EP0490989B1

EP0490989B1 - Conductive polymer device

Info

Publication number: EP0490989B1
Application number: EP90914344A
Authority: EP
Inventors: Arthur F. Emmett
Original assignee: Raychem Corp
Current assignee: Raychem Corp
Priority date: 1989-09-08
Filing date: 1990-09-10
Publication date: 1999-11-24
Anticipated expiration: 2010-09-10
Also published as: DE69033364D1; CA2066254A1; KR920704316A; WO1991003822A1; ATE187013T1; JPH05500884A; DE69033364T2; CA2066254C; EP0490989A1

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

BACKGROUND OF THE INVENTION

Field of the Invention

This invention relates to conductive polymer compositions and strip heaters comprising them, in particular self-regulating strip heaters which comprise a pair of elongate metal electrodes embedded in an elongate core of a conductive polymer composition which exhibits PTC behavior.

Introduction to the Invention

Conductive polymer compositions and electrical devices comprising such compositions are well known. A conductive polymer composition comprises a polymeric component and, dispersed or otherwise distributed therein, a particulate conductive filler. Strip heaters, particularly self-regulating strip heaters comprising conductive polymers, are also well-known. In this application, the term strip heater is used to mean a conductive polymer resistive element into which elongate electrodes are embedded. In operation, such strip heaters provide a varying level of heat in response to changes in the thermal environment. Under normal operating conditions, this self-regulating feature serves to limit the maximum temperature which the heater achieves, thus providing reliability and safety. However, under certain circumstances where the busbars are exposed by external damage or by faulty installation, and when the heater is electrically powered and exposed to an electrolyte, an arc can occur between the electrodes. Unless the arc is interrupted, the conductive polymer may burn and could possibly result. One way to minimize this danger is to develop appropriate conductive polymer compositions in which the polymer itself is flame-retardant or which contain conventional flame retardant additives to work in conjunction with the strip heaters. Another method to minimize risks from arcing faults is to use fuses or other circuit protection devices, e.g. arc fault interruptors or ground fault detectors, as part of the strip heater circuit in order to remove power from the circuit if an arc should occur.
GB 2 036 754 discloses conductive polymer compositions comprising a polymer, for example, polyethylene, a particulate filler, for example, carbon black, and optionally further fillers which may be non-conductive fillers, for example, antimony trioxide.
US 4,591,700 discloses conductive polymer compositions for use in self-limited strip heaters. The compositions comprise a mixture of two crystalline polymers of different melting points, the higher of which is at least 160°C, and a particulate filler, for example, carbon black. The compositions have good thermal stability and do not increase in resistivity by a factor of more than 2 when maintained at 150°C for 1000 hours.

SUMMARY OF THE INVENTION

I have now found that the presence of a non-conductive filler in the conductive polymer composition in a strip heater can reduce the trip time of a fuse which forms part of a strip heater circuit, and thus reduce the danger that the 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 a first conductive polymer composition which is melt-extrudable and which comprises:
(a) a polymeric component which is a polyethylene polymer matrix, having distributed therein
(b) a particulate conductive filler which is carbon black,
(c) 4.0 to 8.0% by weight of a particulate conductive filler which is antyimonytrioxide, and, optionally,
(d) decabromodiphenyloxide;
(B) two electrodes which can be connected to a source of electrical power to cause current to flow through the resistive element
and which
(1) when tested following the procedure of UL test VW-1 either (a) does not pass the test or (b) has a performance which is similar to that of a second heater which is made from a second conductive polymer composition which is the same as the first composition except that it does not comprise 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 than is required by the second heater, and (b) trips the fuse in less than 30 seconds.
In a second aspect the invention discloses a strip heater assembly which comprises
(A) A strip heater which comprises
(1) A resistive element which is composed of a first conductive polymer composition which is melt-extrudable and which comprises:
(a) a polymeric component which is a polyethylene polymer matrix, having distributed therein
(b) a particulate conductive filler which is carbon black,
(c) 4.0 to 8.0% by weight of a particulate non-conductive filler which is Sb₂O₃, and, optionally,
(d) decabromodiphenyloxide;
(2) two electrodes which can be connected to a source of electrical power to cause current to flow through the resistive element,
(B) a fuse,

₂

₃

In a third aspect the invention discloses a strip heater circuit which comprises
(A) a strip heater which comprises
(1) a resistive element which is composed of a conductive polymer composition which is mel-extrudable and which comprises:
(a) a polymeric component which is a polyethylene polymer matrix, having distributed therein
(b) a particulate conductive filler which is carbon black, and
(c) 4.0 to 8.0% by weight of a particulate non-conductive filler which is antimony trioxide, and, optionally,
(d) decabromodiphenyloxide;
(2) two electrodes which can be connected to a source of electrical power to cause current to flow through the resistive element,
(B) a fuse, and
(C) a power supply,

₂

₃

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a cross-sectional view of a standard strip heater of the invention;
Figure 2 is a top view of a strip heater of the invention; and
Figure 3 is a cross-sectional view of a strip heater along line 3-3 of Figure 2.

