CA1240407A

CA1240407A - Electrical devices comprising cross-linked conductive polymers

Info

Publication number: CA1240407A
Application number: CA000504006A
Authority: CA
Inventors: Andrew N. Au; Stephen M. Jacobs; Timothy E. Fahey; Marguerite E. Deep
Original assignee: Raychem Corp
Current assignee: Raychem Corp
Priority date: 1985-03-14
Filing date: 1986-03-13
Publication date: 1988-08-09
Also published as: AU587237B2; JPS61218117A; JP2793790B2; KR860007682A; JPH0845703A; JP2608878B2; AU5475586A; US4724417A; EP0198598A3; KR940004366B1; EP0198598B1; EP0198598A2; DE3680229D1; ATE65341T1; IN167049B

Abstract

ELECTRICAL DEVICES COMPRISING
CROSS-LINKED CONDUCTIVE POYLYMERS

Andrew Au Marguerite Deep Tim Fahey Stephen Jacobs ABSTRACT OF THE DISCLOSURE

Electrical devices containing PTC conductive polymers which have been cross-linked in two steps, preferably by radiation. The conductive polymer is heat-treated above its melting point between the two cross-linking steps, and/or one of the cross-linking steps is effected on part only of the conductive polymer. The process is particularly useful for the preparation of circuit protection devices for use at high voltages.

Description

MP1044-USl BACKGROUND OF THE :[NV:ENTION
__ _ ~ _ Field of the Invention _ _ _ ____ ___ This inven-tion relates to electrica~ devices comprising PTC conductive polymers.

Introduction to the Invention ____~_ Conductive polymer compositions exhibiting PTC
behavior, and electrical devices comprising them, are well known. Reference may be made, for example, to U.S. Patent Nos. 2~952,761; 2,978,665, 3,243,753;
; 3,351,8~2; 3,571,777; 3,757,086; 3,793,716; 3,823,217;
3,~5~,144; 3,861,029; 3,950,604; 4,017,715; 4,0~2,848;
4,085,286; 4,117,312; 4,177,376; 4,177,446; 4,188,276;
4,237,4~1; 4,242,573; 4,246,468; 4,250,400; 4,252,692, 4,255,698, 4,271,350, 4,27~,471, 4,304,987, ~,309,596, 4,3Q9,597, 4,314,230, 4,314,231, 4,315,237, 4,317,027~
4,31~,881, 4,327,351, 4,330,704, 4,33~,351, 4,352,083, 4,3~1,799, 4,38~,607, 4,39~,084, 4,413,301, ~,425,397, ~,426,339, 4,426,633, 4,427,877, 4,435,639, 4,429,216, 4,442,139, 4,459,473, 4,473,~50, 4,481,498, 4,502,9~9, 4,514,&20, 4,517,~49, 4,529,866, 4,534,889, and 4,560,498; J. Applied Polymer Science 19, 813-815 (1975), Klason and Kubat; Polymer Engineering and Science 18, 649-653 (1978), Narkis et al; and European Application Nos. 38,713, 38,714, 38~718; 74,281, 92,406, 119,807, 134,145, 133,748, 144,187, and ; 30 158,410.

Par-ticularly useful devices comprising PTC conduc-tive polymers are self-regulating heaters and circuit protection devices. Self-regulating heaters are relatively ho~ and have relatively high resistance under normal operating conditions. Circuit protection devices are relatively cold and have a relatively low resistance under normal operating conditions, but are "tripped", i.e. converted into a high resistance state, when a fault condition, e.g, excessive current or temperature, occurs. When the device is tripped by excessive current, -the current passing through the PTC
element causes it to self-heat to an elevated temperature at which it is in a high resistance state.
Circuit protection devices and PTC conduc-tive polymer compositions for use in theml are described or example in U.S. Patent3 Nos. 4,237,411, 4,238,812; 4,255,698;
~,315,237; 4,317,027; 4,329,726; 4,352,083; 4/413,301;
4,450,496; 4,475,138; and 4,481,498; in European Patent Publication Nos. 38,713, 134,145, and 158,410, and in Canadian Patent Application Nos. 504,001, 504,008, and 504,0~9.

