Classifications

• • 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
• • 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

Definitions

• This invention relates to conductive polymer compositions and electrical devices comprising them.
• Conductive polymer compositions which exh.ibit PTC (positive temperature coefficient of resistance) behavior"" are particularly useful for self-regulating strip heaters and circuit protection devices. These electrical devices utilize the PTC anomaly, i.e. an anomalous rapid increase in resistance as a function of temperature, to limit the heat output of a heater or the current flowing through a circuit.
• Compositions which exhibit PTC anomalies and comprise carbon black as the conductive filler have been disclosed in a number of references.
• U.S. Patent No. 4,237,441 discloses suitable carbon blacks for use in PTC compositions with resistivities less than 7 ohm-cm.
• a large number of carbon blacks are suitable for use in conductive compositions.
• the choice of a particular carbon black is dictated by the physical and electrical properties of the carbon black and the desired properties, e.g. flexibility or conductivity, of the resulting composition.
• the properties of the carbon blacks are affected by such factors as the particle size, the surface area, and the structure, as well as the surface chemistry. This chemistry can be altered by heat or chemical treatment, either during the production of the carbon black or in a post-production process, ' e.g. • by oxidation. Oxidized carbon blacks frequently have a low surface pH value, i.e. less than 5.0, and may have a relatively high volatile content.
• oxidized carbon blacks When compared to nonoxidized carbon blacks of similar particle size and structure, oxidized carbon blacks have higher resistivities. It is known that carbon blacks which are oxidized provide improved flow characteristics in printing inks, improved wettability in certain polymers, and improved reinforcement of rubbers.
• this invention provides an electrical device which comprises
• a PTC element comprising a conductive polymer composition which exhibits PTC behavior, which has a resistivity at 20°C R C pr and which comprises
• the electrical device having a resistance Ri at 20°'C and being such that if the device is maintained at a temperature equal to T m for a period of 50 hours and is then cooled to 20°C, its resistance at 20°C, Rfso** is from 0.25Ri to 1.7.5Ri ⁇
• the physical and electrical properties of the carbon black may be used to determine suitable fillers for use in compositions of the invention. Therefore, in a second aspect the invention provides a conductive polymer composition which exhibits PTC behavior and which comprises
• the carbon blacks useful in the conductive polymer compositions of this invention have pH values of less than 5.0, preferably less than 4.0, particularly less than 3.0.
• the pH is a measure of the acidity or alkalinity of the carbon black surface.
• a pH of 7.0 indicates a chemically neutral surface; values less than 7.0 are acidic, those higher than 7.0 are basic.
• Low pH carbon blacks generally have a relatively high volatile content, volatile content being a measure of the amount of chemisorbed oxygen which is present on the surface of the carbon black. The amount of oxygen can be increased by oxidation in a post-production process. The resulting carbon black will have ' a higher surface activity.
• the terms "low pH carbon black” and "oxidized carbon black” are used as equivalent terms.
• the pH of the carbon black is that which is measured prior to mixing the carbon black with the polymer.
• the low pH carbon blacks of this invention are used in conductive polymer compositions which exhibit PTC (positive temperature coefficient) behavior in the temperature range of interest when connected to a source of electrical power.
• PTC behavior and “composition exhibiting PTC behavior” are used in this specification to denote a composition which has an R14 value of at least 2.5 or an R 100 value of at least 10, and preferably both, and .
• R ⁇ _4 is the ratio of the resistivities at the end and the beginning of a 14°C range, 100 ⁇ - s 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.
• ZTC behavior is used to denote a composition which increases in resistivity by less than 6 times, preferably less than 2 times in any 30°C temperature range within the operating range of the heater.
• Carbon blacks with suitable size, surface area and structure for use in PTC compositions are well-known. Guidelines for selecting such carbon blacks are found in U.S Patent Nos. 4,237,441 (van Konynenburg et al. ) and 4,388,607 (Toy et al. ) . In general, carbon blacks with a relatively large particle size, D (measured in nanometers), e.g. greater than 18 run, and relatively high structure', e.g. greater than about 70 cc/100 g, are preferred for PTC compositions.
• D measured in nanometers
• structure' e.g. greater than about 70 cc/100 g