DETAILED DESCRIPTION OF THE INVENTION

The first conductive polymer composition used in this invention comprises a polymeric component which has a polyethylene polymer matrix. The polymeric component may comprise a blend of polyethylene with one or more further, crystalline organic polymers. Suitable crystalline polymers for use in such blends include polymers of one or more olefins, copolymers of at least one olefin and at least one monomer copolymerisable therewith such as ethylene/acrylic acid, ethylene/ethyl acrylate, and ethylene/vinyl acetate copolymers; and melt-shapeable fluoropolymers such as polyvinylidene fluoride and ethylene tetrafluoroethylene. Suitable polymers and compositions comprising 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 Publication Nos. 38,713, 38,718, 74,281, 197,759 and 231,068.
Crystalline polymers are particularly preferred, although not required, when it is desired that the composition exhibit PTC (positive temperature coefficient) behaviour. The term "PTC behaviour" is used in this specification to denote a composition or an electrical device which has an R₁₄ value of at least 2.5 or an R₁₀₀ value of at least 10, and preferably both, and particularly one which has an R₃₀ value of at least 6, where R₁₄ is the ratio of the resistivities at the end and the beginning of a 14°C range, R₁₀₀ is the ratio of the resistivities at the end and the beginning of a 100°C range, and R₃₀ is the ratio of the resistivities at the end and the beginning of a 30°C range.
The composition also comprises a particulate conductive filler, carbon black, which is dispersed or otherwise distributed in the polymer. The particulate conductive filler is present in the composition in an amount suitable for achieving the desired resistivity, normally 5 to 50% by weight of the composition, preferably 10 to 40% by weight, particularly 15 to 30% by weight.
The particulate non-conductive filler comprises a material which is electrically insulating, i.e. has a resistivity of greater than 1 x 10⁹ ohm-cm. The non-conductive filler should have a melting temperature of less than 1000°C. As already mentioned, the composition comprises 4.0 to 8.0% by weight of Sb₂O₃ as filler, that material being easily reduced. In this application, easily reduced means that the material has a reduction potential of less than +0.5 volts, preferably less than +0.4 volts, particularly less than +0.375 volts. For ease of dispersion in the polymer matrix, the filler is preferably in the form of particles which have a particle size of 0.01 to 50 µm, particularly 0.05 to 50 µm, especially 0.10 to 10 µm. The non-conductive filler may optionally further comprise decabromodiphenyl-oxide. Although a blend of Sb₂O₃ and decabromodiphenyloxide (also known as decabromodiphenylether), DBDPO, is commonly used as a flame retardant package in polymers, the presence of the DBDPO or any other halogenated material is not necessary for satisfactory performance in the compositions.
The conductive polymer composition may also comprise inert fillers, antioxidants, prorads, stabilizers, dispersing agents, or other components. Mixing is preferably effected by melt-processing, e.g. melt-extrusion. Subsequent processing steps may include extrusion, molding, or another procedure in order to form and shape the composition. The composition may be crosslinked by irradiation or chemical means.
The conductive polymer composition may be used, further to the strip of the invention, in any current carrying electrical device, e.g. a circuit protection device, a sensor, or, most commonly, another heater. The heater may be in the form of a laminar sheet in which the resistive element comprises the composition. Strip heaters of the invention may be of any cross-section, e.g. rectangular, elliptical, or dumbell ("dogbone"). Appropriate electrodes, suitable for connection to a source of electrical power, are selected depending on the shape of the electrical device. Electrodes may comprise metal wires or braid, e.g. for attachment to or embedment into the conductive polymer, or they may comprise metal sheet, metal mesh, conductive (e.g. metal- or carbon-filled) paint, or other suitable materials.
In order to provide environmental protection and electrical insulation, it is common for the resistive element to be covered by a dielectric layer, e.g. a polymeric jacket (for strip heaters) or an epoxy layer (for circuit protection devices). The dielectric layer may comprise flame retardants or other fillers. For some strip heater applications, a metallic grounding braid is present over the dielectric layer in order to provide physical reinforcement and a means of electrically grounding the strip heater.
The compositions are particularly useful when, in the form of strip heaters, they are used in conjunction with a fuse and act to "trip" the fuse faster than strip heaters comprising conventional materials. A fuse "trips" when the current in the circuit comprising the fuse exceeds the rated value of the fuse. Fuses are categorized based on their overload fusing characteristics, i.e. the relationship between the value of current through the fuse and the time for the fuse to open as described in Bulletin SFB, "Buss Small Dimension Fuses", May 1985. Of the major categories (slow blowing, non-delay, and very fast acting), it is very fast acting fuses which are most useful in this invention. These fuses have little, if any, intentional delay in the overload region. Although the selection of a specific fuse is dependent on the normal operating conditions of the strip heater and the anticipated fault conditions, fuses which are particularly preferred are very fast-acting ceramic ferrule fuses with a current rating of 10 amperes and a voltage rating of 125/250 volts. Such fuses are available, for example, from the Bussman Division of Cooper Industries under the name Buss GBB™-10. The fuse may be an independent component in the circuit or it may be in a fused plug assembly, i.e. an assembly in which the fuse is part of the plug which connects the strip heater to the power source, e.g. an outlet or a power supply.
