EP0371059A1 - Composition polymere conductrice - Google Patents

Composition polymere conductrice

Info

Publication number
EP0371059A1
EP0371059A1 EP19880906793 EP88906793A EP0371059A1 EP 0371059 A1 EP0371059 A1 EP 0371059A1 EP 19880906793 EP19880906793 EP 19880906793 EP 88906793 A EP88906793 A EP 88906793A EP 0371059 A1 EP0371059 A1 EP 0371059A1
Authority
EP
European Patent Office
Prior art keywords
filler
polymer
composition
matrix
resistivity
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Withdrawn
Application number
EP19880906793
Other languages
German (de)
English (en)
Inventor
Pradeep Barma
Chi-Ming Chan
Manoochehr Mohebban
Nachum Rosenzweig
Eugen L. Kurjatko
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Raychem Corp
Original Assignee
Raychem Corp
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Priority claimed from US07/075,929 external-priority patent/US5106540A/en
Application filed by Raychem Corp filed Critical Raychem Corp
Publication of EP0371059A1 publication Critical patent/EP0371059A1/fr
Withdrawn legal-status Critical Current

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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
    • H01C7/06—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 including means to minimise changes in resistance with changes in temperature
    • C—CHEMISTRY; METALLURGY
    • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
    • C08K—Use of inorganic or non-macromolecular organic substances as compounding ingredients
    • C08K3/00—Use of inorganic substances as compounding ingredients
    • C08K3/01—Use of inorganic substances as compounding ingredients characterized by their specific function
    • C08K3/013—Fillers, pigments or reinforcing additives
    • C—CHEMISTRY; METALLURGY
    • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
    • C08K—Use of inorganic or non-macromolecular organic substances as compounding ingredients
    • C08K9/00—Use of pretreated ingredients
    • C08K9/10—Encapsulated ingredients
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01B—CABLES; CONDUCTORS; INSULATORS; SELECTION OF MATERIALS FOR THEIR CONDUCTIVE, INSULATING OR DIELECTRIC PROPERTIES
    • H01B1/00—Conductors or conductive bodies characterised by the conductive materials; Selection of materials as conductors
    • H01B1/20—Conductive material dispersed in non-conductive organic material
    • H01B1/22—Conductive material dispersed in non-conductive organic material the conductive material comprising metals or alloys
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01B—CABLES; CONDUCTORS; INSULATORS; SELECTION OF MATERIALS FOR THEIR CONDUCTIVE, INSULATING OR DIELECTRIC PROPERTIES
    • H01B1/00—Conductors or conductive bodies characterised by the conductive materials; Selection of materials as conductors
    • H01B1/20—Conductive material dispersed in non-conductive organic material
    • H01B1/24—Conductive material dispersed in non-conductive organic material the conductive material comprising carbon-silicon compounds, carbon or silicon
    • 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 comprising a particulate conductive filler which is distributed in an organic polymer.
  • Conductive compositions comprising a particulate conductive filler distributed in an organic polymer (this term being used herein to include polysiloxanes) are known. Such compositions are known as "conductive polymer compositions". Documents describing conductive polymer compositions and devices comprising them include U.S. Patents Nos.
  • Conductive polymer compositions can be used as current-carrying com ⁇ ponents, eg. in heaters and circuit protection devices, as shielding or stress-grading components for high voltage cables and other high voltage electrical equipment, and as antistatic materials. They may exhibit what is known as PTC (positive temperature coefficient), ZTC (zero temperature coefficient) or NTC (negative temperature coefficient) behavior.
  • PTC behavior is used in this specification to denote a composition which, in the operating temperature range, eg. 0° to 200°C, has an R t.
  • R14 is the ratio of the resistivities at the end and the beginning of the 14°C temperature range showing the greatest increase in resistivity
  • R30 is the ratio of the resistivities at the end and the beginning of the 30°C temperature range showing the greatest increase in resistivity.
  • NTC behavior is used in this specification to denote a composition which does not show PTC behavior in the operating temperature range, and whose resistivity at 0°C is at least 2 times, preferably at least 5 times, its resistivity at a higher temperature in the operating range.
  • ZTC behavior is used in this specification to denote a composition which does not show either PTC behavior or NTC behavior; ZTC compositions can exhibit PTC behavior at temperatures above the operating temperature range of the composition.
  • the conventional method of preparing conductive polymer compositions comprises dispersing a homogeneous conductive particulate filler in a heated polymeric matrix (the term "homogeneous filler” is used herein to denote a filler in which each particle has a single phase, e.g. carbon black, graphite, a metal, a metal oxide, a ceramic or another conductive inorganic material).
  • This conventional method can be used to make a wide variety of products.
  • a graph of the log of the resistivity of the- composition against the volume per cent of the filler invariably has a short relatively flat upper portion corresponding to the resistivity of the matrix polymer and then falls steeply until it flattens out as the resis ⁇ tivity of the composition approaches an asymptotic value.
  • Such a loading curve is shown as Curve 1 of Figure 1. If the desired resistivity falls on the steep portion of the loading curve, the resistivity of the product can change very significantly if there are small changes in the process conditions or the starting materials.
  • a resistivity on the steep part of the loading curve is desired where the conductive polymer is a carbon black loaded, polymeric PTC composition for use in a PTC heater which comprises a laminar PTC heating element sandwiched between laminar electrodes and which is powered by a relatively high voltage (typically greater than 100V) power source.
  • a resistivity (at 23°C) of I0**-- to 10*-- ohm.cm is desirable, inter alia to avoid high and damaging "ih-rush" currents when the composition is first powered.
  • Another known method of preparing a conductive polymer composition is to dry blend carbon black and a powdered polymer, and to sinter the resulting blend so that the polymer particles coalesce but do not lose their identity.
