CA1106890A - Electrical devices comprising conductive polymer compositions - Google Patents
Electrical devices comprising conductive polymer compositionsInfo
- Publication number
- CA1106890A CA1106890A CA292,832A CA292832A CA1106890A CA 1106890 A CA1106890 A CA 1106890A CA 292832 A CA292832 A CA 292832A CA 1106890 A CA1106890 A CA 1106890A
- Authority
- CA
- Canada
- Prior art keywords
- conductive polymer
- process according
- composition
- polymer composition
- electrode
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
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Classifications
-
- H—ELECTRICITY
- H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
- H05B—ELECTRIC HEATING; ELECTRIC LIGHT SOURCES NOT OTHERWISE PROVIDED FOR; CIRCUIT ARRANGEMENTS FOR ELECTRIC LIGHT SOURCES, IN GENERAL
- H05B3/00—Ohmic-resistance heating
- H05B3/10—Heater elements characterised by the composition or nature of the materials or by the arrangement of the conductor
- H05B3/12—Heater elements characterised by the composition or nature of the materials or by the arrangement of the conductor characterised by the composition or nature of the conductive material
- H05B3/14—Heater elements characterised by the composition or nature of the materials or by the arrangement of the conductor characterised by the composition or nature of the conductive material the material being non-metallic
- H05B3/146—Conductive polymers, e.g. polyethylene, thermoplastics
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01C—RESISTORS
- H01C7/00—Non-adjustable resistors formed as one or more layers or coatings; Non-adjustable resistors made from powdered conducting material or powdered semi-conducting material with or without insulating material
- H01C7/02—Non-adjustable resistors formed as one or more layers or coatings; Non-adjustable resistors made from powdered conducting material or powdered semi-conducting material with or without insulating material having positive temperature coefficient
- H01C7/027—Non-adjustable resistors formed as one or more layers or coatings; Non-adjustable resistors made from powdered conducting material or powdered semi-conducting material with or without insulating material having positive temperature coefficient consisting of conducting or semi-conducting material dispersed in a non-conductive organic material
Abstract
ABSTRACT
The invention relates to electrical devices comprising an electrode and a conductive polymer composition, preferably a PTC com-position, in contact with the electrode.
The process of the invention comprises contacting the com-position and the electrode while the composition is at a temperature (Tp) above its melting point (Tm) and the electrode is at a tempera-ture (Te) above the melting point of the composition, Tp and Te being the same or different, for a time which is sufficient to reduce the contact resistance between the electrode and the composition but which is not sufficient substantially to reduce the resistivity of the composition, for example, 2 time less than 5 minutes. Preferably both Tp and Te are at least 20°C, especially at least 55°C, above Tm. In a preferred process the composition is melt-extruded over one of more pre-heated electrodes. It has been found that the process of the invention substantially reduces the tendency of the resistance of the device to increase when the device is subject to thermal cycling.
The invention is particularly useful in connection with self-regulating heaters, especially strip heaters comprising a pair of stranded wire electrodes embedded in an elongate core of a PTC
material comprising carbon black dispersed in a crystalline polymer.
The invention relates to electrical devices comprising an electrode and a conductive polymer composition, preferably a PTC com-position, in contact with the electrode.
The process of the invention comprises contacting the com-position and the electrode while the composition is at a temperature (Tp) above its melting point (Tm) and the electrode is at a tempera-ture (Te) above the melting point of the composition, Tp and Te being the same or different, for a time which is sufficient to reduce the contact resistance between the electrode and the composition but which is not sufficient substantially to reduce the resistivity of the composition, for example, 2 time less than 5 minutes. Preferably both Tp and Te are at least 20°C, especially at least 55°C, above Tm. In a preferred process the composition is melt-extruded over one of more pre-heated electrodes. It has been found that the process of the invention substantially reduces the tendency of the resistance of the device to increase when the device is subject to thermal cycling.
The invention is particularly useful in connection with self-regulating heaters, especially strip heaters comprising a pair of stranded wire electrodes embedded in an elongate core of a PTC
material comprising carbon black dispersed in a crystalline polymer.
Description
11(~6890 .i This invention relates to electrical devices in which an electrode is in contact with a conductive polymer comFosition.
