example 2 Example 3 45 PN LR PN LR PN LR None 1 1 3 1 1 1 1 1 50 500 0 5 1 6 1 3 1 1 1100 0 3 2 1 1 2 1 1 1700 1 1 1 1 1 1 55 "Comparative Example 1 600 257 5 Examples 4-7 These Examples, which are summarised in Table 2 below, demonstrate the effect of pre-heating the electrodes on the Linearity Ratio of the product.

TABLE 2 5 Example No % Carbon Black Linearity Ratio 4 22 1 6 10 22 1 0 6 23 1 35 15 7 23 1 1 Comparative Example WHAT WE CLAIM IS: 20 1 A strip heater comprising ( 1) an elongate core of a conductive polymer composition which exhibits PTC behaviour, which comprises carbon black, and in which, if the content (L) of carbon black in percent by weight is less than 15, L and the resistivity (R) of the composition in ohm cm at room temperature are such that 25 2 L + 5 10 g 1,( R > 45; 30 ( 2) at least two longitudinally extending electrodes embedded in said composition parallel to each other; and ( 3) an outer layer of a protective and insulating composition; the average linearity ratio (as hereinbefore defined) between any pair of electrodes being at most 1 2 35 2 A strip heater according to claim 1, wherein the average linearity ratio between any pair of electrodes is at most 1 15.
3 A strip heater according to claim 2, wherein the average linearity ratio between any pair of electrodes is at most 1 10.
4 A strip heater according to any one of claims 1 to 3, wherein the linearity ratio at all 40 points between any pair of electrodes is at most 1 2.
A strip heater according to claim 4, wherein the linearity ratio at all points between any pair of electrodes is at most 1 15.
6 A strip heater according to claim 5, wherein the linearity ratio at all points between any pair of electrodes is at most 1 10 45
7 A strip heater according to any one of claims 1 to 6, wherein the electrodes are silver-coated copper wires or nickel-coated copper wires.
8 A strip heater according to any one of claims 1 to 7, wherein the two electrodes are separated by a distance of from 0 15 to 1 cm.
9 A strip heater according to any one of claims 1 to 8, wherein the core of conductive 50 polymer has a major transverse dimension of not more than 3 times its smallest dimension.
A strip heater according to claim 9, wherein the core of conductive polymer has a major transverse dimension which is not more than 1 5 times its smallest dimension.
11 A strip heater according to any one of claims ito 10, wherein the core of conductive polymer is of round cross-section 55
12 A strip heater according to any one of claims 1 to 11, wherein the conductive polymer composition comprises at least 15 % by weight of carbon black.
13 A strip heater according to any one of claims 1 to 12, wherein the conductive polymer composition is cross-linked.
14 A strip heater according to claim 13, wherein the conductive polymer composition is 60 irradiation cross-linked.

6 1 600 257 6 A strip heater according to claim 1, substantially as described in any one of examples 2, 3, 5 and 7.

ABEL & IMRAY, Chartered Patent Agents, 5 Northumberland House, 303-306 High Holborn, London WC 1 V 7 LH.

Printed for Her Majesty's Stationery Office, by Croydon Printing Company Limited, Croydon, Surrey 1981.

Published by The Patent Office 25 Southampton Buildings, London, WC 2 A l AY, from which copies may be obtained.