  • Such methods are very useful for the production of ZTC conductive polymers based on polymers which cannot be melt processed, e.g. ultra-high molecular weight polyethylene (see for example Serial No. 720,11.7, Docket No. MP0922-US2), but are not otherwise widely used.
  • U.S. Patent No. 3,591,526 (Kawashima) and Japanese Patent Publication Nos. 51-32983 and 51-32984 disclose conductive polymer compositions in which the conductive filler is not a homogeneous material, but rather is a composite filler made by melt-blending carbon black with a thermoplastic polymer to make a PTC composition, and then reducing the blend to finely divided form.
  • the composite fillers disclosed in these references contain high loadings of the carbon black, and the compositions contain high loadings of the composite filler. Consequently the compositions have low resistivities both on an absolute scale (for carbon black containing con ⁇ ductive polymers), i.e.
  • compositions are disclosed as being useful as resistors.
  • the particulate conductive filler is at least in part a composite filler, this term being used herein to denote a particulate con ⁇ ductive filler in which each particle comprises an organic polymer and, distributed therein, a homogeneous conductive filler.
  • the conductive polymer compositions in question comprise
  • a continuous matrix comprising a first organic polymer (also referred to herein as the matrix polymer), and (b) a first particulate conductive filler (often referred to herein as the composite filler) which is distributed * in the matrix and main ⁇ tains its identity therein, and each particle of which comprises a second organic polymer (often referred to herein as the filler polymer) and a second particulate conductive filler which is distributed in the second polymer.
  • a first organic polymer also referred to herein as the matrix polymer
  • a first particulate conductive filler often referred to herein as the composite filler
  • the loading curve for a composite filler can have a different shape from the conventional loading curve described above; in particular, the steep part of the curve is interrupted by an intermediate portion which is much less steeply sloped and may be substantially " flat, for example has a slope between -0.25 and zero.
  • Such a loading curve is shown as Curve 2 in Figure 1.
  • Conductive polymer compositions which fall on or above the intermediate portion of such a loading curve, and conductive polymer compositions which form part of such a loading curve, particularly those in which the conductive filler is a composite filler, are novel and form part of the present invention.
  • the intermediate portion of the loading curve occurs at a value which is at least 100 times, generally at least 1,000 times, e.g. from 1,000 to 10,000 times the resistivity of the composite filler.
  • such a loading curve results from the use of a matrix polymer/conductive filler combination such that two different conduction mechanisms are possible, with one mechanism dominating the other (or being the only mechanism) at lower filler loadings above the intermediate portion and causing the resistivity to change rapidly, as the filler content is increased, until no more conductive paths can be set up by that mechanism, and the other mechanism providing relatively few (or no) conductive paths until the filler content reaches the higher levels below the intermediate portion, at which levels the other mechanism causes the resistivity again to change rapidly, as the filler content is increased, until no more conductive paths can be set up by that other mechanism, at which point the resistivity tends towards the resistivity of the filler itself.
  • the matrix can form at least a partial coating around a substantial proportion of the particles of the composite filler, for example through the choice of a matrix having a suitable spreading coefficient on the composite filler.
  • compositions comprising a composite filler can have non-linear properties, ie. a resistivity which is dependent on voltage stress, for example a resistivity which (over some useful ranges of voltage stress) decreases as the voltage stress increases, thus making the compositions very valuable as stress grading materials in those useful ranges of voltage stress.
  • Novel and useful compositions can be made through the use of composite fillers which exhibit ZTC or NTC behavior.
  • Novel and useful compositions can be made by distributing two or • more composite fillers, or a composite filler and one or more homogeneous fillers, in the matrix.
  • Novel and useful compositions can be made by distributing a par ⁇ ticulate filler (which may or may not be a composite filler) which changes its resistivity in response to a change in temperature, voltage stress or frequency, in a sintered polymer matrix.
  • a par ⁇ ticulate filler which may or may not be a composite filler
  • Novel and useful composite fillers can be made by mixing a homogeneous conductive filler, e.g. carbon black, with a cross-linkable polymer, preferably a thermoplastic polymer, under conditions which cause cross-linking of the polymer while the mixing is taking place and which result in a particulate product.
  • the ingredients which are mixed together preferably comprise a chemical cross-linking agent which decompose ⁇ at the mixing temperature.
  • Novel and useful compositions can be made by a process as described in (8) above in which a composite filler is used instead of a homogeneous filler, provided that the mixing is carried out un er * conditions such that the composite filler retains its identity.