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 Coefficient) behavior. The terminology which has been used in the past to describe PTC behavior is variable and often imprecise. In this specification, the terms "composition exhibiting PTC behavior" and "PTC
composition" are used to denote a composition having at least one temFerature range (hereinafter called a "critical range") which is within the limits of -100C and about 250C; at the beginning of which the ccmposition has a resistivity below about 105 ohm. cm.; and in which the composition has an ~ 4 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, ~ 0O 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 ccmposed of a PTC composition as defined above. A plot of the log of the resistance of a ~TC element, measured between two electrodes in contact with the el~ment, against temperature, will often, though by no ` means invariably, show a sharp change in slope over a part of the critical t~mperature 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 si~e of the portion showing the sharp change in slope. The PTC composition in such a PTC element is described herein as having "a useful T ". The ov s TS is preferably between l~æ and 175C., e.g. 50C to 120CC.
:` ~
.,~,.
.''' ~1~6890 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 Folymer composition around or against the electrode or electrodes. In the known methods, the electrode is not heated prior to contact with the polymer comFosition or is heated only to a limited extent, for example to a temperature well below the melting point of the composition, for example not more than 65C as in conventional wire-coating techniques. [Temperatures are in C throughout this specification.] 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 longit~dinally extending electrodes, generally of stranded wire~ embedded in the core near the edges thereof, and an outer layer of a protective and insulating comFosition.
Particularly useful heaters are those in which the composition exhibits PTC behavior, and which are therefore self-regulating. 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 resistivity, to anneal the heater for an extended period such that - 2L + 5 log10 R< 45 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 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.
i89~
It is known that variations, 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 de~ices and to the accurate measurement of the resistivity of such rubbers, particularly at high resistivities and low voltages; and it has been sus~gested that the same is true of other conductive polymer compositions. Various methods have been suggested ~or 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, 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 R~bbers 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.
' rw'e 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 t'.^,at 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 comFosition, the contact resistance between them is reduced. The term "melting point of the ccmposition" is used herein to denote the temperature at which the composition bes~ins 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 ~i~)6890 minutes do not result in any substantial further reduction of contact resistance, and often times less than 1 minute are quite adequate and are therefore preferred. Thus the treatment time is of a quite different order from that re~ui~ed by the known annealing treatments to decrease the resistivity of the composition, as described for example in U.S. Patents Nos. 3,8a3,217 and 3,914,363.
In one aspect, therefore, the invention provides a process for the preparation or modiiication of~a device comprising an electrode and a conductive polymer composition i~ contact w1th the electrode, which process comprises contacting, or maintaining contact between, the conductive polymer composition and the electrode while the conductive polymer composition is at a temperature (Tp) above its melting point (Tm) and the electrode is at a temperature (Te) abcve the melting point of the conductive composition, Tp and Te being the same or different, lS for a time which is sufficient to reduce the contact resistance between the electrode and the conductive polymer comFosition but which is not sufficient substantially to reduce the resistivity of the conductive polymer. Preferably both Tpand Te are at least 20C, especially at least 55C, abcve Tm. It is often preferable that both Tp and Te should be above the Ring-and-Ball softening temperature of the polymer composition.
Preferably the conductive polymer camposition is subjected to pressure to assist in bringing it into close-conformity with the electrode.
The pressure is generally at least 14 kg/cm2, preferably at least 21 kg/cm2, for example 21 to 200 kg/cm2, especially at least 35 kg/cm2, e.g. 35-70 kg/om .
)6890 We have also found that the contact resistance can be correlated with the force needed to pull the electrode from the polymer composition. Accordingly the invention further provides a device comprising an electrode in contact with a conductive polymer composition, especially a stranded wire electrode embedded in a conductive polymer composition, the pull strength (P) of the electrode from the device being equal to at least 1.4 times P0, where P0 is the pull strength of an identical electrode from a device which comprises an identical electrode in contact with an identical conductive polymer composition and which has been prepared by a process which consists of contacting the electrode, while it is at a temperature not greater than 24C, with the molten conductive polymer ccmposition, and allowing the polymer ccmposition to cool in contact with the electrode. The pull strengths P and P0 are determined at 21C as described in detail below A 5.1 cm long sample of the heater strip (or other device), containing a straight 5.1 cm length of the electrodes, is cut off. At one end of the sample, 2.5 cm of the electrode is stripped bare of polymer. The bared electrode is passed downwardly through a hole slightly larger than the electrode in a rigid metal plate fixed in the horizontal plane. The end of the bared electrode is firmly clamped in a movable clamp below the plate, and the other end of the sample is li~htly clamped above the plate, so that the electrode is vertical.
The movable clamp is then moved vertically downwards at a speed of 5.1 cm~min., and the peak force needed to pull the conductor out of the sample is measured.