• Carbon blacks which are particularly preferred for compositions of the invention are those which meet the criteria that the ratio of the resistivity of the carbon black (in powder form) to the particle size (in nanometers) is less than or equal to 0.1, preferably less than or equal to 0.09, particularly less than or equal to 0.08.
• the resistivity of the carbon black in ohm-cm is determined by following the procedure described in Columbian Chemicals Company bulletin "The Dry Resistivity of Carbon Blacks" (AD1078). In this test, 3 grams of carbon black are placed inside a glass tube between two brass plungers. A 5 kg weight is used to compact the carbon black. Both the height of the compacted carbon black and the resistance in ohms between the brass plunger electrodes are noted and the resistivity is calculated.
• the ratio is useful for carbons which are tested in their powder, not pelletized, form. While most nonoxidized carbon blacks fulfill the require ⁇ ments of this ratio, the carbon blacks particularly useful in this invention are those which both meet the ratio and have a pH of less than 5.0.
• conductive fillers may be used in combination with the designated carbon black. These fillers may comprise nonoxidized carbon black, graphite, metal, metal oxide, or any combination of these.
• a nonoxidized carbon black i.e. a carbon black with a pH of at least 5.0
• the pH of the nonoxidized carbon black be at least 1.0 pH unit greater than the pH of the oxidized carbon black.
• the low pH carbon black be present at a level of at least 5% by weight, preferably at least 10% by weight, particularly at least 20% by weight of the total conductive filler r e.g. 25 tq 100%*••• by weight of the total conductive filler.
• the presence of the solvent is neglected and the content of the solid components, e.g. carbon black and polymer, is considered the total composition.
• nonoxidized carbon blacks may be treated, e.g. by heat or appropriate oxidizing agents, to produce carbon blacks with appropriate surface chemistry.
• the conductive polymer composition comprises an organic polymer which has a crystallinity of at least 5%, preferably at least 10%, particularly at least 15%, e.g. 20 to 30%.
• Suitable crystalline polymers include polymers of one or more olefins, particularly polyethylene; polyalkenamers such as polyoctenamer; copolymers of at least one olefin and at least one monomer copolymerisable therewith such as ethylene/acrylic acid, ethylene/ethyl aerylate, and ethylene/vinyl acetate copolymers; melt-shapeable fluoro- polymers such as polyvinylidene fluoride, ethylene/tetra- fluoroethylene copolymers, and terpolymers of vinylidene fluoride, hexafluoropropylene, and tetrafluoroethylene; and blends of two or more such polymers.
• fluoropolymer is used herein to denote a polymer which contains at least 10%, preferably at least 25%, by weight of fluorine, or a mixture of two or more such polymers.
• fluoropolymer is used herein to denote a polymer which contains at least 10%, preferably at least 25%, by weight of fluorine, or a mixture of two or more such polymers.
• the blend must have a crystallinity of at least 5%.
• the crystallinity, as well as the melting point T m are determined from a DSC(differential scanning calorimeter) trace on the conductive polymer composition.
• the T m is defined as the temperature at the peak of the melting curve. If the composition comprises a blend of two or more poly ⁇ mers, T m is defined as the lowest melting point measured for the composition (often corresponding to the melting point of the lowest melting component).
• the composition may comprise additional components, e.g. inert fillers, antioxidants, flame retardants, prorads, stabilizers, dispersing agents.
• Mixing may be conducted by any suitable method, e.g. melt-processing, sintering, or solvent-blending. Solvent-blending is particularly preferred when the conductive polymer composition comprises a polymer thick film ink.
• the composition may be cross- linked by irradiation or chemical means.
• the conductive polymer composition of the invention is used as part of a PTC element in an electrical device, e.g. a heater, a sensor, or a circuit protection device.
• the resistivity of the composition is dependent on the function of the electrical device, the dimensions of the PTC element, and the power source to be used.
• the resistivity may be, for example, from 0.01 to 100 ohm-cm for circuit protection devices which are powered at voltages from 15 to 600 volts, 10 to 1000 ohm-cm for heaters powered at 6 to 60 volts, or 1000 to 10,000 ohm-cm or higher for heaters powered at. voltages of at least 110 volts.
• the PTC element may be of any shape to meet the requirements of the application.
• Circuit protection devices and laminar heaters frequently comprise laminar PTC elements, while strip heaters may be rectangular, elliptical, or dumbell- ("dogbone-") shaped.
• the PTC element may be screen-printed or applied in any suitable configuration.
• Appropriate electrodes, suitable for connection to a source of electrical power, are selected depending on the shape of the PTC element. Electrodes may comprise metal wires or braid, e.g. for attachment to or embedment into the PTC element, or they may comprise metal sheet, metal mesh, conductive (e.g. metal- or carbon-filled) paint, or any other suitable material.