  • the present invention provides a composition which comprises
  • the composition has a resistivity which is at least 100 times, preferably at least 1,000 times, e.g. 1,000 to 100,000, par ⁇ ticularly 1,000 to 10,000 times, the resistivity of the first filler; this condition reflects two facts, first that at such resistivities, the composition will be on or above any relatively flat intermediate part of the loading curve, and second that prior art compositions containing composite fillers have much lower resistivities;
  • the first particulate filler has a hot modulus (measured as ' described below) of at least 250 psi, preferably at least 350 psi, particularly at least 450 psi; this condition reflects the fact that the first filler is preferably extensively cross-linked, so that it can be subjected to mixing conditions which involve heating above the melting point of the second polymer, and/or extensive shearing, without allowing substantial amounts of the second conductive filler to escape into the matrix; (3) the first polymer has a viscosity, at a temperature above its melting or softening point, which is no more than 0.2 times the viscosity of the first filler at the same temperature; this condition reflects the fact that if the first filler is relatively viscous at at least some temperatures above the melting point of the first polymer, the first polymer can be melt-mixed with the first filler without the particles of the first filler losing their identity;
  • the second polymer has a melting or softening point which is at least 30°C higher, preferably at least 60°C higher, than the melting or softening point of the first polymer; this condition reflects the fact that if the melting or softening point of the second polymer is sufficiently above the melting or softening point of the first polymer, the first polymer can be melt-mixed with the first filler without the particles of the first filler losing their identity;
  • the number of particles of the second filler in the matrix is less than 450 particles, preferably less than 400 particles, per 100 micron- ⁇ ; the number of particles being measured by the procedure given in Example 1 below; this condition reflects the desirability of limiting the extent to which the particles of the second filler escape into the matrix;
  • composition comprises a third particulate filler which is distributed in the matrix; this condition reflects that very use ⁇ ful compositions can be made by adding one or more additional particulate fillers, which may be conductive or non-conductive;
  • the matrix comprises particles of the first organic polymer which have been sintered together without completely losing their identity; this condition reflects the fact that novel sintered compositions can be made through the use of composite fillers; (8) the first particulate filler exhibits ZTC or NTC behavior; this condition reflects the fact that novel compositions can be made through the use of composite fillers which ex biir ZTC ' or * NTC behavior; and
  • the composition is in the form of a heat-recoverable article, or is in the form of a flexible tape or sheet which is free from electrodes, or is secured to or contained by a substrate which is deformable from a first configuration to a second configuration, whereby the conductive composition can be applied to a second substrate, or is associated with high voltage equipment so that it can limit electrical stress in a region of high electric field strength;
  • this condition reflects the fact that, having discovered that compositions containing composite fillers are non-linear, it now makes sense to use such compositions to limit electrical stress in high voltage equipment, and to make such compositions into articles which can be used to apply such compositions to high voltage equipment.
  • the invention provides a composition which comprises
  • a first particulate conductive filler which is distributed in the matrix and maintains its identity therein, and which is preferably a composite filler
  • composition having a resistivity which is .less than 0.1 times the resistivity of the first polymer and at least 100 times the resistivity of the first filler, and the method by which said composition has been prepared, and the ingredients in said composition being such that, if
  • compositions are prepared by the same method and using the same ingredients except that the volume percent of the first filler is slightly greater or slightly smaller, (ii) the resistivities of said composition and of the other compositions are measured, and
  • the slope of the graph, at the volume per cent filler of said composition is between -0.25 and zero, preferably between -0.15 and zero.
  • the invention provides a composition which has a resistivity of at least 1,000 ohm.cm and which comprises
  • a first particulate conductive filler which is distributed in the matrix and maintains its identity therein, preferably a composite " filler
  • compositions are prepared by the same method and using the same ingredients except that the volume percent of conductive filler is varied from a very small amount to the maximum amount that can be distributed in the matrix,
  • a graph is made, for said composition and the other compositions, of log resistivity on the vertical axis against volume per cent of the first conductive filler on the horizontal axis, there are two distinct regions of the graph in which the resistivity is less than 0.1 times the resistivity of the first polymer and the slope of the graph is between -0.25 and zero, said distinct regions being separated by a region in which the slope of the graph is less than -0.5 and in which the resistivity changes by the smaller of at least K)3 ohm.cm and a factor of at least 100, preferably at least 1,000.
  • This aspect of the invention reflects the fact that compositions which form part of a loading curve having a relatively flat intermediate portion are novel and useful.
  • the present invention provides a composite par ⁇ ticulate conductive filler in which each particle has a size less than 10 mesh, preferably less than 60 mesh, particularly less than 80 mesh (mesh sizes are standard sieve sizes), and comprises an organic polymer and a second particulate conductive filler which is distributed in the organic polymer, said composite filler having at least one of the following properties:
  • the polymer can be melt-processed and has a melting or softening point of at least 250°C;
  • This aspect of the invention reflects the fact that many of the composite fillers used in making the compositions of the invention are novel per se by virtue of possessing one or more of the. properties (1) to (4) enumerated above.
  • the present invention provides a process for preparing a composition which comprises mixing together
  • »2 is the surface tension of the first filler under the mixing conditions
  • the amount of the first filler is such that the composition has a resistivity which is less than 0.1 times the resistivity of the first polymer and at least 100 times the resistivity of the first filler;
  • the first particulate filler has a hot modulus of at least 250 psi
  • the first polymer has a viscosity which is no more than 0.2 times the viscosity of the first filler; (4) the temperature is lower than the melting or softening point of the second polymer;
  • a third particulate filler is also mixed with the first polymer
  • the process comprises mixing the first filler with particles of the first organic polymer and then sintering the mixture so that the particles of the first polymer are sintered together without completely losing their identity;
  • the first particulate filler exhibits ZTC or NTC behavior.
  • this aspect of the invention reflects, in terms appropriate to the preparation of the compositions, the same conditions as in the first aspect of the invention.
  • Condition (C) reflects a different consideration, namely the desirability of using a matrix polymer whose spreading coefficient, ie. the quantity 2 - ( + & ⁇ 2 ), is such that, according to the theory widely employed in adhesives technology, the matrix polymer will coat the first filler particles. It may be noted that the spreading coefficient is to be ascertained under the particular process conditions. However, the stated condition is usually met under the mixing condiitions if it is met at about 23°C, at which temperature the surface and interfacial tensions can be ascertained from the literature or can be more easily measured.
  • the present invention provides a process for the preparation of a composition which comprises
  • step (3) cross-linking and comminuting the extrudate from step (2) to form a composite particulate conductive filler
  • step (3) dispersing the composition from step (3) in a second organic polymer, which is preferably compatible with the first polymer;
  • ingredients of the composition and the conditions of the process being such that, if
  • a graph is made for said composition and for the other com ⁇ positions of log resistivity on the vertical axis against cross- linking level on the horizontal axis, the cross-linking level being expressed in Mrads if the cross-linking is effected by ionising radiation and in weight percent of the cross-linking agent if the cross-linking is effected by a chemical cross-linking agent,
  • the slope of the graph, at the cross-linking level in step (3) is between -0.25 and zero, preferably between -0.15 and zero.