.. . .. . . . .
8g(~
T~e 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 invention further provides a strip heater comprising:
(1) an elongate core of a conductive polymer cvmposition which exhibits PTC behavior, which comprises carbon black, and in which, if the content (L) of carbon blac~ in percent by weight is less than 15, L and the resistivity R of the cc~osition in ohm. cm are such that 2L + S logl0 R > 45;
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 Coefficient) behavior. The terminology which has been used in the past to describe PTC behavior is variable and often imprecise. In this specification, the terms "composition exhibiting PTC behavior" and "PTC
composition" are used to denote a composition having at least one temFerature range (hereinafter called a "critical range") which is within the limits of -100C and about 250C; at the beginning of which the ccmposition has a resistivity below about 105 ohm. cm.; and in which the composition has an ~ 4 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, ~ 0O 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 ccmposed of a PTC composition as defined above. A plot of the log of the resistance of a ~TC element, measured between two electrodes in contact with the el~ment, against temperature, will often, though by no ` means invariably, show a sharp change in slope over a part of the critical t~mperature 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 si~e of the portion showing the sharp change in slope. The PTC composition in such a PTC element is described herein as having "a useful T ". The ov s TS is preferably between l~æ and 175C., e.g. 50C to 120CC.
:` ~
.,~,.
.''' ~1~6890 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 Folymer composition around or against the electrode or electrodes. In the known methods, the electrode is not heated prior to contact with the polymer comFosition or is heated only to a limited extent, for example to a temperature well below the melting point of the composition, for example not more than 65C as in conventional wire-coating techniques. [Temperatures are in C throughout this specification.] 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 longit~dinally extending electrodes, generally of stranded wire~ embedded in the core near the edges thereof, and an outer layer of a protective and insulating comFosition.
Particularly useful heaters are those in which the composition exhibits PTC behavior, and which are therefore self-regulating. 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 resistivity, to anneal the heater for an extended period such that - 2L + 5 log10 R< 45 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 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.
i89~
It is known that variations, 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 de~ices and to the accurate measurement of the resistivity of such rubbers, particularly at high resistivities and low voltages; and it has been sus~gested that the same is true of other conductive polymer compositions. Various methods have been suggested ~or 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, 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 R~bbers 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.
' rw'e 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 t'.^,at 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 comFosition, the contact resistance between them is reduced. The term "melting point of the ccmposition" is used herein to denote the temperature at which the composition bes~ins 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 ~i~)6890 minutes do not result in any substantial further reduction of contact resistance, and often times less than 1 minute are quite adequate and are therefore preferred. Thus the treatment time is of a quite different order from that re~ui~ed by the known annealing treatments to decrease the resistivity of the composition, as described for example in U.S. Patents Nos. 3,8a3,217 and 3,914,363.
In one aspect, therefore, the invention provides a process for the preparation or modiiication of~a device comprising an electrode and a conductive polymer composition i~ contact w1th the electrode, which process comprises contacting, or maintaining contact between, the conductive polymer composition and the electrode while the conductive polymer composition is at a temperature (Tp) above its melting point (Tm) and the electrode is at a temperature (Te) abcve the melting point of the conductive composition, Tp and Te being the same or different, lS for a time which is sufficient to reduce the contact resistance between the electrode and the conductive polymer comFosition but which is not sufficient substantially to reduce the resistivity of the conductive polymer. Preferably both Tpand Te are at least 20C, especially at least 55C, abcve Tm. It is often preferable that both Tp and Te should be above the Ring-and-Ball softening temperature of the polymer composition.
Preferably the conductive polymer camposition is subjected to pressure to assist in bringing it into close-conformity with the electrode.
The pressure is generally at least 14 kg/cm2, preferably at least 21 kg/cm2, for example 21 to 200 kg/cm2, especially at least 35 kg/cm2, e.g. 35-70 kg/om .