• the electrical devices of the invention show improved stability under thermal aging and electrical stress.
• the resistance at 20°C measured after aging i.e. fso * - will differ from the initial resistance at 20°C, i.e. Rj., by no more than 75%, preferably no more than 60%, particularly no more than 50%, producing an Rfso of from 0.25 i to 1.75 R , preferably from 0.40 Ri to 1.60 R , particularly from 0.50 i to 1.50 R ⁇ .
• the change in resistance will be less than 50%, preferably less than 40%, particularly less than 30%, producing a resistance at 20°C after 300 hours, Rf300 / of from 0.50 Ri to 1.50 R , preferably from 0.60 Ri to 1.40 Ri, particularly from 0.70 Ri to 1.30 Ri. It is to be understood that if a device meets the resistance require ⁇ ment when tested at a temperature greater than T m , it will also meet the requirement when tested at T m . Similar results will be observed when the device is actively powered by the application of voltage. The change in resistance may reflect an increase or decrease in device resistance.
• the resistance will first decrease and then increase during the test, possibly reflecting a relaxation of mechanically-induced stresses followed by oxidation of the polymer.
• Particularly preferred compositions comprising fluoropolymers may exhibit stability which is better than a 30% change in resistance.
• the invention is illustrated by the following examples.
• an ink was prepared by blending the designated percent by weight (of solids) of. the appropriate carbon black with dimethyl formamide in a high shear mixer.
• the solution was then filtered and powdered KynarTM 9301 (a terpolymer of vinylidene fluoride, hexafluoropropylene r and tetrafluoroethylene with a melting point of about 88°C, available from Pennwalt) in an amount equal to (100 - % car ⁇ bon black) was added to the filtrate and allowed to dissolve over a period of 24 to 72 hours. (Approximately 60% solvent and 40% solids was used in making the ink).
• ElectrodeagTM 461SS available from Acheson Colloids
• Samples of each ink were aged in ovens at temperatures of -65, 85, 107 and 149°C. Periodically, the samples were removed from the oven and the resistance at room temperature (nominally 20°C), R , was measured. Normalized resistance, R n , was determined by dividing R by the initial room temperature resistance, Ri. The extent of instability was determined by the difference between R n and 1.00.
• Those inks which comprised carbon blacks with a pH of less than 5 were generally more stable than the inks comprising higher pH blacks.
• Wt% CB indicates the percent by weight of carbon black used in each ink.
• inks were prepared using KynarTM 9301 as a binder and incorporating the carbon blacks listed in Table IV.
• the resistance vs. temperature characteristics were measured by exposing samples of each ink to a temperature cycle from 20°C to 82°C.
• the height of the PTC anomaly was determined by dividing the resistance at 82°C R ⁇ 2> by the resistance, at 20°C (R20- 1 - It was apparent that at comparable resistivity values the PTC anomaly was higher for the oxidized carbon blacks than for the nonoxidized carbon blacks.
• (1) D represents the particle size of the carbon black in run.
• S.A. represents the surface area of the carbon black in m 2 /g as measured by a BET nitrogen test.
• DBP is a measure of the structure of the carbon black and is determined by measuring the amount in cubic centimeters of dibutyl phthalate absorbed by 100 g of carbon black.
• Wt% represents the percent by weight of the total solids content of the ink that is carbon black.
• Rho is the resistivity of the ink in ohm-cm..
• PTC Height is the height of the PTC anomaly as determined by R ⁇ 2/ R 20• -
• RCB is tne drv resi stivity of the carbon black in powder form under a 5 kg load.
• RCB D is the ratio of the dry resistivity of the carbon black to the particle size.
• Fibers were prepared by blending 55% by weight ElvaxTM 250 (ethylene vinyl acetate copolymer with a melting point of 60°C, available from Dow) and 45% by weight RavenTM 22 (carbon black with a pH of 7.0, a particle size of 62 nm, a surface area of 25 m 2 /g, and a DBP of 113 cc/100 g, available from Columbian Chemicals).
• An ink was prepared by dissolving the fibers in xylene. After 813 hours at 52°C, the. R n value was 0.94. ' . -. -.
• Fibers were prepared from 76% by weight PFATM 340 (a copolymer of tetrafluoroethylene and a perfluorovinyl ether with a T m of 307°C, available from du Pont) and 24% by weight RavenTM 600 (carbon black with a pH of 8.3, particle size of 65 nm, DBP of 82 cc/lOOg, and surface area of 34 m 2 /g, available from Columbian Chemicals) as in Example 15. Samples tested at 311°C for 50 hours had an R n of 0.55.
• PFATM 340 a copolymer of tetrafluoroethylene and a perfluorovinyl ether with a T m of 307°C, available from du Pont
• RavenTM 600 carbon black with a pH of 8.3, particle size of 65 nm, DBP of 82 cc/lOOg, and surface area of 34 m 2 /g, available from Columbian Chemicals

