EP0490989B1 - Conductive polymer device - Google Patents

Abstract

A melt-extrudable conductive polymer composition which contains a polymer, a particulate conductive filler and a particulate non conductive filler. When a standard strip heater is made from the composition and tested in a UL VW-1 test, it has comparable performance to a heater made from a second composition which is the same as the composition but which does not contain the non conductive filler. When tested in a standard arcing fault test, the standard heater will trip a fuse in less time than is required by the second heater, i.e. in less than 30 seconds. A preferred nonconductive filler is Sb2O3.

Description

BACKGROUNDOFTHEINVENTIONFieldoftheInvention
• This invention relates to conductive polymer compositionsand strip heaters comprising them, in particular self-regulatingstrip heaters which comprise a pair of elongate metal electrodesembedded in an elongate core of a conductive polymer compositionwhich exhibits PTC behavior.
IntroductiontotheInvention
• Conductive polymer compositions and electrical devicescomprising such compositions are well known. A conductivepolymer composition comprises a polymeric component and,dispersed or otherwise distributed therein, a particulateconductive filler. Strip heaters, particularly self-regulatingstrip heaters comprising conductive polymers, are also well-known.In this application, the term strip heater is used tomean a conductive polymer resistive element into which elongateelectrodes are embedded. In operation, such strip heatersprovide a varying level of heat in response to changes in thethermal environment. Under normal operating conditions, thisself-regulating feature serves to limit the maximum temperaturewhich the heater achieves, thus providing reliability andsafety. However, under certain circumstances where the busbarsare exposed by external damage or by faulty installation, andwhen the heater is electrically powered and exposed to anelectrolyte, an arc can occur between the electrodes. Unlessthe arc is interrupted, the conductive polymer may burn andcould possibly result. One way to minimize this danger is to develop appropriate conductive polymer compositions inwhich the polymer itself is flame-retardant or whichcontain conventional flame retardant additives to work inconjunction with the strip heaters. Another method tominimize risks from arcing faults is to use fuses or othercircuit protection devices, e.g. arc fault interruptors orground fault detectors, as part of the strip heater circuitin order to remove power from the circuit if an arc shouldoccur.
• GB 2 036 754 discloses conductive polymer compositionscomprising a polymer, for example, polyethylene, aparticulate filler, for example, carbon black, andoptionally further fillers which may be non-conductivefillers, for example, antimony trioxide.
• US 4,591,700 discloses conductive polymer compositionsfor use in self-limited strip heaters. The compositionscomprise a mixture of two crystalline polymers of differentmelting points, the higher of which is at least 160°C, anda particulate filler, for example, carbon black. Thecompositions have good thermal stability and do notincrease in resistivity by a factor of more than 2 whenmaintained at 150°C for 1000 hours.
SUMMARY OF THE INVENTION
• I have now found that the presence of a non-conductivefiller in the conductive polymer composition in a stripheater can reduce the trip time of a fuse which forms partof a strip heater circuit, and thus reduce the danger thatthe heater will burn and cause damage. In a first aspect,this invention discloses a strip heater which comprises
• (A) a resistive element which is composed of afirst conductive polymer composition whichis melt-extrudable and which comprises:
• (a) a polymeric component which is apolyethylene polymer matrix, having distributed therein
• (b) a particulate conductive filler whichis carbon black,
• (c) 4.0 to 8.0% by weight of a particulateconductive filler which isantyimonytrioxide, and, optionally,
• (d) decabromodiphenyloxide;
and
• (B) two electrodes which can be connected to asource of electrical power to cause currentto flow through the resistive element
  and which