)6890 We have also found that the contact resistance can be correlated with the force needed to pull the electrode from the polymer composition. Accordingly the invention further provides a device comprising an electrode in contact with a conductive polymer composition, especially a stranded wire electrode embedded in a conductive polymer composition, the pull strength (P) of the electrode from the device being equal to at least 1.4 times P0, where P0 is the pull strength of an identical electrode from a device which comprises an identical electrode in contact with an identical conductive polymer composition and which has been prepared by a process which consists of contacting the electrode, while it is at a temperature not greater than 24C, with the molten conductive polymer ccmposition, and allowing the polymer ccmposition to cool in contact with the electrode. The pull strengths P and P0 are determined at 21C as described in detail below A 5.1 cm long sample of the heater strip (or other device), containing a straight 5.1 cm length of the electrodes, is cut off. At one end of the sample, 2.5 cm of the electrode is stripped bare of polymer. The bared electrode is passed downwardly through a hole slightly larger than the electrode in a rigid metal plate fixed in the horizontal plane. The end of the bared electrode is firmly clamped in a movable clamp below the plate, and the other end of the sample is li~htly clamped above the plate, so that the electrode is vertical.
The movable clamp is then moved vertically downwards at a speed of 5.1 cm~min., and the peak force needed to pull the conductor out of the sample is measured.
.. . .. . . . .
8g(~
T~e 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 invention further provides a strip heater comprising:
(1) an elongate core of a conductive polymer cvmposition which exhibits PTC behavior, which comprises carbon black, and in which, if the content (L) of carbon blac~ in percent by weight is less than 15, L and the resistivity R of the cc~osition in ohm. cm are such that 2L + S logl0 R > 45;
(2) at least two longitudinally extending electrodes embedded in said comFosition parallel to each other; and
(3) an outer layer of a protective and insulating comFosition;
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 l.lS, especially at most 1.10. The linearity ratio of a strip heater is defined as Resistance at 30 MV.
Resistance at 100 V.
the resistances being measured at 21C between tw~ electrodes which are contacted by probes pushed thro~gh 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 to l, the lower the contact resistance.
The invention is useful with any type of electrode, for metal example!plates, strips or wires, but particularly so with electrodes having an irregular surface, e.g. stranded wire electrodes as conventionally used in strip heaters, braided wire electrodes (for example as described in German Offenlegungschrift No. 2,635,000.5) ~1~6890 and expandable electrodes as described in German Offenlegungschrift ~o.
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 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 behavior. The resistivity of the composition is generally less than 50,000 ohm.cm at 21C, for example 100 to 50,000 ohm.cm. For strip heaters designed to be powered by A.C. of 115 volts or more, the ccmposition generally has a resistivity of 2,000 to 50,000 ohm.cm, e.g.
2,000 to 40,000 ohm.cm. m e composition is preferably thermoplastic.
However, it may be lightly cross-linked, or be in the process of being cross-linked, provided that it is sufficiently fluid under the contacting conditions to conform closely to the electrode surface.
The polymer is preferably a crystalline polymer.
The strip heaters with which the invention is concerned generally have tw~ electrodes separated by a distance of 0.15 to 1 cm, :-: but greater separations, e.g. up to 2.5 cm. or even re, can be used.
The core of conductive polymer can be of conventional shape, but preferably it has a cross-section which is not more than 3 times, especially not more than 1.5 times, e.g. not more than 1.1 times, its smallest dimension, especially a round cross-section.
li~6890 In one preferred em~odiment of the invention, the electrode and the polymer composition are heated separately before being contacted. In this embcdiment, it is preferred that the comFosition ~;hould be melt-extruded over the electrode, e.g. by extrusion around a pair of spaced-apart wire electrodes using a cross-head die. The electrode is pre-heated to a temperature (Te) which may be greater or less than the melt temperature of the polymer composition (Tp) but is generally more than (Tp-55) and preferably more than (Tp-30). Tp will normally be substantially above the melting point of the comE~osition, for example 30 to 80C above. Of course, neither the electrode nor the cc ~ osition should be heated to a temperature at which it undergoes substantial oxidation or other degradation.
In another embodiment of the invention, the composition is shaped in contact with the electrode (without pre-heating the electrode) and the electrode and the oom~osition are then heated, while in contact with each other, to a temperature above the melting point of the comFosition. Care is needed to ensure a useful reduction in the contact resistance by this method. The optimum conditions will depend uFon the electrode and the composition, but increased time, temperature and pressure help to achieve the desired result. The - pressure may be applied for example in a press or by means of nip rollers. This embcdiment of the invention is particularly useful when the need for, or desirability of, an annealing treatment does not arise at all, for example, when the comosition has a carbon black content greater than 15~ by weight, e.g. greater than 17% or 20~ by weight, or when only a limited annealing treatment is carried out, such that at the end of the annealing the content of carbon black (L) and the resistivity ~R) are such that 2L + 5 logl0 > 45.
One way of heating the electrode and the comFosition surrounding it is to pass a high current through the electrode and thus produce the desired heat by resistance heating of the electrode.