In many devices, and especially in circuit protec tion devices, it is desirable or necessary for the PTC
conductive polymer to be cross-linked, preferably by means of radiation. The effect of the cross-linking depends on, among other things, the polymer and the conditions during the cross-linking step, in particular the extent of the cross~linking, as discussed for example in U.S. Patent No. 4,534,889 and European Patent Publication No. 63,440. When a conductive polymer element is irradiated, the radiation dose absorbed by a particular part of the element in a given .

time depends upon its distance from the surface of the element exposed to the source, and the intensity, energy and type of the radiation. For a relatively thin element and a highly penetrating source (e.g. a Cobalt 60 source), the variation of dose with thickness is negligible. However, when using an electron beam, the variation in dose with thickness can be substan-tial; this variation can be offset by exposing the ele-ment to radiation from different directions, ey. by traversing the element past the source twice, irra-diating it first on one side and then on ~he other.
Depending upon the energy of the beam and the thickness of the element (which can of course vary, depending upon its shape), the radiation dose can be higher at the surEaces exposed to radiation than at the middle, ~ or substantially uniform across the thickness of the element, or higher at the middle than at the surfaces exposed to radiation. In addition, the radiation dose near the surface exposed to the radiation can be less than expected because of surface scattering, and the radiation dose in the vicinity of the elec~rodes is affected by the shielding effect and the scattering effect of the electrodes.

It has now been dlscovered that a PTC conductive polymer based on a crystalline polymer has substan~
tially improved electrical properties, in particular when subjected to high voltage stress, if it is cross-linked in two steps and is heated between the cross-linking steps, to a tempeature above the temperature at which the crystals begin to melt ~referred to herein as .'.

TI), and preferably above the temperature at which melting of the crystals is complete (referred to herein as T~). For example, if two identical circuit protec-tion devices are irradiated to the same total dose, one in two steps with no intermediate heat-treatment step, and the other in two steps with an intermediate heat-treatment above TM, the latter product has substan-tially better tolerance to repeated "tripping" at high voltages (e.g. at 600 volts hC and 1 amp~ and the PTC
element does not get as hot during the "tripping" pro-cess. It i5 theorized ~hat the new process results in a different cross-linked structure such that the resistivity/temperature curve of the conductive polymer is changed so that at least at some elevated resistances, a particular device resistance is reached at a lower temperature.

It has also been discovered that a PTC conductive polymer device has improved properties, for example a broader hot lioe and/or a more rapid response, if it is cross-linked in such a way that a center section bet-ween the electrodes absorbs a radiation dose which is at least 1.5 times the radiation dose absorbed by por-tions of the PTC element adjacent the electrodes.

Particularly useful results are obtained when these two discoveries are combined. For example, in this way it is possible to produce circuit protection devices which ~ill withstand repeated -tripping at 1 amp and 600 volts AC and which, for a particular r~sistance, will trip more rapidly than a similar device in which the whole o the PTC element is irradiated in both steps.

:,, _5_ MP1044 In its first aspect~ this invention provides a pro-cess for the preparation of an electrical device whichcomprises (1) a PTC element composed of a cross-linked con-ductive polymer composition which exhibits PTC
behavior and which comprises a polymeric co~-ponent comprising a crystalline polymer and, dispersed in the polymeric component, a par-ticulate conductive filler; and

(2) two electrodes which are electrically con-nected to the PTC element and which are con-nectable to a source of electrical power to cause current to pass through the PTC element, which process comprises the steps of:

(a) subjecting at least part of the PTC ale-ment to a first cross-linking step, (b) heating at least part of the cross-linked PTC element to a temperature above TI, where TI is the temperature at which the conductive polymer starts to melt, ; ~ (c) cooling the cross-linked and heated PTC
: element to recrystallize the polymer, and (d~ subjecting at lea.st part of the cross-linked, heated and cooled PTC element to a second cross-linking step to effect further cross-linking thereof.