  • This aspect of the invention reflects the fact that more reproduceable results are obtained if the composite filler has been extensively cross-linked.
  • the invention provides a process for the preparation of a composition which comprises
  • step (3) comminuting the extrudate from step (2) to form the first filler
  • step (4) subjecting the blend obtained in step (4) to heat and pressure such that
  • the particles of the first polymer are sintered together so that they coalesce but do not completely lose their identity, with the first filler particles being present substantially only at the boundaries of the coalesced particles, or
  • the invention reflects the fact that very useful results can be obtained through processes in which the composite filler is not subjected to shearing, since shearing promotes migration of the second filler from the first filler into the matrix.
  • the invention provides a conductive composition which comprises.
  • the conductive composition as a whole also changes its resistivity in response to the change in the variable.
  • This aspect of the invention reflects the fact that the sintering process represents a useful route to the manufacture of a wide range of novel compositions.
  • the invention provides a composition which comprises
  • first particulate conductive filler has been crosslinked under dynamic mixing conditions.
  • This aspect of the invention reflects the fact that the first particulate filler can be easily produced by chemically crosslinking at an elevated temperature under * relatively high shear conditions.
  • the invention further includes electrical devices which comprise at least one electrode, and usually two, and a conductive polymer composition • as defined through which current passes in use of the device.
  • Figure 1 is a graph diagrammatically showing the loading curves for a composition according to the present invention (Curve 2) and a composition of the prior art (Curve 1),
  • Figure 2 is a schematic cross-section through a composition according to the invention, comprising composite filler particles 4 and third conductive filler particles 6 distributed in a matrix 5,
  • Figure 3 is a graph of log resistivity against radiation cross- linking level for compositions in which the only variable is the degree of cross-linking of the composite filler
  • Figure 4 is a graph of the number of particles of the second filler which have migrated into the matrix against radiation cross- linking level for compositions in which the only variable is the degree of cross-linking of the composite filler, and
  • Figure 5 is a schematic cross-section through a sintered composition of the invention showing composite filler particles 4 and a third conductive filler particles 6 distributed around the peripheries of sintered matrix polymer particles 7.
  • the hot. modulus values referred, to herein are measured at 150°C for polymers which do not have a melting point and at a temperature 20°C above the melting point (i.e. the peak of a differential scanning calorimeter curve) for polymers having a melting point.
  • the test employed measures the stress required to elongate a sample by 100% (or to cause it to break) ' and the modulus (or M * Q0 Value) is calculated from
  • the hot modulus value of the composite filler cannot be measured directly on a composition of the invention. However, providing that the composition is not cross-linked after the matrix polymer and the composite filler have been mixed, and providing that the composite filler, if it is cross-linked, is cross-linked prior to grinding, the hot modulus value can be ascertained directly from the starting material, since it will not be changed by the mixing and shaping process. In other circumstances, or if the starting material is not available, the hot modulus value can be ascertained indirectly by designing one or more test processes which will make an article which can be tested and which has a substantially identical composition to the composite filler and by measuring the hot modulus value of the products of those test processes.
  • the loading curves referred to herein are drawn in a conventional way, with the logarithm of the resistivity on the vertical axis and the volume percent of the filler on the horizontal axis.
  • the logarithms given herein are logarithms to the base 10.
  • resistivities referred to herein are measured at 23°C and at a field strength of 100 volts/cm, using a pulse technique with a pulse duration of 100 microseconds, and a repetition rate of one per second; where the resistivity of" one composition is expressed in terms of the resistivity of another composition, or the resistivities of different com ⁇ positions are compared, both (or all) of the resistivities must be measured at the same field strength.
  • the results of comparing resistivi ⁇ ties are also true at field strengths of 1 volt/cm and/or 10 volt/cm.
  • the method of choice will be to cross-link the second conductive filler.
  • any combination of matrix and filler polymer including matrix and filler polymers which are compatible with each other, in par ⁇ ticular the same, and to shape the composition by any process, including processes in which the molten composition is subjected to shear forces, e.g. melt-extrusion, injection molding and flow molding.
  • the composite filler used in this invention is preferably made ' by preparing an intimate mixture comprising the second polymer and at least one homogeneous conductive particulate filler, and optionally one or more other ingredients (e.g. non-conductive fillers, antioxidants, chemical cross-linking agents, radiation cross-linking agents, etc.) cross-linking the mixture (which increases its hot-modulus), and grinding or otherwise comminuting the mixture.
  • suitable mixtures include those disclosed in the documents incorporated by reference therein.
  • one or both of the first and second polymers has a melting point of at least 250°C, for example is a pol (aryl)etherketone or a polyetheretherketone or another polyarylene.
  • Such polyarylenes can be cross-linked with the aid of sulfur (see for example U.S. Patent No. 4,616,056, Chan et al., the disclosure of which is incorporated herein by reference).
  • the mixing is preferably carried out by a process which comprises blending the homogeneous filler with the hot filler polymer, e.g. in a melt-extrusion apparatus or on a mill.
  • Preferably the comminution of the mixture is carried out after the* cross-linking.
  • Cross-linking can be effected by chemical cross-linking, or by irradiation with electrons or gamma rays, or otherwise, depending on the polymer employed.
  • the cross-linking is pre ⁇ ferably such that the cross-linked composite filler has a hot modulus of at least 250 psi (17.5 kg/cm 2 ), particularly at least 350 psi (24.5 kg/cm 2 ), especially at least 450 psi (31.5 kg/cm 2 ).
  • the cross-linking is substantially uniform throughout the filler.