In another embodiment of the invention, the conductive polymer co~position is initially in the form of a shaped article, e~g.
one or more pills or pelle~s, which has not been shaped in contact with the electrode, and the electrode and the comFosition are heated together under pressure, for example in a compression mould.
Particularly when the conductive polymer composition exhibits PTC behavior, it is often desirable that in the final product the composition should be cross-linked. Cross-linking can be carried out as a separate step after the treat~ent to reduce contact resistance; in this case, cross-linking with aid of radiation is preferred.
Alternatively cross-linking can be carried out simultaneously with the said treatment, 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 lS of which are comparative Examples.
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 115C. The com~osition was melt-extruded at a melt temperature of about 180C through a cross-head die having a circular orifice 0.36 cm in diameter over a pair of stranded silver-coated 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 800C. The temperature of the wires entering the die was 82C in the comparative Examples 1, 4 and 6, in which the speed of the wires through the oven and the die was 21m/min, 165C in Examples 2 and 7 and 193C in Examples 3 and 5.
The extrudate was then given an insulating jacket by melt-e!xtruding around it a layer 0.051 cm thick of chlorinated polyethylene or an ethylene/tetrafluoroethylene copolymer. The coated extrudate was then irradiated in order to cross-link the conductive polymer ccmposition.
These Examples, in which Example 1 is a cc~parative Example, demonstrate the 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 c~cle, being raised ~rom -37C to 65C c~ver a period of 90 seconds and then reduced to -37C again over the next 90 seconds.
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 ccmposition used in Examples 2 and 3 contained about 22% carbon black.
.
li~)6890 The results obtained are shown in Table 1 below.
~BLE 1 l~o. of Cycles *Example 1 Example 2 Example 3 PN LR PN PN
None 1 1.3 11.1 500 0.5 1.6 1.3 - 1 1 1100 0.3 2.1 1.2 1700 - - 1.1 1.1 *Comparative Example EX~MPLES 4-7 These Examples, which are summarised in Table 2 below, demonstrate the effect of pre-heating the electrodes on the Linearity Ratio and Pull Strength of the product.
I~B~E 2 Example No. % Carbon Black Linearity Ratio -; *4 22 1.6 22 1.0 *6 23 1.35 ~i.
` 7 23 1.1 20*Comparative Example , .
The ratio of the pull strengths of the heater strips of Examples 7 and 6 (P/PO) was 1.45.
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 l.lS, especially at most 1.10. The linearity ratio of a strip heater is defined as Resistance at 30 MV.
Resistance at 100 V.
the resistances being measured at 21C between tw~ electrodes which are contacted by probes pushed thro~gh 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 to l, the lower the contact resistance.
The invention is useful with any type of electrode, for metal example!plates, strips or wires, but particularly so with electrodes having an irregular surface, e.g. stranded wire electrodes as conventionally used in strip heaters, braided wire electrodes (for example as described in German Offenlegungschrift No. 2,635,000.5) ~1~6890 and expandable electrodes as described in German Offenlegungschrift ~o.
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 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 behavior. The resistivity of the composition is generally less than 50,000 ohm.cm at 21C, for example 100 to 50,000 ohm.cm. For strip heaters designed to be powered by A.C. of 115 volts or more, the ccmposition generally has a resistivity of 2,000 to 50,000 ohm.cm, e.g.
2,000 to 40,000 ohm.cm. m e composition is preferably thermoplastic.
However, it may be lightly cross-linked, or be in the process of being cross-linked, provided that it is sufficiently fluid under the contacting conditions to conform closely to the electrode surface.
The polymer is preferably a crystalline polymer.
The strip heaters with which the invention is concerned generally have tw~ electrodes separated by a distance of 0.15 to 1 cm, :-: but greater separations, e.g. up to 2.5 cm. or even re, can be used.
The core of conductive polymer can be of conventional shape, but preferably it has a cross-section which is not more than 3 times, especially not more than 1.5 times, e.g. not more than 1.1 times, its smallest dimension, especially a round cross-section.
li~6890 In one preferred em~odiment of the invention, the electrode and the polymer composition are heated separately before being contacted. In this embcdiment, it is preferred that the comFosition ~;hould be melt-extruded over the electrode, e.g. by extrusion around a pair of spaced-apart wire electrodes using a cross-head die. The electrode is pre-heated to a temperature (Te) which may be greater or less than the melt temperature of the polymer composition (Tp) but is generally more than (Tp-55) and preferably more than (Tp-30). Tp will normally be substantially above the melting point of the comE~osition, for example 30 to 80C above. Of course, neither the electrode nor the cc ~ osition should be heated to a temperature at which it undergoes substantial oxidation or other degradation.
