Conductive polymer composition

Abstract: Conductive polymer compositions comprising a matrix polymer and a conductive particulate filler dispersed in the matrix polymer. At least some of the filler particles comprise a polymer and a homogeneous filler, eg. carbon black, dispersed therein. Preferably the filler is highly cross-linked so that it has a hot modulus of at least 250 psi (l7.5 kg/cm²) and is then dispersed in a melt-shapeable polymer. In this way it is possible to make conductive polymer compositions of high resistivity, eg. at least l,000 ohm-cm, with a high degree of reproduceability. Alternatively, the matrix polymer has a hot modulus of at least 250 psi (l7.5 kg/cm²) and is sintered.

Description: This invention relates to conductive polymer compositions. Conductive polymer compositions are well known. They comprise a particulate conductive filler which is dispersed in, or otherwise held together by, an organic polymer (this term being used to include polysiloxanes). They can be used as current-carrying components, 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. The term "PTC behavior" is used in this specification to denote a composition which, in the operating temperature range, eg. 0° to 200°C, has an R₁₄ value of at least 2.5 or an R₁₀₀ value of at least 10, preferably both, and which preferably has an R₃₀ value of at least 6, where 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, R₁₀₀ is the ratio of the resistivities at the end and the beginning of the 100°C temperature range showing the greatest increase in resistivity, and 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. The term "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. The term "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. Documents describing conductive polymer compositions and devices comprising them include U.S. Patents Nos. 2,952,761, 2,978,665, 3,243,753, 3,351,882, 3,571,777, 3,658,976, 3,757,086, 3,793,716, 3,823,217, 3,858,144, 3,861,029, 3,950,604, 4,017,715, 4,072,848, 4,085,286, 4,117,312, 4,151,126, 4,177,376, 4,177,446, 4,188,276, 4,237,441, 4,242,573, 4,246,468, 4,250,400, 4,252,692, 4,255,698, 4,271,350, 4,272,471, 4,304,987, 4,309,596, 4,309,597, 4,314,230, 4,314,231, 4,315,237, 4,317,027, 4,318,881, 4,327,351, 4,330,704, 4,334,351, 4,352,083, 4,361,799, 4,388,607, 4,398,084, 4,413,301, 4,425,397, 4,426,339, 4,426,633, 4,427,877, 4,435,639, 4,429,216, 4,442,139, 4,459,473, 4,470,898, 4,481,498, 4,476,450, 4,502,929; 4,514,620, 4,517,449, 4,534,889, and 4,560,498; J. Applied Polymer Science 19 , 813-815 (1975), Klason and Kubat; Polymer Engineering and Science 18 , 649-653 (1978), Narkis et al; European Application Nos. 38,713, 38,714, 38,718, 74,281, 92,406, 119,807, 133,748, 134,145, 144,187, 157,640, 158,410, 175,550 and 176,284; and Japanese Published Patent Application No. 59-122,524. 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, eg. carbon black, graphite or a conductive inorganic material). This conventional method can be used to make a wide variety of products, but for many combinations of polymeric matrix and conductive filler, it is difficult to obtain reproduceable results. The reason for this is that a graph of the filler concentration against the resistivity of the composition often has a very steep slope in the region of desired resistivity; consequently the resistivity of the product can change very significantly if there are small changes in the process conditions or the starting materials. 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. Such methods are very useful for polymers which cannot be melt processed, eg. ultra-high molecular weight polyethylene, but are not generally applicable. U.S. Patent No. 3,591,526 (Kawashima) discloses a composite conductive particulate filler which is 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 resulting composite filler is mixed with a molding compound, eg. a thermosetting polymer, a thermoplastic polymer, or a synthetic or natural rubber; and the mixture is molded by conventional means such as a press, an injection molding machine, a screw extruder or a roll mill, to make a shaped article exhibiting PTC behavior. Kawashima states that "the electrical and mechanical characteristics of a product obtained in this way differ substantially from the characteristics of a material produced by merely mixing a conductive material with a plastic material and then molding the resulting mixture" and that "it appears that the first mixture provides a conductive material having a non-linear, temperaturesensitive electrical resistance characteristic and that the second mixture forms a protective matrix for the first mixture which not only preserves the conductive characteristics of the first mixture but also enhances the temperature-sensitivity thereof and improves its mechanical and heat-resisting properties". We have found that if Kawashima's process is employed to manufacture a shaped article by a process in which the composition is subjected to shear forces, satisfactory results are not obtained unless the matrix polymer