US3858144A - Voltage stress-resistant conductive articles - Google Patents

Voltage stress-resistant conductive articles Download PDF

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Publication number
US3858144A
US3858144A US00319492A US31949272A US3858144A US 3858144 A US3858144 A US 3858144A US 00319492 A US00319492 A US 00319492A US 31949272 A US31949272 A US 31949272A US 3858144 A US3858144 A US 3858144A
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percent
matrix
article
weight
conductive
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US00319492A
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R Bedard
A Kampe
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Raychem Corp
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Raychem Corp
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Priority to US00319492A priority Critical patent/US3858144A/en
Priority to GB2343876A priority patent/GB1456048A/en
Priority to GB5963773A priority patent/GB1456047A/en
Priority to FR7346976A priority patent/FR2212617B1/fr
Priority to BE139428A priority patent/BE809287A/xx
Priority to CA189,180A priority patent/CA1017395A/en
Priority to DE2364947A priority patent/DE2364947C2/de
Application granted granted Critical
Publication of US3858144A publication Critical patent/US3858144A/en
Priority to CA262,825A priority patent/CA1020998A/en
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    • 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—Heating elements characterised by the composition or nature of the materials or by the arrangement of the conductor
    • H05B3/12—Heating 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—Heating 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
    • H01B—CABLES; CONDUCTORS; INSULATORS; SELECTION OF MATERIALS FOR THEIR CONDUCTIVE, INSULATING OR DIELECTRIC PROPERTIES
    • H01B3/00—Insulators or insulating bodies characterised by the insulating materials; Selection of materials for their insulating or dielectric properties
    • H01B3/18—Insulators or insulating bodies characterised by the insulating materials; Selection of materials for their insulating or dielectric properties mainly consisting of organic substances
    • H01B3/30—Insulators or insulating bodies characterised by the insulating materials; Selection of materials for their insulating or dielectric properties mainly consisting of organic substances plastics; resins; waxes
    • H01B3/44—Insulators or insulating bodies characterised by the insulating materials; Selection of materials for their insulating or dielectric properties mainly consisting of organic substances plastics; resins; waxes vinyl resins; acrylic resins
    • 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
    • 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
    • H05B3/02—Details
    • H05B3/06—Heater elements structurally combined with coupling elements or holders