• (1) when tested following the procedure ofUL test VW-1 either (a) does not passthe test or (b) has a performance whichis similar to that of a second heaterwhich is made from a second conductivepolymer composition which is the sameas the first composition except that itdoes not comprise the particulate non-conductivefiller, and
• (2) when tested in a standard arcing faulttest (a) the time it requires to trip afuse is less than is required by thesecond heater, and (b) trips the fusein less than 30 seconds.
• In a second aspect the invention discloses a stripheater assembly which comprises
• (A) A strip heater which comprises
• (1) A resistive element which is composed of afirst conductive polymer composition whichis melt-extrudable and which comprises:
• (a) a polymeric component which is apolyethylene polymer matrix, having distributed therein
• (b) a particulate conductive filler whichis carbon black,
• (c) 4.0 to 8.0% by weight of a particulatenon-conductive filler which is Sb₂O₃,and, optionally,
• (d) decabromodiphenyloxide;
and
• (2) two electrodes which can be connected to asource of electrical power to cause currentto flow through the resistive element,
and
• (B) a fuse,
the particulate non-conductive filler, Sb₂O₃, being suchthat when the composition is made into a standard stripheater and the standard heater is tested in a standardarcing fault test it trips the fuse in less than 30seconds.
• In a third aspect the invention discloses a stripheater circuit which comprises
• (A) a strip heater which comprises
• (1) a resistive element which is composed of aconductive polymer composition which is mel-extrudableand which comprises:
• (a) a polymeric component which is apolyethylene polymer matrix, having distributed therein
• (b) a particulate conductive filler whichis carbon black, and
• (c) 4.0 to 8.0% by weight of a particulatenon-conductive filler which is antimonytrioxide, and, optionally,
• (d) decabromodiphenyloxide;
and
• (2) two electrodes which can be connected to asource of electrical power to cause currentto flow through the resistive element,
• (B) a fuse, and
• (C) a power supply,
the particulate non-conductive filler, Sb₂O₃, being suchthat when the composition is made into a standard stripheater and the standard heater is tested in a standardarcing fault test it trips the fuse in less than 30seconds.
BRIEF DESCRIPTION OF THE DRAWINGS
• Figure 1 is a cross-sectional view of a standard stripheater of the invention;
• Figure 2 is a top view of a strip heater of theinvention; and
• Figure 3 is a cross-sectional view of a strip heateralong line 3-3 of Figure 2.
DETAILED DESCRIPTION OF THE INVENTION
• The first conductive polymer composition used in thisinvention comprises a polymeric component which has apolyethylene polymer matrix. The polymeric component maycomprise a blend of polyethylene with one or more further,crystalline organic polymers. Suitable crystallinepolymers for use in such blends include polymers of one ormore olefins, copolymers of at least one olefin and atleast one monomer copolymerisable therewith such asethylene/acrylic acid, ethylene/ethyl acrylate, andethylene/vinyl acetate copolymers; and melt-shapeablefluoropolymers such as polyvinylidene fluoride and ethylenetetrafluoroethylene. Suitable polymers and compositionscomprising them may be found in US Patent Nos. 4,188,276,4,237,441, 4,388,607, 4,470,898, 4,514,620, 4,534,889,4,560,498, 4,591,700, 4,624,990, 4,658,121, 4,774,024,and 4,775,778; and European Patent PublicationNos. 38,713, 38,718, 74,281, 197,759 and 231,068.
• Crystalline polymers are particularly preferred,although not required, when it is desired that thecomposition exhibit PTC (positive temperature coefficient)behaviour. The term "PTC behaviour" is used in thisspecification to denote a composition or an electricaldevice which has an R₁₄ value of at least 2.5 or an R₁₀₀value of at least 10, and preferably both, and particularlyone which has an R₃₀ value of at least 6, where R₁₄ is theratio of the resistivities at the end and the beginning ofa 14°C range, R₁₀₀ is the ratio of the resistivities at theend and the beginning of a 100°C range, and R₃₀ is theratio of the resistivities at the end and the beginning ofa 30°C range.
• The composition also comprises a particulateconductive filler, carbon black, which is dispersed orotherwise distributed in the polymer. The particulateconductive filler is present in the composition in anamount suitable for achieving the desired resistivity,normally 5 to 50% by weight of the composition, preferably10 to 40% by weight, particularly 15 to 30% by weight.