and the filler polymer (ie. the thermoplastic polymer containing the carbon black) have substantially different chemical natures. In particular, if the two polymers are chemically similar, the Kawashima process suffers from the same disadvantage as the conventional process, namely that it is sensitive to small changes and is, therefore, difficult to carry out reproduceably. This is a serious disadvantage, since the processes in which the composition is subjected to shear forces (eg. extrusion, injection molding and blow molding) are the most useful processes for making many products, and it is highly desirable to use a matrix polymer which is chemically similar to (and preferably identical to) the filler polymer. We have discovered that the above disadvantage can be overcome through the use of a composite filler which, prior to distributing it in the matrix polymer, has been cross-linked to a high level such that the filler has a hot modulus of at least 1724 kPA (17.5 kg/cm², 250 psi); furthermore we have discovered that the filler polymer can be any polymer, not merely a thermoplastic, and that the filler need not exhibit PTC behaviour, as required by Kawashima. Although this discovery is of particular value for compositions in which the matrix and filler polymers are chemically similar, and which are shaped by a process which the composition is subjected to shear forces, the highly cross-linked composite fillers can also be used in conjunction with dissimilar matrix polymers and/or in manufacturing processes which do not make use of shear forces. US-A-4 388 607 discloses a conductive polymer composition in which a composite filler comprising an organic polymer and carbon black is distributed in another polymer, and which has good voltage stability and which exhibits PTC behaviour. Accordingly there is provided a process for preparing a shaped article composed of an electrically conductive material, the process comprising (1) mixing together (a) a first organic polymer and (b) a first particulate conductive filler which comprises a second organic polymer and a second particulate conductive filler which is distributed in the second organic polymer, and (2) shaping the mixture from step (1), whereby the first organic polymer forms a matrix in which the first particulate conductive filler is distributed, characterised in that the first particulate conductive filler, prior to step (1), has been cross-linked so that it has a hot modulus of at least 1724 kPA (17.5 kg/sq, 250 psi). In another aspect, the invention provides a process for the preparation of a particulate conductive filler which is suitable for use in the process of the invention, said process comprising (1) preparing an intimate mixture of (a) an organic polymer and (b) a homogenous particulate conductive filler, preferably by a process in which the polymer is softened by heating; (2) cross-linking the mixture so that it has a hot modulus of at least 1724 kPa (17.5 kg/cm², 250 psi); and (3) comminuting the mixture, preferably after the cross-linking step. 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 (ie. 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 M₁₀₀ Value) is calculated from 1 or, if the sample breaks before 100% elongation, The hot modulus values of the matrix polymer and the composite filler are difficult or impossible to measure directly on a shaped article 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 values can be ascertained directly from the starting materials, since they will not be changed by the mixing and shaping process. In other circumstances, or if the starting materials are not available, the hot modulus values can be ascertained indirectly by designing one or more test processes which will make a substantially identical shaped article and/or one or more articles which will have the same hot modulus as the matrix polymer and/or the composite filler, and by measuring the hot modulus values of the starting materials and/or the finished products of those test processes. The composite filler used in this invention is preferably made by preparing an intimate mixture of the second polymer and a homogeneous conductive particulate filler, cross-linking the mixture (which increases its hot modulus), and grinding or otherwise comminuting the mixture. The mixing is preferably carried out by a process which comprises blending the homogeneous filler with the hot filler polymer, eg. 