Strip heaters of the invention are commonly used in a strip heater assembly which comprises the strip heater and a fuse. Alternatively, the strip heater is a component of a strip heater circuit which comprises the strip heater, a power supply, and a fuse. The power supply can be any suitable source of power including portable power supplies and mains power sources. Other components, such as resistors, thermostats, and indicating lights, may also be present in the circuit.
In this specification, a "standard strip heater" is defined for testing purposes. A "standard strip heater" is one in which a conductive polymer composition is melt-extruded around two 22 AWG stranded nickel/copper wires to produce a strip heater of flat, elliptical shape as shown in Figure 1. The heater has an electrode spacing of 0.10 inch (0.25 cm) from the center of one electrode to the center of the second electrode. The thickness of the heater at a point centered between the electrodes is 0.08 inch (0.20 cm). The heater is jacketed with a composition which contains 31.9% by weight of a standard flame retardant package 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 is connected in a circuit to a power supply and a 10A, 125/250V fuse. An arc is initiated between two exposed electrodes of the heater and the time to interrupt the current and extinguish the arc by means of tripping the fuse is recorded. I have found that a standard strip heater which comprises the composition of the invention (i.e. a first conductive polymer composition) trips the fuse faster than a second strip heater which comprises a second conductive polymer composition, i.e. a composition which is the same as the first composition except that it does not comprise the nonconductive particulate filler. The time to trip a fuse for the standard heater generally will be at least two times as fast, preferably at least three times as fast, particularly at least five times as fast, e.g. five to eight times as fast as the second heater. Thus the standard heater will trip the fuse in at most half the time required to trip the fuse in a circuit which comprises a second heater. When tested in the standard arcing fault test, a standard strip heater of the invention normally will trip the fuse in less than 30 seconds, preferably in less than 25 seconds, particularly in less than 20 seconds, e.g. in 5 to 10 seconds. An additional aspect of the invention is that the addition of the nonconductive particulate filler results in an increase in the number of current spikes observed during the arcing fault test. Even if the amplitude of the spikes is similar for both types of heaters, there generally will be at least 2 times, preferably at least 3 times, particularly at least 4 times as many current spikes in a given period, e.g. 30 seconds, for the heater comprising the first composition.
A second test which is conducted on heaters comprising the first composition of the invention is the UL VW-1 vertical-wire flame test (Reference Standard for Electrical Wires, Cables, and Flexible Cords, UL 1581, No. 1080, August 15, 1983). In this test, a heater sample is held in a vertical position while a flame is applied. In order to pass the test, the sample cannot "flame" longer than 60 seconds following any of five 15-second applications of the test flame. The period between sequential applications of the test flame is either 15 seconds (if the sample ceases flaming within 15 seconds) or the duration of the sample flaming time if the flaming lasts longer than 15 seconds. In addition, combustible materials in the vicinity of the sample cannot be ignited by the sample during the test. In this specification, when the performance in this test of the heater of the invention is said-to be "similar" to that of a second heater which comprises a second conductive polymer composition, it means that if ten different samples of the standard heater are tested, eight of them (i.e. 80%) must have the same result (i.e. pass or fail) as ten samples of the second composition.
The invention is illustrated by the drawing in which Figure 1 shows a cross-section of a standard strip heater 1. Electrodes 5, 7 are embedded in the first conductive polymer composition 3 (the resistive element). A polymeric jacket 9 surrounds the heater core. Figure 2 shows a top view of strip heater 1 which has been prepared for the arcing fault test described below. A V-shaped notch 11 is cut through the polymeric jacket 9 and the conductive polymer composition 3 on one surface of the heater in order to expose electrodes 5 and 7. The cross-sectional view of the heater along line 3-3 is shown in Figure 3. Electrodes 5, 7 remain partially embedded in the conductive polymer 3.
The invention is illustrated by the following examples.