In its second aspect, this invention provides a circuit protection device which has aL resistance of less than 100 ohms and which can be prepared by process as defined above and which comprises (1) a PTC element composed of a cross-linked con-ductive polymer composition which exhibits PTC
behavior and which comprises a polymeric com-ponent comprising a crystalline polymer and, dispersed in the polymeric component, a par-ticulate conductive filler; and (2) two electrodes which are electrically con-nected to the PTC element and which are con-nectable to a source of electrical power to cause current to pass through the PTC element;

said PTC element, if said circuit protection device is converted into an equilibrium high temperature, high resistance state by passing through the device a current of 1 amp from a power source of 600 volts AC, having a maximum temperature in the equilibrium state which ~t most 1.2 times TM, where TM is the temperature in C at which melting of the conductive polymer is complete. The maximum temperature referred to here and elsewhere in this specification is the maximum tem-perature on the surface of the PTC element.

In its third aspect this invention provides a pro-cess ~r the preparation of an electrical device which comprises (1) a PTC element composed of a cross-linked con-: ductive polymer composition which exhibits PTC

,, behavior and which comprises a polymeric com-ponent and, dispersed in the polymeric com-ponent, a particulate conductive filler; and ~2) two electrodes which are electrically con-nected to the PTC ~lement and which are con-nectable to a source of electrical power to cause current to pass through the PTC element, which process comprises subjecting the PTC element to radiation cross~linking su~h that in the resulting pro-duct, the geometrically shortest current path between the electrodes through the PTC element comprises in sequence a first section which has absorbed a first lS dose D1 Mrad, a second section which has absorbed a second dose D2 Mxad, and a third section which has absorbed a third dose D3 Mrad, wherein the ratio D2/D
is at least 1.5 and the ratio D2/D3 is at least 1.5, D
and D3 being the same or different. In this process, the cross-linking is preferably carried out in two steps, part only of the PTC element being irradiated in at least one of the steps. However, the invention includes other processes in which different parts of the PTC element absorb different amounts of radiation, for e~ample because the density of the PTC element varies or the amount of cross-linking agent in the PTC
element varies.

RIEF DESCRIPTXON OF THE DRAWING
The invention is illustrated in the accompanying drawing in which Figures 1, 2 and 3 are front, plan and side view5 of a circuit protection device o:E the invention, and Figures 4 shows resistivity/temperature curves for devices which have been cross-linked in accordance with the prior art and in accordance wi~h the invention.

DETAILED DESCRIPTION OF THE INVENTION

The cross-linkin~ of the PTC conduc~ive polymer i5 preferably effected by means of r~diation in both steps, and will be chiefly described herein by reference to such cross-linking. However, it is to be understood that the invention is al50 applicable, to the extent appropriate, to processes which involve chemical cross-linking, for example processes in which the first step involves chemical cross-linking and the ~ second step involves radiation. Depending upon the radiation source and the thickness of the PTC element, each step can (for reasons outlined above) involve exposing the element to the source one or more times from different directions. Radiation doses given in this specification for the PTC element are the lowest doses absorbed by any efEective part of the elemen~l the term "effective part" being used to denote any part of the element in which the radiation dose is substan-tially unaffected by surface scattering of the radiation, or by shielding by the eIectrodes~ or by scattering by the electrodes, and through which current passes in operation of the device. For example, where this specification states that the radiation dose in step (a) is 5 to 60 Mrad, this means that the lowest dose received by any effective part of the element is in the range of 5 to 60 Mrad, and does not exclude the possibility that other effective parts of the element _g_ have received a dose greater than 60 Mrad. Preferably~
however, all efEective parts of the PTC element receive a dose within the specified range.

When part only of the PTC element is irradiated in one of the cross~linking steps, this can be achieved for example by making use of a narrow radiation source, or by means of masks. The desired effect can be achieved by irradiating different but overlapping parts of the device in the two steps, or by irradiating a first part only of the PTC element in one of the steps and irrad-iating at least a second part of the PTC element in the other step, the second part being larger than and including at least some of the first part. It is pre-ferred to cross-link the whole of the PTC element in the first step and part on~y of the PTC element, inter-mediate the electrodes, in the second step. The radiation is preferably such that, in the product, the geometrically ~hortest electrical path between the electrodes through the PTC element, and preferably each electrical path between the electrodes through the PTC
element, comprises in sequence a fir~t section which has absorbed a first dose Dl Mrad, a second section which has absorbed a second dose D2 Mrad, and a third section which has absorbed a third dose D3 Mrad, Dl and D3 preferably being the same, and D2/Dl and D2/D3 being at least 1.5, particularly at least 2.0, especially at least 3.0, e.g. 4.0 or more. As noted above~ the known cross-linking procedures can produce some variation in cross-linking density, but not a variation as large as 1.5:1. Furthermore it was not recoynized that any advantage could be derived from any such variation, nor was it known to heat~treat the conductive polymer between the cross-linking step~.