In another embodiment of the invention, the composition is shaped in contact with the electrode (without pre-heating the electrode) and the electrode and the oom~osition are then heated, while in contact with each other, to a temperature above the melting point of the comFosition. Care is needed to ensure a useful reduction in the contact resistance by this method. The optimum conditions will depend uFon the electrode and the composition, but increased time, temperature and pressure help to achieve the desired result. The - pressure may be applied for example in a press or by means of nip rollers. This embcdiment of the invention is particularly useful when the need for, or desirability of, an annealing treatment does not arise at all, for example, when the comosition has a carbon black content greater than 15~ by weight, e.g. greater than 17% or 20~ by weight, or when only a limited annealing treatment is carried out, such that at the end of the annealing the content of carbon black (L) and the resistivity ~R) are such that 2L + 5 logl0 > 45.
One way of heating the electrode and the comFosition surrounding it is to pass a high current through the electrode and thus produce the desired heat by resistance heating of the electrode.
In another embodiment of the invention, the conductive polymer co~position is initially in the form of a shaped article, e~g.
one or more pills or pelle~s, which has not been shaped in contact with the electrode, and the electrode and the comFosition are heated together under pressure, for example in a compression mould.
Particularly when the conductive polymer composition exhibits PTC behavior, it is often desirable that in the final product the composition should be cross-linked. Cross-linking can be carried out as a separate step after the treat~ent to reduce contact resistance; in this case, cross-linking with aid of radiation is preferred.
Alternatively cross-linking can be carried out simultaneously with the said treatment, 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 lS of which are comparative Examples.
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 115C. The com~osition was melt-extruded at a melt temperature of about 180C through a cross-head die having a circular orifice 0.36 cm in diameter over a pair of stranded silver-coated 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 800C. The temperature of the wires entering the die was 82C in the comparative Examples 1, 4 and 6, in which the speed of the wires through the oven and the die was 21m/min, 165C in Examples 2 and 7 and 193C in Examples 3 and 5.
The extrudate was then given an insulating jacket by melt-e!xtruding around it a layer 0.051 cm thick of chlorinated polyethylene or an ethylene/tetrafluoroethylene copolymer. The coated extrudate was then irradiated in order to cross-link the conductive polymer ccmposition.
These Examples, in which Example 1 is a cc~parative Example, demonstrate the 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 c~cle, being raised ~rom -37C to 65C c~ver a period of 90 seconds and then reduced to -37C again over the next 90 seconds.
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 ccmposition used in Examples 2 and 3 contained about 22% carbon black.
.
li~)6890 The results obtained are shown in Table 1 below.
~BLE 1 l~o. of Cycles *Example 1 Example 2 Example 3 PN LR PN PN
None 1 1.3 11.1 500 0.5 1.6 1.3 - 1 1 1100 0.3 2.1 1.2 1700 - - 1.1 1.1 *Comparative Example EX~MPLES 4-7 These Examples, which are summarised in Table 2 below, demonstrate the effect of pre-heating the electrodes on the Linearity Ratio and Pull Strength of the product.
I~B~E 2 Example No. % Carbon Black Linearity Ratio -; *4 22 1.6 22 1.0 *6 23 1.35 ~i.
` 7 23 1.1 20*Comparative Example , .
The ratio of the pull strengths of the heater strips of Examples 7 and 6 (P/PO) was 1.45.
Claims (20)
1. 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 temperature (Te) 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.
(2) heating an electrode, out of contact with the conductive polymer composition, to a temperature (Te) 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.
2. A process according to Claim 1 wherein the conductive polymer composition exhibits PTC behaviour.
3. A process according to Claim 1 or Claim 2 wherein the polymeric composition is thermoplastic.
4. A process according to Claim 1 wherein Te is at least (Tp-55)°C.
5. A process according to Claim 1 wherein both Tp and Te are at least 20°C above Tm.
6. A process according to Claim 5 wherein both Tp and Te are at least 55°C above Tm.
7. A process according to Claim 1 wherein both Tp and Te are above the Ring-and-Ball softening point of the conductive polymer composition.
8. A process according to Claim 1 wherein the conductive polymer composition is melt-extruded over at least two spaced-apart electrodes.