Definitions

  • ABSTRACT Described herein are methods and means by which conductive carbon black-containing resistive heaters which are self-temperature limiting by reason of their positive temperature coefficient of resistance are stabilized against long-term resistance variation under high voltage stress, variously by (a) increasing the proportion of carbon black at the electrode interface relative to that of the remainder of the semi-conductive material of which the article is comprised; and (b) providing at the electrode interface a material selected from the group consisting of carboxylic acid group containing polymersof acid number greater than 3, their ammonium, alkali or alkaline earth metal salts, or a polymeric amine of amine number greater than 3.
  • compositions comprised of conductive carbon black dispersed in an interconnected array of current-carrying channels in a polymeric matrix have heretofore found wide application in resistance heating. Such compositions as exhibit a steep-sloped positive temperature coefficient of resistance have found particular application in the manufacture of selftemperature regulating heating elements, exemplary of which are those described in the aforesaid application Ser. Nos. 287,442, 287,443 and 287,444, the disclosures of which are incorporated herein by reference to illuminate the background of this invention.
  • the formation of localized regions of high current density at the electrode interface is apparently discouraged by precoating the metallic electrodes with a conductive black-containing composition such that following disposition of the electrodes in spaced-apart relation electrically continuous through a black-loaded polymeric matrix or core (as by extruding the core onto a number of electrodes), the percent by weight conductive black contained at the electrode interface is at least about 1.5 times that of the midpoint between adjoining electrods.
  • the objectives of the invention are achieved by uniformly deploying in the matrix or core electrically connecting adjoining electrodes an effective resistance-stabilizing amount of a material selected from the group consisting of carboxylic acid group-containing polymers of acid number greater than about 3, the ammonium, alkali or alkaline earth metal salts of such polymers, and polymeric amines of amine number greater than about
  • a material selected from the group consisting of carboxylic acid group-containing polymers of acid number greater than about 3, the ammonium, alkali or alkaline earth metal salts of such polymers, and polymeric amines of amine number greater than about
  • a further object of the invention is to provide selftemperature regulating conductive articles free from excessive resistance increase upon subjection to high voltage stress early in their service life.
  • FIGS. 1 and 2 respectively, graphically depict, for comparison purposes, voltage-induced resistance variance with time in the case of a self-temperature regulating article formed according to one embodiment of this invention and that with a control without the scope of the invention;
  • FIG. 3 graphically depicts resistance variance with temperature data employed in normalizing resistance values employed in constructing FIGS. 1, 2 and 4-7;
  • FIGS. 4, 6 and 7 similarly permit comparison of timerelated resistance variation of other embodiments of the invention with control results depicted in FIG. 5.
  • Amine number refers to that quantity obtained according to the standard ASTM D-20- 74-66 determination. Acid number is the mg. KOH.
  • Particular preferred carboxylic acid groupcontaining polymer addends are ethylene-acrylic acid copolymers (preferably the sodium or zinc-neutralized salts thereof such as those sold by E. I. du Pont de Nemours & Company, Inc. under the tradename Surlyn) and terpolymers of ethylene, vinyl acetate (24-29 percent) and a minor proportion (e.g., 5 percent) of an 04-3 ethylenically unsaturated carboxylic acid such as acrylic or methacrylic acid.
  • Exemplary of the latter category is the E1vax" family of acid terpolymers (acid number of about 6) available from E. I. du Pont de Nemours and Company, Inc.
  • an especially suitable amine addend may be mentioned poly (2,2,4-trimethyl- 1,2-dihydroquinoline) such as that available from R. T. Vanderbilt and Company under the name AgeRite Resin D.
  • the semi-conductive core electrically connecting adjoining electrodes contains from about 0.1 to about l5, preferably 0.1 to 8 percent, by weight amine or acid additive, based on total weight of the additive-containing semi-conductive material.
  • amine or acid additive based on total weight of the additive-containing semi-conductive material.
  • optional results may be obtained with as little as from 0.1 to 2 percent.
  • the adverse effects of voltage stress are diminished by providing the electrodes with a coating containing sufficient carbon black to ensure that at the electrode surface the enveloping conductive material contains at least about l.5 times that amount of carbon contained at the midpoint between adjoining electrodes where the percent by weight carbon at that midpoint is at least about 9 to 15 percent. Percent by weight carbon at the electrode interface desirably is within the range from about 1.5 to about 7 times that of the midpoint. Most preferably, the electrode coating when dry contains from about 30 to percent by weight carbon black. This differentiation in relative content of carbon is, most conveniently achieved by coating onto the electrode a carbon blackrich aqueous composition, preferably one comprised of conductive black in deionized water.
  • the coating composition may optionally contain fillers such as colloidol silica for strength enhancement, etc.
  • Electrodes include copper, tinned copper, and nickeland silver-plated copper.
  • the electrodes may vary conventionally in configuration, e.g., flat, round, solid, stranded, etc.
  • the polymeric matrix in which conductive black is dispersed in whatever proportion should exhibit overall an appropriately non-linear coefficient of thermal expansion, for which reason a degree of crystallinity is believed useful.
  • Polymers exhibiting at least about 20 percent crystallinity as determined by x-ray diffraction are suited to the practice of the invention.
  • polystyrene resin such as low, medium and high density polyethylenes and polypropylene, polybutene-l, poly (dodecamethylene pyromellitimide), ethylenepropylene copolymers and terpolymers with non-conjugated dienes, polyvinylidine fluoride, polyvinylidine fluoride-tetrafluoroethylene copolymers, etc.
  • polyolefins such as low, medium and high density polyethylenes and polypropylene, polybutene-l, poly (dodecamethylene pyromellitimide), ethylenepropylene copolymers and terpolymers with non-conjugated dienes, polyvinylidine fluoride, polyvinylidine fluoride-tetrafluoroethylene copolymers, etc.
  • blends of polymeric substances may also be employed as the matrices in which the carbon black is dispersed.
  • the minor polymeric blend component is chosen for superior compatibility with carbon black relative to the blend component present in major proportion, while the latter component is selected for the particular physical properties desired in the overall extrudate.
  • the principal blend component is preferably present in at least about 3:1 weight ratio relative to the minor component with which the black is first mixed.
  • the blends most preferred have a polyethylene as the principal component, the other being an ethylene-vinyl ester copolymer, such as ethylene-vinyl acetate or ethylene-ethylacrylate copolymers.
  • An especially preferred extrudate contains about 70:20 polyethylene: ethyleneethyl acetate copolymer by weight.
  • limiting temperatures tailored to the application intended may be obtained by appropriate selection of polymeric matrix material.
  • elements which self-limit at temperatures on the order of F, F, F, F and 250F may be produced with, respectively, wax-poly (ethylenevinyl acetate) blends, low density polyethylene, high density polyethylene, polypropylene and polyvinylidene fluoride.
  • Other criteria of polymer selection will, in particular instances, include desired elongation, en-
  • the carbon blacks employed are those conventionally used in conductive plastics, e.g., high structure varieties such as furnace and channel blacks.