Cross-lin~ing a PTC conductive polymer generally increases its resistivity as well as increasing its electrical stability. The increase in resistivity is acceptable in some cases, but in other cases restric-tions on the resistance and/or dimensions of the device make it impossible to cross-link the conductive polymer to the extent desired. Especially under these cir-cumstances, it is useful to have a relatively small section of the PTC element, intermediate the electrodes, which has been more highly irradiated than the remainder, thus increasing the stability of the element in the critical "hot zone" area, while not excessively increasing the resistance of the device.

The radiation dose in the first cross-linking step is prefera`bly less than the dose in the second cross-linking step. The dose in the first step is preferably 5 to 60 Mrad, particularly 10 to 50 Mrad, especially 15 to 40 Mrad. The dose in the second step is preferably at least 10 Mrad, more preferably at least 20 ~rad, particularly at least 40 Mrad, especially 50 to 180 Mrad, e.g. S0 to 100 Mrad.

When, as is preferred, at least part of the cross-linked PTC conductive polymer is heated to a tem-perature above TI, and preferably above TM, between the two cross-linking steps, that temperature is preferably maintained for at least the time required to ensure that equilibrium is reached, e.g. for at least 1 minute, e.g. 2 to 20 minutes. The whole of the PTC
element which has been cross-linked in the first step can be heated in this way. Alternatively only part of the element is so heated; this can result in variations between different parts of the PTC element which can be desirable or undesirable depending on circumstancesO

The T~ and TM of the conductive polymer a~ defined herein can be ascertained from a curve generated by a differential scanning calorimeter, TI being the tem-perature at which the curve departs from the relatively straight baseline because the composition has begun to undergo an endothermic transition, and TM being the peak of the curve. If there is more than one peak on the curve, TI and TM are taken from the lowest of the peaks. For further details, reference should be made to ASTM D-3417-83. The heating of the PTC element, which is preferahly carried out in an inert, e.g.
nitrogen, atmosphere, can be effected by external heating, e.g. in an oven, in which the whole of the PTC
element will normally be uniformly heated; or by means of internally generated heat, e.g. by passing a current through the device which is sufficient to make it trip, in which case the heating will normally be confined to a narrow zone of the PTC element between the electrodes~

After it has been heated above TI, the PTC element is cooled to recrystallize the polymer, prior to the second cross-linking stepO The cooling is preferably effected slowly, e.g. at a rate less than 7C/minute, particularly less than 4C/minute, especially less than 3C/minute, at least over the temperature range over which recrystallization takes place. Similar heat treatments, again preferably with slow cooling, are preferably carried out before the first cross-linking step and after the second cross~linking step.

MPl044 There can be some overlap between the different steps of the process. For example the irradiation of the PTC element ean be continued while the element is heated to a temperature above TI.

The PTC conductive polymer comprises a polymeric component and a particulate conductive Eiller. The polymeric component can eonsist essentially of one or more crystalline polymers, or it can al50 contain amorphous polymers, e.g. an elastomer, preferably :in minor amount, e.g. up to 15% by weight. The erystalline polymer preferably has a erystallinity of at least 20%, particularly at least 30%, especially at least 40~, as measured by DSC. Suitable polymers inelude polyolefins, in partieular polyethylene;
copolymers of olefins with copolymerisable monomers, e.g. eopolymers of ethylene and one or more fluorinated monomers e.g. tetraEluoroethylene, or one or more earboxyl- or ester-eontaining monomers, e.g. ethyl acrylate or aerylie acid; and other fluoropolymers, e.g. polyvinylidene fluoride. The conductive filler preferably eonsists of or contains carbon blaek. The composition can also contain non-conductive fillers, including are-suppression agents, radiation cross-linking agents, antioxidants and other adjuvants.