• The particulate non-conductive filler comprises amaterial which is electrically insulating, i.e. has aresistivity of greater than 1 x 10⁹ ohm-cm. The non-conductivefiller should have a melting temperature of lessthan 1000°C. As already mentioned, the compositioncomprises 4.0 to 8.0% by weight of Sb₂O₃ as filler, thatmaterial being easily reduced. In this application, easilyreduced means that the material has a reduction potentialof less than +0.5 volts, preferably less than +0.4 volts,particularly less than +0.375 volts. For ease ofdispersion in the polymer matrix, the filler is preferablyin the form of particles which have a particle size of 0.01to 50 µm, particularly 0.05 to 50 µm, especially 0.10 to 10µm. The non-conductive filler may optionally furthercomprise decabromodiphenyl-oxide. Although a blend of Sb₂O₃ and decabromodiphenyloxide (also known asdecabromodiphenylether), DBDPO, is commonly used as a flameretardant package in polymers, the presence of the DBDPO orany other halogenated material is not necessary forsatisfactory performance in the compositions.
• The conductive polymer composition may also compriseinert fillers, antioxidants, prorads, stabilizers,dispersing agents, or other components. Mixing ispreferably effected by melt-processing, e.g. melt-extrusion.Subsequent processing steps may includeextrusion, molding, or another procedure in order to formand shape the composition. The composition may becrosslinked by irradiation or chemical means.
• The conductive polymer composition may be used, further to the strip of the invention, in anycurrent carrying electrical device, e.g. a circuitprotection device, a sensor, or, most commonly, another heater.The heater may be in the form of alaminar sheet in which the resistive element comprises thecomposition. Strip heaters of the invention may be of anycross-section, e.g. rectangular, elliptical, or dumbell("dogbone"). Appropriate electrodes, suitable forconnection to a source of electrical power, are selecteddepending on the shape of the electrical device.Electrodes may comprise metal wires or braid, e.g. forattachment to or embedment into the conductive polymer, orthey may comprise metal sheet, metal mesh, conductive (e.g. metal- or carbon-filled)paint, or other suitable materials.
• In order to provide environmental protection andelectrical insulation, it is common for the resistiveelement to be covered by a dielectric layer, e.g. apolymeric jacket (for strip heaters) or an epoxy layer (forcircuit protection devices). The dielectric layer maycomprise flame retardants or other fillers. For some stripheater applications, a metallic grounding braid is presentover the dielectric layer in order to provide physicalreinforcement and a means of electrically grounding thestrip heater.
• The compositions are particularlyuseful when, in the form of strip heaters, they are usedin conjunction with a fuse and act to "trip" the fuse fasterthan strip heaters comprising conventional materials. Afuse "trips" when the current in the circuit comprising thefuse exceeds the rated value of the fuse. Fuses arecategorized based on their overload fusing characteristics,i.e. the relationship between the value of current throughthe fuse and the time for the fuse to open as described inBulletin SFB, "Buss Small Dimension Fuses", May 1985.Ofthe major categories (slow blowing, non-delay, and very fastacting), it is very fast acting fuses which are most usefulin this invention. These fuses have little, if any, intentionaldelay in the overload region. Although the selectionof a specific fuse is dependent on the normal operating conditionsof the strip heater and the anticipated fault conditions,fuses which are particularly preferred are veryfast-acting ceramic ferrule fuses with a current rating of 10 amperes and a voltage rating of 125/250 volts. Suchfuses are available, for example, from the Bussman Divisionof Cooper Industries under the name Buss GBB™-10. The fusemay be an independent component in the circuit or it may bein a fused plug assembly, i.e. an assembly in which the fuseis part of the plug which connects the strip heater to thepower source, e.g. an outlet or a power supply.