  • the cross-linking level is such that it lies on a relatively flat part of a graph of log resistivity against hot modulus, and/or on a relatively flat part of a graph of log resistivity against cross-linking level (expressed in Mrads if the cross-linking is effected by ionising radiation and in weight percent of cross-linking agent if the cross-linking is effected by a chemical cross-linking linking agent), such relatively flat part preferably having a slope of more than -0.5, e.g. between -0.5 and zero, particularly more than -0.25, especially more than -0.15.
  • Comminution of the mixture can be carried out in any convenient way, including cryogenic grinding, and is preferably such that the average particle size (and more preferably the maximum particle size) of the composite filler, is less than 425 microns, eg. 100 to 425 microns.
  • the comminuted mixture can be sieved eg. through an 80 mesh screen, to ensure that particles over a particular size are excluded.
  • a chemical crosslinking agent it may be possible to achieve the crosslinking and comminution steps in one step by crosslinking under dynamic mixing conditions.
  • crosslinking agent may be incorporated during the mixing of the second polymer with the second particulate conductive filler or it may be blended with pellets of previously mixed second polymer- and second particulate filler and the blend, then mixed under * conditions which cause decomposition of the cross-linking agent.
  • chemical crosslinking agent i.e. at " least 3% by weight, preferably at least 4% by weight, particularly at least 5% by weight, based on the weight of the total composition, the extent of crosslinking will be such that the material will be brittle.
  • the level of crosslinking agent is substantially higher than the 1-2% conventionally used for crosslinking.
  • the blend of chemical crosslinking agent, second polymer and second particulate filler is mixed at elevated temperature under dynamic conditions for sufficient time for the crosslinking to reach a sufficiently high level, the resulting mixture will fragment. Under sufficient shear conditions, the fragments will be ground into a coarse powder.
  • the powder may be used without further treatment as the first particulate conductive filler, or it may be further ground to produce a finer particulate, or otherwise modified.
  • useful first particulates have been produced by mixing the second polymer, second particulate filler, and chemical crosslinking agent in a Banbury mixer, although other relatively high shear dynamic mixing equipment may also be used.
  • the proportion of homogeneous conductive filler in the composite filler can vary widely, but is preferably selected so that it lies on a relatively flat part of a loading curve for the homogeneous filler in the second polymer, preferably a part of the graph whose slope is more than -0.5, particularly more than -0.3.
  • the filler polymer and the homogeneous filler should be selected having regard to the desired temperature/resistivity relationship (e.g. PTC or ZTC) in the composite filler and in the final product, as disclosed in the documents incorporated herein by reference.
  • PTC desired temperature/resistivity relationship
  • higher carbon black loadings e.g. 40 to 60% by weight, are. preferred.
  • the filler polymer and the matrix polymer should be selected having regard to the desired physical, electrical and chemical properties of the product. Preferably they are compatible with each other (i.e. are completely miscible over a wide range of proportions when both polymers are uncross-linked).
  • the two polymers preferably comprise similar or identical substituents, e.g. polar groups, and/or similar or identical repeating units; each polymer contains for example at least 25 mole %, preferably at least 50 mole %, particularly at least 80 mole %, of the same repeating unit. It is particularly preferred that the two polymers should be chemically identical, e.g. both the filler polymer and the matrix polymer are polyethylene.
  • each of the filler polymer and the matrix polymer should be a crystalline thermoplastic.
  • both the filler polymer and the matrix polymer should be an elastomer.
  • the composite conductive filler can also be a filler obtained by comminuting a composition of the invention comprising a matrix filler and a composite conductive filler comprising a homogeneous conductive filler.
  • a composition of the invention comprising a matrix filler and a composite conductive filler comprising a homogeneous conductive filler.
  • the average particle size of the further conductive filler, if present, is preferably at least 1 ⁇ m, e.g. 5 to 100 nm.
  • an inert filler e.g. zinc oxide, titanium dioxide, glass, alumina or other material, an organic material, e.g. a polymer which is incompatible with the second polymer, or additional conductive filler may be added to the composite conductive filler.
  • additional filler increases the brittleness of the composite filler and improves the ease of grinding, particularly for soft or elastomeric polymers or those which have low melting temperatures.
  • Particulates which are produced from compositions comprising an inert filler often have a uniform size and shape when ground * .
  • the amount of composite filler which is present in the compositions of the invention can vary widely, particularly if the composition also has a homogeneous conductive filler distributed therein.
  • the conductive filler content is such that it lies on a relatively flat part of the loading curve, preferably a part of the graph whose slope is more than -0.5, particularly more than -0.3, especially more than -0.25, e.g. more than -0.15.
  • the content thereof may be for example 40 to 80% by weight, preferably 55 to 75% by weight.
  • the content thereof is preferably at least 20% by volume.
  • the content of composite filler may for example be 1 to 40% by volume, preferably 15 to 25% by volume, and the content of homogeneous filler may be for example up to 10% by volume, e.g. 3 to 5% by volume.
  • the composition can also contain other ingredients distributed in the matrix polymer, eg. one or more non-conductive fillers.
  • the composition if it is to be used for stress control, it-may contain one or more ingredients appropriate to such compositions such as for example silicon carbide, iron oxide, aluminum flakes, carbon black and the other fillers referred to in U.S. Patent No. 4,470,898 incorporated herein by reference.
  • the composite filler is preferably dispersed in the first polymer by melt-mixing, and the mixture is shaped by melt-shaping.
  • the dry blend of first polymer particles and composite filler is preferably formed into a layer on a substrate which supports the layer while the first polymer is molten.
  • the substrate is preferably a sheet electrode, eg- a metal foil.
  • a second sheet electrode eg. a metal foil, is placed on- top or " the layer-prior to subjecting the assembly to heat and pressure.