This invention is particularly useiul in the pro-duetion of eireuit proteetion deviees, espeeially those whieh are subjeet to high voltage fauLts and whieh must be eapable of repeated ~'tripping". Such devices generally have a resistanee of less than lO0 ohms, .

often less than 50 ohms, at 23C, and usually make use of PTC conductive polymers having a resistivity of less than 100 ohm.cm, preferably less than 50 ohm.cm at room temperature. Preferred protection devices for this invention comprise two parallel electrodes which have an electrically active surface of generally columnar shape and which are embedded in, and in physical con-tact with, the PTC element. The device can have a shape or other characteristic which ensures that when the device is tripped, the hot zone forms at a location away from the electrodes (see in particular U.S. Patent Nos. 4,317,027 and 4,352,083) and when one of the cross-linking steps is carried out on part only of the PTC elemen~, intermediate the electrodes, this can create or enhance such characteristic.

As noted above, the sequence of cross-link, heat above TM, cool, and cross-link again, results in a device which, when it is tripped (and especially when it is tripped at high voltage), has a cooler "hot zone"
than a device which has been cross-linked in a conventional way. The reduction in the maximum temperature of the PTC element is highly significant improvement since it increases the nllmber of times that the device can be tripped before it fails. This improvement can be demonstrated with the aid of the tests described below, in which the device is tripped by means of a current of 1 amp from a 600 vQlt AC power source.

The device is made part of a circuit which consists of a 600 volt AC power source, a switch, the device, and a resistor in series with the device, the device MP~044 being in still air at 23C and the resistor being of a size such that when the switch is closed, the initial current is 1 amp. The switch is then closed~ and after about 10 seconds (by which time the clevice is in an equilibrium state) an infra-red thermal imaging system i5 used to determine the maximum temperature on the surface of tha PTC element. Devices according to the invention have a maximum temperature which is less than 1.2 times T~, preferably less than 1.1 times TM, particularly less than TM. Rnown devices have substantially higher maximum temperatures, e.g. at least 1.25 times ~M If the temperature of the PTC
element is monitored while ~he device is being tripped, it is sometimes found that small sections of the lS element reach a temperature greater than 1.2 times TM
for a limited time; however, it is preferred that no part of the surface of the PTC element should reach a temperature greater than 1.2 times TM while the device is being tripped.

The test circuit described above can also be used to test the voltage withstand performance of the device. In this test the switch is closed for 1 second (which i5 sufficient to trip the device), and the device is then allowed to cool for 90 seconds before the switch is again closed for 1 seoond. This sequence is continued until the device fails (as evidenced by visible arcs or flames or by signi~icant resistance increase). Preferred devices of the invention have a survival life of at least 100 cycles, preferably at least 125 cycles/ particularly at least 150 cycles, in this test.

Preferred circuit protection devices of the inven~
tion are particularly useful Eor providing secondary protection in subscriber loop interface circuits in telecommunication systems.

Referring now to the drawing, Figures 1, 2 and 3 show face, plan and side views of a circuit protec-tion device comprising columnar electrodes l and 2 embedded in, and in physical contact wi-th, a PTC conductive polymer element 3 which has a central section of reduced cross-sec-tion by reason of res-triction 31. The height of the PTC element is l, the maximum width of the PTC element is y, the distance between the electrodes is t, and the wid-th of the electrodes is w.

The invention is illustrated in the following Examples, in which Examples l and 2 are comparative ~xamples.

Exam~le_l The ingredients listed in Table l were preblended, mixed in a Banbury mixer, pelletized ana dried. Circuit Circuit protection devices as illu~trated in Figures 1-3 (1 = 0.300 inch, t = 0.200 inch, x = O.O9Z inch, y =
O.Q60 inch, and w = 0.032 inch) were made by injection molding the dried pellets around two 20 AWG tin-coated copper wires which have been coated with a graphite emulsion (Electrodag [trademark] 502, sold by Acheson).
The devices were heat-trea-ted in a nitrogen atmosphere by increasing the temperature to 150C at 10C/min.;
maintaining them for 1 hour at 150C; cooling them to 110C at ~C/min; maintaining them for 1 hour at 110C, .