9. A process according to Claim 8 wherein the conductive polymer composition is extruded over a pair of stranded wire electrodes.
10. A process according to Claim 9 wherein the electrodes are separated by a distance of 0.15 to 1 cm.
11. A process according to any one of Claims 8 to 10 wherein the electrodes are silver-coated copper wires or nickel-coated copper wires.
12. A process according to any one of Claims 8 to 10 wherein the conductive polymer composition is extruded as an extrudate having a cross-section in which the largest dimension is not more than 3 times the smallest dimension.
13. A process according to Claim 1 wherein the conductive polymer composition exhibits PTC behaviour and has a resistivity at 21°C of 100 to 50,000 ohm. cm.
14. A process according to Claim 1 wherein the conductive polymer composition contains at least 15% by weight of carbon black.
15. A process according to Claim 1 wherein the conductive polymer composition contains carbon black dispersed in a crystalline polymer and exhibits PTC behaviour.
16. A process according to Claim 1 which also comprises the further step of cross-linking the conductive composition.
17. A process according to Claim 16 wherein the cross-linking is effected by irradiation.
18. A strip heater comprising (1) an elongate core of a conductive polymer composition which exhibits PTC behaviour, which comprises carbon black 3 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 2L + 5 log10 R . 45;
(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 between any pair of electrodes being at most 1.2.
(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 between any pair of electrodes being at most 1.2.
19. A strip heater according to Claim 18 wherein the average linearity ratio between any pair of electrodes is at most 1.10.
20. A strip heater according to Claim 18 or 19 wherein the electrodes are silver-coated wires or nickel-coated copper wires.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US75014976A | 1976-12-13 | 1976-12-13 | |
| US750,149 | 1976-12-13 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1106890A true CA1106890A (en) | 1981-08-11 |
Family
ID=25016715
Family Applications (2)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA292,832A Expired CA1106890A (en) | 1976-12-13 | 1977-12-12 | Electrical devices comprising conductive polymer compositions |
| CA000488467A Expired CA1206507B (en) | 1976-12-13 | 1985-08-09 | Electrical devices comprising conductive polymer compositions |
Family Applications After (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000488467A Expired CA1206507B (en) | 1976-12-13 | 1985-08-09 | Electrical devices comprising conductive polymer compositions |
Country Status (10)
| Country | Link |
|---|---|
| JP (4) | JPS6057192B2 (en) |
| AU (1) | AU515034B2 (en) |
| BE (1) | BE861776A (en) |
| CA (2) | CA1106890A (en) |
| DE (1) | DE2755077A1 (en) |
| FR (1) | FR2392572A1 (en) |
| GB (2) | GB1600256A (en) |
| NL (1) | NL185545C (en) |
| NO (1) | NO147735C (en) |
| SE (3) | SE434587B (en) |
Families Citing this family (12)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US4238812A (en) * | 1978-12-01 | 1980-12-09 | Raychem Corporation | Circuit protection devices comprising PTC elements |
| NZ193661A (en) * | 1979-05-10 | 1983-06-17 | Sunbeam Corp | Heating element conductive and ptc material |
| CA1156300A (en) * | 1980-04-01 | 1983-11-01 | Gordon S. Carlson | Electric blanket safety circuit |
| US4591700A (en) * | 1980-05-19 | 1986-05-27 | Raychem Corporation | PTC compositions |
| CA1168433A (en) * | 1980-05-19 | 1984-06-05 | Umesh K. Sopory | Ptc conductive polymers and devices comprising them |
| US4309596A (en) * | 1980-06-24 | 1982-01-05 | Sunbeam Corporation | Flexible self-limiting heating cable |
| ATE77155T1 (en) | 1983-06-30 | 1992-06-15 | Raychem Corp | METHOD OF DETECTING AND OBTAINING INFORMATION ABOUT THE CHANGES OF VARIABLES. |