  • the foil is preferably one having a rough surface adjacent the conductive polymer, as disclosed in Serial No.
  • steps 787,218 can be accomplished on a continuous basis by drawing the sheet electrodes from rolls, applying the dry blend to one of the electrodes, and then passing the electrode bearing the layer and the other electrode through heated nip rollers.
  • an insulating apertured separating member is preferably included between the electrodes, for example is placed on the electrode before the dry blend is applied thereto.
  • composition After the composition has been shaped, it can if desired be cross-linked, preferably by irradiation, in order to improve its electrical and mechanical stability, particularly at elevated temperatures.
  • This section is concerned with the compositions made by sintering a dry blend of the matrix polymer and the composite filler.
  • the conductive particles are present substantially only at the particle boundaries. This means that when an electric current is passed through the composition, ohmic heating occurs substantially only at the boundaries and not inside the matrix particles. This enhances the thermal stability of the matrix and hence the composition as a whole. Also to achieve a desired conductivity level in a conductive polymer composition, less filler is required in a sintered composition, in which the filler is concentrated at the particle boundaries, than in a non-sintered composition, in which the filler is more uniformly dispersed.
  • the sintered composition according to the invention comprises a second particulate filler which (i) is conductive, and
  • first particulate filler and the second particulate filler are stated herein to be present substantially only at or near the boundaries of the coalesced particles, and in preferred compositions they are the sole conductive particles in the composition.
  • the invention includes compositions which ' contain additional conductive particles; for example, the sintered particles themselves may comprise particles which are composed of an organic polymer having conductive particles uniformly dispersed therein, and those conductive particles can be the same as or different from either the first or the second particulate filler.
  • the composition When the first particulate filler changes its resistivity in response to a change in temperature of another variable, the composition also changes its resistivity.
  • the resistivity change of the overall composition may be the same as, or different from, the change in resistivity of the first particulate filler.
  • the composition preferably also shows a sharp change . in its overall resistivity at the same value of the variable.
  • the first particulate filler may comprise any suitable material which changes its resistivity in response to a change in a variable.
  • the filler comprises a conductive composition composed of a conductive polymer comprising a conductive filler dispersed in a crystalline organic polymer.
  • the conductive polymer of the filler is one which exhibits PTC behavior.
  • Such a filler can be used in a composition which changes its resistivity in response to a change in temperature.
  • the first particulate filler (a) is composed of a conductive polymer which exhibits PTC behavior with a switching temperature, which can be designated T s particle, and (b) maintains its integrity within the matrix at temperatures up to T s particle and above.
  • the conductive polymer composition of the first particulate filler comprises carbon black dispersed in a crystalline organic polymer.
  • the first filler comprises a non polymeric material which exhibits PTC behavior, for example barium titanate.
  • a composition according to the invention containing a first particulate filler comprising another filler (for example carbon black) dispersed in an organic polymer may itself be pulverized, and used as the first particulate filler in another composition according to the invention. That new composition may itself be pulverized and used as the first filler in yet another composition according to the invention: and so on.
  • another filler for example carbon black
  • the first particulate filler may be obtained by any suitable route. In one preferred embodiment it is made by pulverizing a melt-extruded conductive polymer composition.
  • the sintered matrix polymer may comprise any suitable polymer.
  • polymers that may be used there may be mentioned poly- ethylenes, including polyethylenes having a weight average molecular weight in the range 50,000 to 8 million.
  • a preferred polymer to use is cross-linked polyethylene having a molecular weight of about 100,000, or polyethylene, either cross-linked or uncross-1inked, having a molecular weight in the range 3 to 6 million.
  • fluoropolymers for example poly- tetrafluoroethylene, polyvinylidene fluoride (Kynar), polyphenylene- sulfide, polyetherether ketones (PEEK), polyaryleneetherketones and polyimides.
  • the polymer therein which is designated the filler polymer
  • the filler polymer can be a single polymer or a mixture of polymers
  • the particulate conductive filler therein can be a single filler or a mixture of fillers.
  • Suitable conductive polymers are disclosed. in the documents incorporated by reference herein.
  • the s ntered'matrix polymer and the filler polymer are preferably compatible with each other. We have found that the greater the degree of compatibility, the more closely the change in resistance of the composition as a whole follows the change in resistivity of the filler. Particularly is this so when a second filler, e.g.
  • the filler polymer will "wet" the second filler.
  • Compatibility between the filler and matrix polymers can be achieved in ⁇ different ways, including in particular the use of polymer which comprise similar or identical substituents, e.g. polar groups, and/or repeating units.
  • the filler and matrix polymers can, for example, each comprise at least 25%, preferably at least 50%, particularly at least 80%, of the same repeating unit.
  • Particularly preferred compositions are those in which the filler and matrix polymers are chemically substantially identical, e.g. both are polyethylene; in this case, the filler and matrix polymers can be of the same or different molecular weights and one or both of them can be cross-!inked.
  • the first filler should preferably be selected so that the first filler maintains its physical identity in preparation and use of the composition sufficiently to ensure that the electrical characteristics of the composition remain substantially unchanged in use.
  • examples of combinations where different polymers are used are polyvinylidene fluoride based particles contained in a polytetrafluoroethylene matrix, polyphenylenesulfide based particles contained in a polyetheretherketone matrix, and polyetheretherketone based particles contained in a high molecular weight polyi ide matrix.
  • the first particulate filler can maintain its integrity within the matrix, even when it comprises a conductive polymer which is the same as the polymer of the matrix. If desired the integrity of each of the polymeric components can be enhanced by controlling the viscosities of each of the polymers. This may be achieved, for example, by cross-linking one or both of the polymers. Where the first particulate filler comprises a polymeric material, that, material may have a higher viscosity than the matrix polymer, or * vice versa.