~2~

and cooling them to 23C at 2C/min. The devices were then cross-linked by means of a 1 Mev electron beam;
the devices were exposed to a dose of 20 Mrad on one side and then to a dose of 20 Mrad on the other side.
The devices were then subjected to a second heat-treatment as described above.

Example 2 The procedure of Example 1 was followed except that the radiation dose was 80 Mrad on each side of the device.

~xamRle 3 The procedure of Example 1 was followed except that after the second heat-treatment, the devices were given a second cross-linking in which the devices were exposed to a dose of 60 Mrad on one side and then to a dose of 60 Mrad on the other side, and then given a third heat treament which was the same as the first and second heat treatments.

Example 4 The procedure of Example 1 was follo~ed except that the devices were exposed to a dose of 60 Mrad on each side in the first cross-linking step and a dose of 20 Mrad on each side in the second cross-linking step.

Example 5 The procedure of Example 1 was followed except that the devices were exposed to a dose oE 140 Mrad on each ~P104 side in the second cross-linking s-tep.

The devices prepared in Examples 1-5 were tested at 600 volts AC and 1 amp by th~ procedures described above, and the results obtained are shown in Table 2 below.

~e~

The ingredients listed in Table 1 were preblended, mixed in a Banbury mixer, pelletized and dried. Using a Brabender cross-head extruder fitted with a dog-bone shaped die, the pellets were melt-extruded at a temperature of about 160C around two 20 AWG 19/32 1~ nickel-coated copper wires which had been coated with a graphite-silicate composition (Electrodag 181 sold by Acheson). The extrudate was cut into 0.46 inch long pieces, and the conduc~ive polymer removed from the bottom 0.20 inch of each piece, to give devices as shown in Figures 1-3 (1 - 0.260 inch, t = 0.1~0 inch, x = 0.090 inch, y = 0.065 inch, and w = 0.040 inch).

The devices were heat-treated as in Example l;
cross-linked a first time by exposing them to a dose of 20 Mrad on one side and then to a dose of 20 Mrad on the other side using a 1.5 Mev electron beam; again heat-treated as in Example 1: cross-linked a second time by exposing them to a dose of 100 Mrad on one side and then to a dose of 100 Mrad on the other side, and again heat-treated as in Example 1.

;

Example 7 The ingredients listed in Table 1 were preblended, mixed in a Banbury mixer, granulated and dried.
Circuit protection devices as illustrated in Figures 1-3 ~1 = 0.375 inch, t = 0~466 inch, x - 0.060 inch, y = 0.034 inch, and w = 0.032 inch) were made by injection molding the ~ranules around 20 AWG
nickel-coated copper wires. The devices were heat-treated as in Example l; cross~linked a first time by exposing them to a dose of 20 Mrad ~on one side only~, using a 1 Mev electron beam; and again heat-treated as in Example 1. Aluminum tape was applied to the devices so as to mask the entire device from electrons except for a strip OoO10 inch wide in the center, parallel to the electrode; the mas~ced - devices were cross-linked a second time by exposing them to a dose of 100 Mrad ~on one side); masking material was removed; and the device was again heat-treated as in Example 1.

The ingredients listed in the Table under Example 8 (Master) were preblended, mixed in a Banbury mixer, granulated and dried. The granules were blended with alumina trihydrate in a volume ratio of 83.5 to 16.5, to give a mixture as listed in Table 1 under Example 8 (Final). Using a Brabender cross-head extruder~ the mixture was melt-extruded around two pre-heated parallel 20 AWG 19/32 stranded nickel-coated copper wires and around a solid 24 ~G nickel-coated copper wire midway between the stranded wires. The extrudate MPl044 was cut into pieces about 1.5 inch long; the conductive polymer was stripped from one end of each piece; and the center wire was withdrawn from each piece, thus producing a circuit protection device consisting of the S stranded wires embedded in a conductive polymer element 1 inch long, 0.4 inch wide and 0.1 inch deep, with a hole through the middle where the center wire had been removed. The devices were cross-linked a first time by irradiating them (on one side only) to a dose of 20 Mrad in a nitrogen atmosphere, using a Cobalt 60 gamma source at a rate of 21.2 Mrad/hour. Aluminum sheet 92 mils thick was then used to mask the device except for a strip 0.062 inch wide in the center, parallel to the electrodes. The masked devices were then cross-linked a second time by irradiating them to a dose of 80 Mrad on one side and then to a dose of 80 Mrad on the other side, using a l Mev electron beam.