• Strip heaters of the invention are commonly used in astrip heater assembly which comprises the strip heater and afuse. Alternatively, the strip heater is a component of astrip heater circuit which comprises the strip heater, apower supply, and a fuse. The power supply can be anysuitable source of power including portable power suppliesand mains power sources. Other components, such asresistors, thermostats, and indicating lights, may also bepresent in the circuit.
• In this specification, a "standard strip heater" isdefined for testing purposes. A "standard strip heater" isone in which a conductive polymer composition is melt-extrudedaround two 22 AWG stranded nickel/copper wires toproduce a strip heater of flat, elliptical shape as shown inFigure 1. The heater has an electrode spacing of 0.10 inch(0.25 cm) from the center of one electrode to the center ofthe second electrode. The thickness of the heater at apoint centered between the electrodes is 0.08 inch (0.20cm). The heater is jacketed with a composition whichcontains 31.9% by weight of a standard flame retardantpackage as described in Example 1. The jacket is 0.030 inch(0.076 cm) thick.
• The standard strip heater is tested by means of a"standard arcing fault test". In this test (which is more fully described in Example 1), a standard strip heater isconnected in a circuit to a power supply and a 10A, 125/250Vfuse. An arc is initiated between two exposed electrodes ofthe heater and the time to interrupt the current andextinguish the arc by means of tripping the fuse isrecorded. I have found that a standard strip heater whichcomprises the composition of the invention (i.e. a firstconductive polymer composition) trips the fuse faster than asecond strip heater which comprises a second conductivepolymer composition, i.e. a composition which is the same asthe first composition except that it does not comprise thenonconductive particulate filler. The time to trip a fusefor the standard heater generally will be at least two timesas fast, preferably at least three times as fast, particularlyat least five times as fast, e.g. five to eighttimes as fast as the second heater. Thus the standardheater will trip the fuse in at most half the time requiredto trip the fuse in a circuit which comprises a secondheater. When tested in the standard arcing fault test, astandard strip heater of the invention normally will tripthe fuse in less than 30 seconds, preferably in less than 25seconds, particularly in less than 20 seconds, e.g. in 5 to10 seconds. An additional aspect of the invention is thatthe addition of the nonconductive particulate filler resultsin an increase in the number of current spikes observedduring the arcing fault test. Even if the amplitude of thespikes is similar for both types of heaters, there generallywill be at least 2 times, preferably at least 3 times, particularlyat least 4 times as many current spikes in a givenperiod, e.g. 30 seconds, for the heater comprising the firstcomposition.
• A second test which is conducted on heaters comprisingthe first composition of the invention is the UL VW-1 vertical-wire flame test (Reference Standard for ElectricalWires, Cables, and Flexible Cords, UL 1581, No. 1080, August 15,1983). In this test, a heater sample is held in a verticalposition while a flame is applied. In order to pass the test,the sample cannot "flame" longer than 60 seconds following anyof five 15-second applications of the test flame. The periodbetween sequential applications of the test flame is either 15seconds (if the sample ceases flaming within 15 seconds) or theduration of the sample flaming time if the flaming lasts longerthan 15 seconds. In addition, combustible materials in thevicinity of the sample cannot be ignited by the sample duringthe test. In this specification, when the performance in thistest of the heater of the invention is said-to be "similar" tothat of a second heater which comprises a second conductivepolymer composition, it means that if ten different samples ofthe standard heater are tested, eight of them (i.e. 80%) musthave the same result (i.e. pass or fail) as ten samples of thesecond composition.