  • each of the conductive fillers are chosen according to the desired resistivity.
  • the particle size of the second filler (when present) has an average dimension of at least 1 nm, particularly in the range 5 nm to 100 nm, and the first filler has an average dimension of at least 1 nm, particularly at least 20 microns.
  • The- average dimension of the first particulate filler is preferably no more than half the size of the sintered matrix particles.
  • the average dimension of the sintered matrix particles is in the range 200 to 500 microns.
  • the second filler (when present) constitutes no more than 10 volume per cent of the composition.
  • the first filler constitutes 1 to 40 volume per cent of the composition.
  • the first particulate filler When there is some second filler present, the first particulate filler preferably constitutes 15 to 25, especially about 20, volume per cent of the composition. When there is no second filler present the first particulate filler preferably constitutes more than 30 volume per cent of the composition. There may be more first filler present than second filler, or vice versa. In one embodiment the composition comprises about 20 volume per cent of first particulate filler, and about 3 to 5 volume per cent of second particulate filler.
  • the first particulate filler is non polymeric, for example a ceramic, it cannot "wet" the surface of the second filler (when present) to interrupt the conductive paths.
  • the particle size of each of the first and second filler is preferably about the same. This enables the second filler to intermingle with the first filler to interrupt the current path.
  • the second particulate filler changes its resistivity in response to a change in a variable.
  • more than one filler which changes its resistivity in response to a change in a variable may be included in the composition. Where more than one such fille -is included the fillers may change their resistivity in response to a change in the same variable, or in response to a change in different ' variables.
  • the matrix polymer comprises sintered ultra high molecular weight polyethylene particles 2.
  • Two fillers are distributed in the matrix along the boundaries of the particles 2.
  • the first filler 4 comprises carbon black
  • the second filler 6 comprises a PTC conductive polymer composition comprising carbon black dispersed in polyethylene (formed by eTt-extrusion).
  • the polymer 2 and the polymer of composition 6 are compatible.
  • a material of the invention was prepared in the following way.
  • a composite filler was prepared in a Banbury mixer by melt-blending 56 wt% high density polyethylene (Marlex 50100, available from Phillips Petroleum) with 43 wt% carbon black (Statex G, available from Columbian Chemicals) and 1 wt% antioxidant. The compound was extruded into strands through a die and irradiated to doses ranging from 10 to 80 Mrad using a 1 MeV electron beam. The strands were then cryogenically pulverized until all the particles were smaller than 250 microns.
  • the modulus ( M 100» i* e - the- value-in psi required to stretch the sample at 150°C to 100% of its original length) was measured on slabs compression-molde ⁇ fro - the PTC particulates following irradiation of the particulates to different beam doses.
  • the M ⁇ QQ value in psi was plotted as a function of irradiation level of the particulates, as shown in Figure 3.
  • the second test determined the amount of carbon black present in the matrix phase of the blended composition. Sections of the extruded tape were cooled in liquid nitrogen and cryosectioned with a microtome to give samples less than 1000 Angstroms thick. Using a transmission electron microscope (TEM) at 5000x magnification, the number of carbon black particles (i.e. black dots) in the polymer matrix was counted. The number of carbon particles in a 100 square micron area of polymer matrix was plotted as a function of beam dose of the PTC particulate, as shown in ' Figure 4.
  • TEM transmission electron microscope
  • Composite filler was obtained as in Example 1, except that ethylene vinyl acetate (Elvax 4260, a copolymer with a vinyl acetate content of 28%, available from DuPont) was used in place of the high density polyethylene. After irradiating to 60 Mrad, the material was ground to a particle size of less than 250 microns and the particulates were blended with high density polyethylene.. The resulting composition had a switching temperature, T s , of 75°C.
  • ethylene vinyl acetate Elvax 4260, a copolymer with a vinyl acetate content of 28%, available from DuPont
  • Composite filler was made as in Example 1, except that low density polyethylene (Petrothene NA 140, available from U.S.I. Chemicals) was used.
  • the composition was irradiated to 60 Mrad, ground to a particle size of less than 250 microns, and then blended with high density polyethylene.
  • the resulting composition had a switching temperature, T s , • of 90°C.
  • Example 2 The- composite filler of Example 1, crosslinked to 60 Mrad, were blended with a low melt index high density polyethylene (FA 113, available from U.S.I. Chemicals, particle size 20 microns, melt index 5 g/10 min).
  • F 113 low melt index high density polyethylene
  • a composite filler was obtained by blending polyetheretherketone powder (available from ICI) with 40 wt% carbon black (Statex G, available from Columbian Chemicals) and 0.25 wt% elemental sulfur. The blend was mixed in a 33 mm Leistritz counterrotating twin screw extruder and was extruded through a pelletizing die. The pellets were heat-treated at 300°C for 4 days to complete the crosslinking reaction and were then ground into a fine powder with a particle size less than 250 microns. Plaques were made by dry blending 70 wt% of the composite filler with 30 wt% of PEEK powder, and compression molding the blend between two electrodeposited nickel foil electrodes. The resulting heaters were powered by a 400 volt AC power source and heated to a switching temperature, T s , of about 335°C.
  • a composite filler as described in Example 1 was prepared and was irradiated to 80 Mrad with a 3.5 MeV electron beam prior to grinding to less than 250 microns. Approximately 35 wt% of this composite filler was blended in a Sigma mixer for approximately 20 minutes at 175°C with a black mastic consisting of polyisobutylene, amorphous polypropylene, a hydrocarbon tackifier, an antioxidant, and carbon black. The mixture was then suspended in a solvent or melted to apply to the inner surface of a tube for stress-grading applications.