The resistance/temperature characteristics of the devices prepared in Examples 2, 3, 7 and 8 were then determined by measuring the resistance of the devices as they were externally heated from 20C to 200C at a rate of 2C/minute. The resistivities of the compositions were then calculated, and the results are presented graphically in Figure 4, in which the flat portions at the top of some of the curves are produce~
by the maximum resistance which could be measured by the test apparatus.

Table 1 Example No. 1-5 6 7 8 _~redients ~parts by volume) (master) (final)____ Polyethylene ll) 53~7 56.7 66.0 55.1 Polyethylene (2) - - 55.0 - -Carbon Black 131.1 - 30.0 32.0 26.7 Carbon Black 2 - 25.1 ~ - -A12O3-3~2O 16.5 Si-coated A12O3.3H2O(1) 13.5 - 13.0 Si-coated Al2O3 3H2o(~) 16.5 Antioxidant 1.7 1.7 2.0 2.0 1.7 NOTES
Po_y~_hylene (1) is high density polyethylene having ~ a peak DSC melting point of about i 135C sold as Marlex 6003. "Marlex"
is a trademark of Phillips Petroleum.
Poly thylene (2) is high density polyethylene having a peak DSC melting point of about 135C sold as Alathon 7050. "Alathon"
is a trademark of duPont.

Carbon Black (1) is carbon black sold as Statex G.
"Statex" is a trademark of Columbian Chemicals.
Carbon Black ~2) is carbon black sold as Sterling SO.
"Sterling" is a trademark of Cabot.
A12O3~3H2O is alumina trihydrate sold as Hydral - - ~ 705. "~ydral" is a trademark o~
Alcoa.
Si-coated A12O3.3H2O(l) is a silane-coated alumina yrate having a particle size o~
about 0.8 microns sold as Solem ; 916SP. "Solem" is a trademark of J. M. Huber.

.

Antioxidant is an oligomer of 4,4-thio bis(3-methyl 1-6-t-butyl phenol) with an average degree of polymerisation of 3-4, as described in U.S. Paten~ No. 3,986,981.
Table 2 Example No. Processing Max Temp.
when cyc:Les ~ e~ survived 1 HT/20,~0/HT 197Cl:L
2 HT/80~80/HT 174C60

3 HT/20,20/HT/60,60/HT 12~C 157

4 HT/60,60/HT/20,20/HT 162C 60 HT/20,20/HT/140,140/HT 135C ~200

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows.

1. A process for the preparation of an electrical device which comprises (1) a PTC element composed of a cross-linked conductive polymer composition which exhibits PTC behavior and which comprises a polymeric component comprising a crystalline polymer and, dispersed in the polymeric component, a particulate conductive filler; and (2) two electrodes which are electrically connected to the PTC element and which are connectable to a source of electrical power to cause current to pass through the PTC element, which process comprises the steps of:

(a) subjecting at least part of the PTC
element to a first cross-linking step, (b) heating at least part of the cross-linked PTC element to a temperature above TI, where TI is the temperature at which the conductive polymer starts to melt, (c) cooling the cross-linked and heated PTC
element to recrystallize the polymer; and (d) subjecting at least part of the cross-linked, heated and cooled PTC

element to a second cross-linking step to effect further cross-linking thereof.

2. A process according to Claim 1 wherein the PTC
element is cross-linked by irradiation in step (a) and in step (d).

3. A process according to Claim 2 wherein the whole of the PTC element is irradiated in step (a) and in step (d).

4. A process according to Claim 2 wherein the whole of the PTC element is irradiated in one of steps (a) and (d), and only a part of the PTC element, intermediate the electrodes, is irradiated in the other of steps (a) and (d).