  • Other conventional addends such as antioxidants, etc, may be employed provided only that their quantities and characteristics do not subvert the objects of the invention.
  • An especially interesting class of beneficial addends are materials such as waxes which, while compatible with the predominant blend component, melt at lower temperature. The result is to permit obtainment of a given wattage at lower temperature, owing to a first peaking effect of the wax on the resistivity-temperature curve.
  • Compounding of the core material intended for extrusion about coated or uncoated electrodes is conventional and generally involves banburying, milling and pelletizing prior to pressure extrusion of the selflimiting element from the melt.
  • a polymeric amine or carboxylic acid group-containing polymeric addend is employed as a component of the polymeric matrix material according to one embodiment of the invention, it may be added at any point in compounding of that material.
  • the blackcontaining matrix 1 is extruded onto a spaced-apart pair of elongate electrodes 2 to form an element rodshaped or, most preferably, dumbbell-shaped in crosssection, the extruded thermoplastic both encapsulating and interconnecting the electrodes.
  • annealed at a temperature greater than about 25 F, preferably at least about 300F, and in any case at or above the melting point or range of the polymeric matrix in which the carbon black is dispersed.
  • the period over which annealing is effected will, it will be appreciated, vary with the nature of the particular matrix and the amount of carbon black contained therein.
  • annealing preferably occurs over a time sufficient to reduce resistivity of the annealed element to satisfaction of the equation 2 L+ 5 logio R S 45, preferably s 40 (L being percent by weight black in the matrix and R resistivity of the extrudate in ohm-cm) and the time necessary in a particular case may be readily determined empirically.
  • annealing is conducted over a period in excess of hours, and commonly at least about a 24 hour anneal is had. Where the element is held at anneal temperature continuously throughout the requisite period, it is advisable to control cooling upon completion of the anneal so that at least about 1% hours are required to regain room temperature. However, control of cooling is substantially less important where the requisite overall annealing residence time is divided into at least about 3 roughly equal stages and the element returned to room temperature between each annealing stage.
  • that extrudate is preferably supplied prior to annealing with an insulative extruded jacket 3 (see FIG. 8) of a thermoplastic material which is shape-retaining when brought to the annealing temperature.
  • Suitable jacketing materials are discussed in length in the aforesaid application S.N.
  • the self-limiting element Upon completion of annealing and optional addition of a further insulative jacket 4 of, e.g., polyethylene, the self-limiting element is desirably subjected to ionizing radiation sufficient in strength to cross-link the black-containing core.
  • Radiation dosage is selected with an eye to achieving cross-linking sufficient to impart a degree of thermal stability requisite to the particularly intended application without unduly diminishing crystallinity of the polymer matrix, i.e., diminution of overall crystallinity of v the cross-linked blackcontaining matrix to less than about 20 percent is preferably avoided.
  • radiation dosage may in particular cases range from about 2 to 15 megarads or more and preferably is about 12 megarads.
  • EXAMPLE 1 A. Preparation of Electrode Coating Composition To a tank were added, with stirring, 200 lbs. colloidal silica (Ludox l-lS-40, E. l. duPont de Nemours & Company, Inc.), 40 lbs. of a 25 percent aqueous solution of Tamol 731 (a sodium salt of a polymerized carboxylic acid available from Rohm & Haas), 60 lbs. dionized water and sufficient ammonium hydroxide to adjust pH to about 9.5.
  • colloidal silica Lidox l-lS-40, E. l. duPont de Nemours & Company, Inc.
  • Tamol 731 a sodium salt of a polymerized carboxylic acid available from Rohm & Haas
  • dionized water 60 lbs. dionized water and sufficient ammonium hydroxide to adjust pH to about 9.5.
  • the two masterbatches prepared above were then mixed together under shear for about 15-20 minutes, and the resulting mixture let stand for about 24 hours. Thereafter, 236 lbs. conductive acetylene black (Shawinigan Company) was added to the mixture under shear and the resulting black-loaded composition let stand for a further 24 hour period. Viscosity was then adjusted for optional coating by the addition of about 61 lbs. dionized water.
  • a conventional tube extrusion method was employed in which a vacuum (e.g., -20 inch H O) is drawn in the molten tube to collapse it about the semiconductive core within about 3 inches of the extrusion head.
  • the jacketed product was next spooled onto aluminum disks (26 inches diameter) and exposed to 300F for 24 hours in a circulating air oven. Following this thermal structuring procedure, the extrudate was cooled to room temperature over about l /z /z hours.
  • a control was prepared following the foregoing procedure save that the core material was extruded about uncoated electrodes.
  • the resistance of a 1 foot section of the'control extrudate was measured at various temperatures with a wheatstone bridge, the resulting values divided by measured resistance at 70, and FIG. 3 prepared'from the resulting values.
  • FIGS. 1 and 2 A comparison of FIGS. 1 and 2 makes plain that the electrode-coated extrudate exhibits stable resistance characteristics under high voltage stress, whereas the control does not.
  • the extrudate prepared according to this invention markedly differs from the control both in terms of resistance stability after extended powering and in freedom from excessive resistance increase in early operational life.
  • Example l the resulting mixture was mixed, milled, extruded and annealed to form a 5 jacketed heating element as in the control of Example l (i.e., the electrodes were uncoated).
  • resistance of each extrudate was periodically measured as the extrudates were subjected to various voltages over a lengthy period, and the result data for Examples 2, 3 and 4 plotted as before, respectively, on FIGS. 4, 6 and 7.
  • FIG. 5 depicts data similarly obtained with a control identical to that of Example 2 save that Surlyn resin was replaced with an equal weight of polyethylene.
  • the plots obtained in Examples 2-4 demonstrate the markedly superior long-term resistance characteristics obtainable according to this invention.
  • an electrically conductive self-regulating article comprised of at least two spaced-apart metallic electrodes electrically interconnected by a composition containing conductive carbon black dispersed in a crystalline polymeric matrix
  • the improvement wherein voltage-induced resistance variance is diminished which comprises providing at the electrode surface an effective resistance-stabilizing amount of a material selected from the group consisting of carboxylic acid group-containing polymers of acid number greater than about 3 and the ammonium, alkali or alkaline earth metal salts of such polymers.
  • composition of claim 4 wherein said material is uniformly dispersed throughout said matrix in the amount of from about 2 to 8 percent by weight based on total weight of the material-containing matrix.
  • an electrically conductive self-regulating article comprised of at least two spaced-apart metallic electrodes electrically interconnected by a composition containing conductive carbon black dispersed in a crystalline matrix
  • voltageinduced resistance variance which comprises providing that the percent by weight conductive black at the electrode surface is at least about 1.5 times that contained in said matrix at the midpoint between adjoining electrodes, the latter constituting at least about 5 percent by weight of the total weight of matrix and conductive black at said midpoint.
  • voltage-induced resistance variance is diminished which comprises providing at the electrode surface an effective resistance-stabilizing amount of poly(2,2,4- trimethyl-1,2-dihydroquinoline).