• The invention is illustrated by the drawing in which Figure1 shows a cross-section of a standard strip heater 1.Electrodes 5, 7 are embedded in the first conductive polymercomposition 3 (the resistive element). Apolymeric jacket 9surrounds the heater core. Figure 2 shows a top view of stripheater 1 which has been prepared for the arcing fault testdescribed below. A V-shaped notch 11 is cut through thepolymeric jacket 9 and theconductive polymer composition 3 onone surface of the heater in order to exposeelectrodes 5 and 7.The cross-sectional view of the heater along line 3-3 is shownin Figure 3.Electrodes 5, 7 remain partially embedded in theconductive polymer 3.
• The invention is illustrated by the following examples.
Example 1 (Comparative Example)
• The ingredients listed in Table I were preblended andthen mixed in a co-rotating twin-screw extruder to formpellets. The pelletized composition was extruded through a1.5 inch (3.8 cm) extruder around two 22 AWG strandednickel/copper wires to produce a strip heater. The heaterhad an electrode spacing of 0.106 inch (0.269 cm) fromcenter-to-center and a thickness of 0.083 inch (0.211 cm) ata center point between the wires. The heater was jacketedwith a 0.030 inch (0.076 cm) layer of a compositioncontaining 10% by weight ethylene/vinyl acetate copolymer(EVA), 36.8% medium density polyethylene, 10.3% ethylene/propylenerubber, 23.4% decabromodiphenyloxide, 8.5%antimony oxide, 9.4% talc, 1.0% magnesium oxide, and 0.7%antioxidant.
• The heater was tested using the standard arcing faulttest described below. The results are shown in Table II.In a related test, the amplitude and frequency of thecurrent spikes produced when a heater was tested followingthe procedure of the arcing fault test but without the useof a fuse were recorded. In this modified arcing faulttest, the samples were allowed to burn for three minutesafter a flame was initiated. The results are shown inTable III.
• The heater was tested following the procedures of theUL VW-1 vertical-wire flame test (Reference Standard forElectrical Wires, Cables, and Flexible Cords, UL 1581, No.1080, August 15, 1983). Of the 10 samples tested, fivepassed the test. These results are shown in Table IV.
Standard Arcing Fault Test
• A standard, jacketed 25 inch- (64 cm-) long strip heaterwas prepared by stripping one inch (2.5 cm) of jacket and corematerial from a first end to expose the two electrodes. Atransverse v-shaped notch was cut half-way through the thicknessof the heater 2 inches (5.1 cm) from the second end and thejacket and core polymer were removed from the top half in orderto expose part of each of the two electrodes. The electrodes atthe first end were connected in a circuit in series with a120V/100A power supply, a contactor relay, a 0.1 ohm/100 wattshunt resistor, and a 10A, 125/250V very fast acting fuse (BussGBB™-10, available from the Bussman Division of CooperIndustries). A chart recorder was connected across the shuntresistor in order to measure the voltage drop. When the relaywas closed, the sample was powered. A sufficient quantity of 10to 20% saline solution was applied to the exposed v-notch untilan arcing fault was initiated. The chart recorder was monitoreduntil the current was interrupted and the arc was extinguished(i.e. until the fuse tripped). Both the time duration of thearc, as determined from the current spikes on the chart, and thedistance of arc fault propagation on the strip heater weremeasured. In some instances, the number of current spikespresent during the arcing fault was also determined.
Examples 2 to 4
• For each example, pellets of the composition of Example 1were preblended with the inorganic materials in the proportionsshown in Table I. After mixing in a co-rotating twin screwextruder and pelletizing, the compositions were extruded to form strip heaters with thesame geometry as that of Example 1 and were jacketed as inExample 1. The results of the arcing fault test and thevertical flame test are shown in Tables II and IV. It isapparent that those compositions which contain Sb₂O₃ havesignificantly faster trip times in the arc fault test thancomparable materials which do not contain the filler.
• A strip heater formed from the composition of Example 2was also tested following the modified arcing fault testdescribed in Example 1. As shown in the results in TableIII, the amplitude of the current spikes and the burn ratewere comparable for both the conventional composition(Example 1) and the composition of the invention (Example2). The major difference occurred in the frequency of thecurrent spikes; the spikes were much more prevalent for thecomposition of the invention than for the conventionalmaterial.