5. A process according to Claim 2 wherein the whole of the PTC element is irradiated in step (a) and only a part of the PTC element, intermediate the electrodes, is irradiated in step (d) and the radiation dose in step (a) is less than the radiation dose in step (d).

6. A process according to Claim 2 wherein the radiation dose in step (a) is 5 to 60 Mrad, and the radiation dose in step (d) is at least 10 Mrad.

7. A process according to Claim 2 wherein the radiation dose in step (a) is 10 to 50 Mrad, and the radiation dose in step (d) is 50 to 180 Mrad.

8. A process according to Claim 2 wherein the radiation dose in Step (a) is 15 to 40 Mrad, and the radiation dose in step (d) is 50 to 100 Mrad.

9. A process according to Claim 1 wherein in step (b) the cross-linked PTC element is heated to a temperature above TM, where TM is the temperature at which melting of the conductive polymer is complete.

10. A process according to Claim 1 wherein in step (c) the cross-linked and heated PTC element is cooled at a rate of less than 4°C per minute over the temperature range in which recrystallization takes place.

11. A process according to Claim 2 wherein the electrical device is a circuit protection device having a resistance of at room temperature of less than 100 ohms and the conductive polymer composition has a resistivity at 23°C of less than 50 ohm.cm.

12. A process according to Claim 11 wherein each of the electrodes has an electrically active surface of a generally columnar shape, and the electrodes are (a) parallel to each other and (b) embedded in, and in physical contact with, the PTC element.

13. A process according to Claim 11 wherein the conductive polymer comprises carbon black dispersed in polyethylene.

14. A process according to Claim 11 wherein the radiation doses in Steps (a) and (b) are such that when the device is converted into a high temperature, high resistance state by passing through the device a current of 1 amp from a power source of 600 volts AC, the PTC element reaches a maximum surface temperature which is at most 1.2 times TM, where TM is the temperature in °C at which melting of the conductive polymer is complete.

15. A circuit protection device which has a resistance of less than 100 ohms and which comprises:

(1) a PTC element composed of a cross-linked conductive polymer composition which exhibits PTC behavior and which comprises a polymeric component comprising a crystalline polymer and, dispersed in the polymeric component, a particulate conductive filler; and (2) two electrodes which are electrically connected to the PTC element and which are connectable to a source of electrical power to cause current to pass through the PTC element;

said PTC element, if said circuit protection device is converted into an equilibrium high temperature, high resistance state by passing through the device a current of 1 amp from a power source of 600 volts AC, having a maximum surface temperature in the equilibrium state which at most 1.2 times TM, where TM is the temperature in °C at which melting of the conductive polymer is complete.

16. A device according to Claim 15 wherein each of the electrodes has an electrically active surface of a generally columnar shape, and the electrodes are (a) parallel to each other and (b) embedded in, and in physical contact with, the PTC element.

17. A device according to Claim 15 wherein said maximum temperature is at most 1.1 times TM.

18. A device according to Claim 16 wherein the geometrically shortest current path between the electrodes through the PTC element comprises in sequence a first section which has absorbed a first dose D1 Mrad, a second section which has absorbed a second dose D2 Mrad, and a third section which has absorbed a third dose D3 Mrad, wherein the ratio D2/D1 is at least 1.5 and the ratio D2/D3 is at least 1.5, D1 and D3 being the same or different.

19. A process for the preparation of an electrical device which comprises (1) a PTC element composed of a cross-linked conductive polymer composition which exhibits PTC behavior and which comprises a polymeric component and, dispersed in the polymeric component, a particulate conductive filler;
and (2) two electrodes which are electrically connected to the PTC element and which are connectable to a source of electrical power to cause current to pass through the PTC element, which process comprises subjecting the PTC element to radiation cross-linking such that the geometrically shortest current path between the electrodes through the PTC element comprises in sequence a first section which has absorbed a first dose D1 Mrad, a second section which has absorbed a second dose D2 Mrad, and a third section which has absorbed a third dose D3 Mrad, wherein the ratio D2/D1 is at least 1.5 and the ratio D2/D3 is at least 1.5, D1 and D3 being the same or different.