  • A- composite.-filler and a polyethylene powder * as described in Example 1 were dry-blended. The blend was then sprayed onto an 0.030 inch (0.076 cm) thick, open-mesh (0.25 inch (0.635 cm) square pores), electrically insulating crosslinked polyethylene or fiberglass fabric placed on top of an electrodeposited copper foil. A second copper foil was laminated to the top of the assembly as it passed through two rolls heated to 232°C, a temperature sufficient to melt and fuse the polyethylene powder. The resulting heater had a polymer cross-section of 0.040 inch (0.102 cm).
  • a composite filler was prepared by melt blending high density polyethylene with 40% by volume of carbon black, Statex G. The mixture was pulverized until more than 90% of the particles were within the size range of 140 to 325 mesh. Then the composite filler was irradiated to 6 megarads by means of an electron beam.
  • Ultra High Molecular Weight Polyethylene (UMHWPE) (Hostalen GUR-212, made by Hoechst) was blended with 3% by volume of Statex-G carbon black and 20% by volume of the PTC powder. The blend was cold compacted, then sintered at 200°C for 20 minutes, and finally cooled under pressure. The product was exposed to 10 megarads of high energy electrons.
  • UHWPE Ultra High Molecular Weight Polyethylene
  • the product had a resistivity of about 100 ohm-cm, at 23°C, about 1000 ohm-cm at 112°C, and about 100,000 ohm-cm at about 120°C.
  • Example 2 The procedure used in Example 2 was carried out, but the volume fractions of the components were: UHMWPE 93.8%
  • the product had a resistivity of about 1300 ohm-cm at 23°C, about 10,000 ohm-cm at 112°C and about 1,000,000 ohm-cm at 120°C.
  • Example 2 The procedure used in Example 2 was carried out, but the volume fractions of the components were
  • the product had a resistivity of about 400 ohm-cm at 23°C, about 1,300 ohm-cm at 112°C and about 9,000 at 120°C.
  • a composite filler was prepared in a Brabender mixer by melt-blending 56 wt% Marlex 50100, 43 wt% Statex GH (carbon black available from Columbian Chemicals) and 1 wt% antioxidant with 5% by weight of the polyme /carbon black total of Luperco 130XL (a chemical crosslinking agent containing peroxide available from Pennwalt Corporation). The mixture was crossl nked by heating at 200°C for 30 minutes and was then ground to produce particles smaller than 250 microns. Three different formulations, A, B, and C, comprising 28, 30, and 32 wt% particulates, respectively, were made by mixing the particulates with Petrothene NA 140.
  • each composition was extruded into a tubular geometry with an inner diameter of 0.59 inch (1.5 cm) and a wall thickness of approximately 0.10 inch (0.25 cm). After irradiating each tube to a level consistent with an M ⁇ gg value of " 25 psi, each tube was pneumatically expanded to various ratios up to 3x expansion. When tested on a 15 KV cable, each expanded tube showed the ability to withstand 110 KV transient impulses.
  • the non-linear behavior as a function of increasing electrical stress is shown in the following data:
  • a composite filler was made by blending 50 wt% Marlex 50100, 49 wt% Statex GH, and 1 wt% antioxidant in a Banbury mixer and pelletizing the composition.
  • the pellets were dry-blended with 5 wt% Luperco 130XL and then were melt-mixed in a Banbury mixer at high speed. After 10 minutes, a large drop in the current of the mixer motor was observed. At this point, the composition was removed from the mixer as small fragments. (Testing of the fragments indicated that they were crosslinked; the peroxide reactions were complete). The fragments were ground into particulates smaller than 250 microns.
  • Various compounds containing 30 to 70 wt% particulates in Marlex 50100 were made. At 36 wt% particulates, one composition had a resistivity of approximately 150,000 ohm-cm.
  • a composite filler was made by blending 46 wt% Marlex 50100, 43 wt% Statex GH, 10 wt% Tipure (titanium dioxide pigment available from New England Resins and Pigments), and 1 wt% antioxidant in a Brabender mixer. The mixture was pelletized and the pellets were irradiated to a dose of 80 Mrad before grinding into particles smaller than 250 microns. The presence of the titanium dioxide made the mixture easier to grind.

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Abstract

Composition comprenant un polymère matriciel dans lequel est répartie une masse de remplissage conductrice particulaire. Au moins certaines des particules de remplissage sont à base d'une composition de polymère conducteur et/ou la matrice comporte un polymère fritté. Le polymère matriciel, la masse de remplissage et le procédé de répartition des particules sont tels que les particules gardent leur identité dans la matrice. Par exemple, les particules sont de préférence fortement réticulées de sorte qu'elles ont un facteur de conversion à chaud d'au moins 250 psi, si la composition est préparée par extrusion à chaud ou par un autre procédé impliquant un degré élevé de cisaillement; on peut également utiliser un procédé à faible cisaillement tel que le frittage. Le polymère matriciel est de préférence capable d'enrober les particules de remplisssage, par exemple, le polymère matriciel et le polymère de remplissage peuvent être chimiquement similaires. L'invention est particulièrement utile pour préparer des compositions ayant des résistances élevées, par exemple de 1000 ohm/cm ou plus, et pouvant être préparées avec un degré élevé de reproductibilité. Les compositions peuvent présenter un comportement PTC (coefficient de température positive), ZTC (coefficient de température zéro) ou NTC (coefficient de température négative) selon la nature de la masse de remplissage conductrice, leur résistance dépendant du champ électrique. Les compositions sont particulièrement utiles comme matériaux de résistance dans des radiateurs en tôle et comme matériaux de protection pour appareils à haute tension.
EP19880906793 1986-01-14 1988-07-21 Composition polymere conductrice Withdrawn EP0371059A1 (fr)

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US81884686A 1986-01-14 1986-01-14
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US07/075,929 US5106540A (en) 1986-01-14 1987-07-21 Conductive polymer composition

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