Example 1 (Comparative Example)

The ingredients listed in Table I were preblended and then mixed in a co-rotating twin-screw extruder to form pellets. The pelletized composition was extruded through a 1.5 inch (3.8 cm) extruder around two 22 AWG stranded nickel/copper wires to produce a strip heater. The heater had an electrode spacing of 0.106 inch (0.269 cm) from center-to-center and a thickness of 0.083 inch (0.211 cm) at a center point between the wires. The heater was jacketed with a 0.030 inch (0.076 cm) layer of a composition containing 10% by weight ethylene/vinyl acetate copolymer (EVA), 36.8% medium density polyethylene, 10.3% ethylene/propylene rubber, 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 fault test described below. The results are shown in Table II. In a related test, the amplitude and frequency of the current spikes produced when a heater was tested following the procedure of the arcing fault test but without the use of a fuse were recorded. In this modified arcing fault test, the samples were allowed to burn for three minutes after a flame was initiated. The results are shown in Table III.
The heater was tested following the procedures of the UL VW-1 vertical-wire flame test (Reference Standard for Electrical Wires, Cables, and Flexible Cords, UL 1581, No. 1080, August 15, 1983). Of the 10 samples tested, five passed the test. These results are shown in Table IV.

Standard Arcing Fault Test

A standard, jacketed 25 inch- (64 cm-) long strip heater was prepared by stripping one inch (2.5 cm) of jacket and core material from a first end to expose the two electrodes. A transverse v-shaped notch was cut half-way through the thickness of the heater 2 inches (5.1 cm) from the second end and the jacket and core polymer were removed from the top half in order to expose part of each of the two electrodes. The electrodes at the first end were connected in a circuit in series with a 120V/100A power supply, a contactor relay, a 0.1 ohm/100 watt shunt resistor, and a 10A, 125/250V very fast acting fuse (Buss GBB™-10, available from the Bussman Division of Cooper Industries). A chart recorder was connected across the shunt resistor in order to measure the voltage drop. When the relay was closed, the sample was powered. A sufficient quantity of 10 to 20% saline solution was applied to the exposed v-notch until an arcing fault was initiated. The chart recorder was monitored until the current was interrupted and the arc was extinguished (i.e. until the fuse tripped). Both the time duration of the arc, as determined from the current spikes on the chart, and the distance of arc fault propagation on the strip heater were measured. In some instances, the number of current spikes present during the arcing fault was also determined.

Examples 2 to 4

For each example, pellets of the composition of Example 1 were preblended with the inorganic materials in the proportions shown in Table I. After mixing in a co-rotating twin screw extruder and pelletizing, the compositions were extruded to form strip heaters with the same geometry as that of Example 1 and were jacketed as in Example 1. The results of the arcing fault test and the vertical flame test are shown in Tables II and IV. It is apparent that those compositions which contain Sb₂O₃ have significantly faster trip times in the arc fault test than comparable materials which do not contain the filler.

A strip heater formed from the composition of Example 2 was also tested following the modified arcing fault test described in Example 1. As shown in the results in Table III, the amplitude of the current spikes and the burn rate were comparable for both the conventional composition (Example 1) and the composition of the invention (Example 2). The major difference occurred in the frequency of the current spikes; the spikes were much more prevalent for the composition of the invention than for the conventional material.

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

A strip heater which comprises

(A) a resistive element which is composed of a first conductive polymer composition which is melt-extrudable and which comprises:

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

(ii) a particulate conductive filler which is carbon black,

(iii) 4.0 to 8.0% by weight of a particulate non-conductive filler which is antimony trioxide, and, optionally,

(iv) decabromodiphenyloxide;

and

(B) two electrodes which can be connected to a source of electrical power to cause current to flow through the resistive element,

and which

(1) when tested following the procedure of UL test VW-1 either (a) does not pass the test or (b) has a performance which is similar to that of a second heater which is made from a second conductive polymer composition which is the same as the first composition except that it does not comprise 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 than is required by the second heater, and (b) trips the fuse in less than 30 seconds.
A heater according to claim 1, wherein the composition exhibits PTC behaviour.
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 such that when the first composition is made into a standard strip heater and the standard heater is tested in a standard arcing fault test it trips the fuse in less than 30 seconds.
A strip heater assembly according to claim 3, wherein the fuse is part of a fused plug assembly.
A strip heater assembly according to claim 3 or 4, wherein the fuse is a very fast acting fuse, preferably a fuse which has a rating of 10A, 125/250 volts.
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 such that when the composition is made into a standard strip heater and the standard heater is tested in a standard arcing fault test it trips the fuse in less than 30 seconds.