| GB8623082D0 (en) * | 1986-09-25 | 1986-10-29 | Raychem Gmbh | Heated conduit |
| DE4024268A1 (en) * | 1990-07-31 | 1992-02-06 | Lehmann & Voss & Co | Electroconductive plastics element for heater or electronic device - contains synergistic mixt. of carbon or graphite powder and fibres and opt. metal fibres |
| DE4307371A1 (en) * | 1993-03-09 | 1994-09-15 | Hit Hillesheim Innovations Und | Heatable line for a flow medium |
| DE4426188A1 (en) * | 1994-07-23 | 1996-01-25 | Mekra Rangau Plastics | Outside mirrors for motor vehicles |
| CN113635534A (en) * | 2021-08-10 | 2021-11-12 | 芜湖佳宏新材料股份有限公司 | Process method for reducing contact resistance of conductive polymer and metal conductor |
Family Cites Families (7)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| GB828334A (en) * | 1956-10-30 | 1960-02-17 | British Insulated Callenders | Improvements in or relating to electrically conductive non-metallic materials |
| US3861029A (en) * | 1972-09-08 | 1975-01-21 | Raychem Corp | Method of making heater cable |
| US3858144A (en) | 1972-12-29 | 1974-12-31 | Raychem Corp | Voltage stress-resistant conductive articles |
| JPS5432173B2 (en) * | 1974-03-29 | 1979-10-12 | ||
| JPS5530669B2 (en) * | 1974-03-29 | 1980-08-12 | ||
| US4177376A (en) * | 1974-09-27 | 1979-12-04 | Raychem Corporation | Layered self-regulating heating article |
| CA1100561A (en) * | 1975-12-08 | 1981-05-05 | Stephen H. Diaz | Apertured deformable laminar heating elements |
-
1977
- 1977-12-07 GB GB50916/77A patent/GB1600256A/en not_active Expired
- 1977-12-07 GB GB26479/80A patent/GB1600257A/en not_active Expired
- 1977-12-09 FR FR7737164A patent/FR2392572A1/en active Granted
- 1977-12-09 AU AU31394/77A patent/AU515034B2/en not_active Expired
- 1977-12-10 DE DE19772755077 patent/DE2755077A1/en active Granted
- 1977-12-12 BE BE183389A patent/BE861776A/en not_active IP Right Cessation
- 1977-12-12 NO NO774258A patent/NO147735C/en unknown
- 1977-12-12 CA CA292,832A patent/CA1106890A/en not_active Expired
- 1977-12-13 SE SE7714126A patent/SE434587B/en not_active IP Right Cessation
- 1977-12-13 NL NL7713800A patent/NL185545C/en not_active IP Right Cessation
- 1977-12-13 JP JP52149792A patent/JPS6057192B2/en not_active Expired
-
1983
- 1983-07-19 SE SE8304042A patent/SE447781B/en not_active IP Right Cessation
-
1985
- 1985-08-09 CA CA000488467A patent/CA1206507B/en not_active Expired
- 1985-10-28 SE SE8505088A patent/SE8505088D0/en not_active Application Discontinuation
-
1989
- 1989-05-22 JP JP1128616A patent/JPH0256886A/en active Granted
- 1989-05-22 JP JP1128617A patent/JPH0256887A/en active Granted
-
1990
- 1990-10-23 JP JP2286933A patent/JPH03257783A/en active Granted
Also Published As
| Publication number | Publication date |
|---|---|
| SE8304042D0 (en) | 1983-07-19 |
| JPH03257783A (en) | 1991-11-18 |
| JPS6057192B2 (en) | 1985-12-13 |
| NO774258L (en) | 1978-06-14 |
| GB1600257A (en) | 1981-10-14 |
| JPH0562439B2 (en) | 1993-09-08 |
| JPH0256886A (en) | 1990-02-26 |
| AU515034B2 (en) | 1981-03-12 |
| FR2392572A1 (en) | 1978-12-22 |
| SE8304042L (en) | 1983-07-19 |
| NL7713800A (en) | 1978-06-15 |
| BE861776A (en) | 1978-06-12 |
| AU3139477A (en) | 1979-06-14 |
| NO147735C (en) | 1983-06-01 |
| JPH0256887A (en) | 1990-02-26 |
| NL185545C (en) | 1995-01-16 |
| SE434587B (en) | 1984-07-30 |
| GB1600256A (en) | 1981-10-14 |
| SE8505088L (en) | 1985-10-28 |
| DE2755077A1 (en) | 1978-06-29 |
| JPS5395298A (en) | 1978-08-21 |
| DE2755077C2 (en) | 1987-06-11 |
| SE7714126L (en) | 1978-06-14 |
| FR2392572B1 (en) | 1984-03-30 |
| JPH053120B2 (en) | 1993-01-14 |
| NL185545B (en) | 1989-12-01 |
| CA1206507B (en) | 1986-06-24 |
| JPH0559557B2 (en) | 1993-08-31 |
| NO147735B (en) | 1983-02-21 |
| SE8505088D0 (en) | 1985-10-28 |
| SE447781B (en) | 1986-12-08 |
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