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• Engineering & Computer Science (AREA)
• Microelectronics & Electronic Packaging (AREA)
• Ceramic Engineering (AREA)
• Physics & Mathematics (AREA)
• Electromagnetism (AREA)
• Chemical & Material Sciences (AREA)
• Dispersion Chemistry (AREA)
• Compositions Of Macromolecular Compounds (AREA)
• Conductive Materials (AREA)
• Thermistors And Varistors (AREA)
• Inks, Pencil-Leads, Or Crayons (AREA)

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Citations (6)

• 1989
  • 1989-09-15 EP EP97201655A patent/EP0803879B1/de not_active Expired - Lifetime
  • 1989-09-15 AT AT89910755T patent/ATE161354T1/de not_active IP Right Cessation
  • 1989-09-15 JP JP1510152A patent/JP2876549B2/ja not_active Expired - Lifetime
  • 1989-09-15 WO PCT/US1989/004010 patent/WO1990003651A1/en active IP Right Grant
  • 1989-09-15 DE DE68929517T patent/DE68929517T2/de not_active Expired - Lifetime
  • 1989-09-15 DE DE68928502T patent/DE68928502T2/de not_active Expired - Fee Related
  • 1989-09-15 EP EP89910755A patent/EP0435923B1/de not_active Expired - Lifetime
  • 1989-09-15 AT AT97201655T patent/ATE262725T1/de not_active IP Right Cessation
  • 1989-09-15 KR KR1019900701040A patent/KR100224945B1/ko not_active IP Right Cessation
  • 1989-09-19 CA CA000611894A patent/CA1334480C/en not_active Expired - Fee Related
• 1998
  • 1998-08-12 JP JP10227984A patent/JP2955281B2/ja not_active Expired - Fee Related
  • 1998-12-21 HK HK98114530A patent/HK1021613A1/xx not_active IP Right Cessation

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