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  • Engineering & Computer Science (AREA)
  • Physics & Mathematics (AREA)
  • Microelectronics & Electronic Packaging (AREA)
  • Chemical & Material Sciences (AREA)
  • Dispersion Chemistry (AREA)
  • Ceramic Engineering (AREA)
  • Electromagnetism (AREA)
  • Spectroscopy & Molecular Physics (AREA)
  • Compositions Of Macromolecular Compounds (AREA)
  • Resistance Heating (AREA)
  • Conductive Materials (AREA)
  • Thermistors And Varistors (AREA)
US00319492A 1972-12-29 1972-12-29 Voltage stress-resistant conductive articles Expired - Lifetime US3858144A (en)

Priority Applications (8)

Application Number Priority Date Filing Date Title
US00319492A US3858144A (en) 1972-12-29 1972-12-29 Voltage stress-resistant conductive articles
GB5963773A GB1456047A (en) 1972-12-29 1973-12-21 Voltage stress-resistant conductive articles
GB2343876A GB1456048A (de) 1972-12-29 1973-12-21
BE139428A BE809287A (fr) 1972-12-29 1973-12-28 Produits manufactures presentant de la resistance electrique
FR7346976A FR2212617B1 (de) 1972-12-29 1973-12-28
CA189,180A CA1017395A (en) 1972-12-29 1973-12-28 Voltage stress-resistant conductive articles
DE2364947A DE2364947C2 (de) 1972-12-29 1973-12-28 Elektrischer Heizwiderstand
CA262,825A CA1020998A (en) 1972-12-29 1976-10-06 Voltage stress-resistant conductive articles

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US00319492A US3858144A (en) 1972-12-29 1972-12-29 Voltage stress-resistant conductive articles

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BE (1) BE809287A (de)
CA (1) CA1017395A (de)
DE (1) DE2364947C2 (de)
FR (1) FR2212617B1 (de)
GB (2) GB1456048A (de)

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DE2364947C2 (de) 1985-04-18
GB1456047A (en) 1976-11-17
BE809287A (fr) 1974-06-28
DE2364947A1 (de) 1974-07-25
GB1456048A (de) 1976-11-17
CA1017395A (en) 1977-09-13
FR2212617A1 (de) 1974-07-26
FR2212617B1 (de) 1979-06-29

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