  CONDUCTIVE POLYMER FORMULATIONS (Components in Percent by Weight)
  Component	1	2	3	4
  EEA	51.7	43.4	49.6	47.5
  CB	30.3	25.5	29.1	27.9
  MDPE	17.2	14.4	16.5	15.8
  HDPE
  Antioxidant	0.8	0.7	0.8	0.8
  Sb2O3		4.3	4.0	8.0
  DBDPO		11.7
  Notes to Table I: EEA is ethylene/ethyl acrylate copolymer. CB is carbon black with a particle size of 28 nm. MDPE is medium density polyethylene.

• Antioxidant is an oligomer of 4,4-thio bis(3-methyl 1-6-t-butyl phenol)with an average degree of polymerisation of 3 to 4, as described in U.S.Patent No. 3,986,981.
  Sb₂O₃ is antimony trioxide with a particle size of 1.0 to 1.8 µm.
• DBDPO is decabromodiphenyl oxide (also known as decabromodiphenylether).

  ARCING FAULT TEST RESULTS
  Example	Circuit Length (feet)	Fuse Response (seconds)	Burn Length (inches)	Burn Rate (in/min)
  1	2	97	2.1	1.30
  	100	180	4.3	1.43
  2	2	6.9	0	--
  	50	8.4	0	--
  	100	19	0.3	0.94
  3	2	9	0	--
  4	2	6	0	--

  MODIFIED ARCING FAULT TEST RESULTS
  Example	Circuit Length (feet)	Amplitude of Current Spikes (amps)	Frequency of Current Spikes (#/0.5 min)	Burn Rate (in/min)
  1	2	31 - 71	8	2.06
  	50	8 - 41	16	2.08
  	100	5 - 21	34	2.52
  2	2	27 - 100	28	1.83
  	50	6 - 40	63	2.00
  	100	4 - 30	88	2.34

  VERTICAL WIRE FLAME TEST (UL VW-1)
  Example	% Pass
  1	50%
  2	100
  3	100
  4	100

Claims (6)

1. A strip heater which comprises

   (A) a resistive element which is composed of a firstconductive polymer composition which is melt-extrudableand which comprises:

   (i) a polymeric component which is apolyethylene polymer matrix, having distributed therein

   (ii) a particulate conductive filler which iscarbon black,

   (iii) 4.0 to 8.0% by weight of a particulatenon-conductive filler which is antimonytrioxide, and, optionally,

   (iv) decabromodiphenyloxide;

   and

   (B) two electrodes which can be connected to a sourceof electrical power to cause current to flowthrough the resistive element,

   and which

   (1) when tested following the procedure of UL testVW-1 either (a) does not pass the test or (b) hasa performance which is similar to that of asecond heater which is made from a secondconductive polymer composition which is the sameas the first composition except that it does notcomprise the particulate non-conductive filler,and

   (2) when tested in a standard arcing fault test (a)the time it requires to trip a fuse is less thanis required by the second heater, and (b) tripsthe fuse in less than 30 seconds.
2. A heater according to claim 1, wherein the compositionexhibits PTC behaviour.
3. A strip heater assembly which comprises

   (A) a strip heater according to claim 1 or 2, and

   (B) a fuse,

   the particulate non-conductive filler, Sb₂O₃, being suchthat when the first composition is made into a standardstrip heater and the standard heater is tested in astandard arcing fault test it trips the fuse in less than30 seconds.
4. A strip heater assembly according to claim 3, whereinthe fuse is part of a fused plug assembly.
5. A strip heater assembly according to claim 3 or 4,wherein the fuse is a very fast acting fuse, preferably afuse which has a rating of 10A, 125/250 volts.
6. A strip heater circuit which comprises

   (A) a strip heater according to claim 1 or 2,

   (B) a fuse, and

   (C) a power supply,

   the particulate non-conductive filler Sb₂O₃, being suchthat when the composition is made into a standard stripheater and the standard heater is tested in a standardarcing fault test it trips the fuse in less than 30seconds.

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