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 such that the cross-linked composite filler has a hot modulus of at least 1724 kPa (17.5 kg/cm², 250 psi), particularly at least 2413 kPa (24.5 kg/cm², 350 psi), especially at least 3103 kPa (31.5 kg/cm², 450 psi). Preferably the cross-linking is substantially uniform throughout the filler. When using a thermoplastic polymer which is readily cross-linked by radiation, such as polyethylene, we have obtained good results using a dosage of at least 250 kGy (25 Mrad), preferably at least 300 kGy (30 Mrad), particularly at least 400 kGy (40 Mrad), with higher dosages of at least 600 kGy (60 Mrad), eg. 600-900 kGy (60 to 90 Mrad), giving yet better results. In the melt-shaped compositions which we have tested, we have found that, other things being equal, the greater the cross-linking of the filler, the lower the resistivity of the final composition, with the rate of change decreasing progressively as the cross-linking increases. Preferably the cross-linking level is such that it lies on a relatively flat part of a graph of resistivity against hot modulus, preferably a part having a slope of less than 0.5, particularly less than 0.3, especially less than 0.15. Comminution of the mixture can be carried out in any convenient way, and is preferably such that the average particle size (and more preferably the maximum particle size) of the composite filler, is less than 425 µm (425 x 10-6 metres), eg. 10 to 425 µm (100 to 425 x 10-6 metres). 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 graph of the weight percent of homogeneous filler (on the horizontal axis against the log₁₀ of the resistivity of the composite filler (on the vertical axis), preferably a part of the graph whose slope is less than 0.5, particularly less than 0.3. The filler polymer and the homogeneous filler should be selected having regard to the desired temperature/resistivity relationship (eg. PTC or ZTC) in the composite filler and in the final product. We prefer to use carbon black as the conductive filler, and for PTC compositions we habe obtained good results using 35 to 50% by weight of carbon black dispersed in a crystalline polymer. 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 (ie. are completely miscible over a wide range of proportions when both polymers are uncross-linked). To this end, the two polymers preferably comprise similar or identical substituents, eg. 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, eg. both the filler polymer and the matrix polymer are polyethylene. For a PTC composition it is preferred that at least one, and preferably each of the filler polymer and the matrix polymer should be a crystalline thermoplastic, and that the PTC composition has a resistivity at 23°C of at least 1000 ohm cm preferably 1000 to 100 000 ohm cm. One or both of the filler and matrix polymers may be a cross-linked elastomer. For a ZTC composition, it is preferred that both the filler polymer and the matrix polymer should be an elastomer. The composite conductive filler can also be filler obtained by comminuting a composition of the invention comprising a matrix filler and a composite conductive filler comprising a homogeneous conductive filler. There can be two or more composite fillers distributed in the matrix polymer. There can also be a homogeneous conductive filler distributed in the matrix polymer; such a homogeneous filler preferably exhibits ZTC behaviour, eg. carbon black or graphite, but can exhibit PTC behaviour, eg. a doped barium titanate or another PTC ceramic. The average particle size of the further conductive filler is preferably at least 1nm, eg. 5 to 100 nm. 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. Preferably the conductive filler content is such that it lies on a relatively flat part of a graph of the weight percent of the conductive filler (on the horizontal axis) against the log₁₀ of the resistivity of the composition (on the vertical axis), preferably a part of the graph whose slope is less than 0.5, particularly less than 0.3. When a composite filler is used alone in a shear-processed composition, the content thereof may be for example 40 to 80% by weight, preferably 55 to 75% by weight. When a composite filler is used alone in a sintered composition, the content thereof is preferably at least 20% by volume when both a composite conductive filler and a homogeneous conductive filler are present in a shear-processed composition, the composite filler may for example be 20 to 35% by volume and the homogeneous filler 15 to 50% by volume. When both a composite filler and a homogeneous conductive filler are present in a shear-processed composition, 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, eg. 3 to 5% by volume. The composition can also have distributed therein one or more non-conductive fillers. 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. The known conductive sintered products, in which carbon black is the sole conductive filler, exhibit ZTC behavior, but we have found that by using a composite filler exhibiting PTC behavior, a sintered composition which exhibits PTC behavior can be obtained. A similar result can be obtained by using a particulate PTC ceramic filler such as doped barium titanate, instead of or in addition to a PTC composite filler. The preferred sinterable polymer for use in this invention is ultra high molecular weight polyethylene (UHMWPE), eg. having a molecular weight of 3 million to 6 million. Other sinterable polymers include fluoropolymers, eg. polytetrafluorethylene and polyvinylidene fluoride, polyphenylene sulfide, polyether ketones, polyaryleneetherketones and polyamides. In the sintered products, the matrix is composed of particles of the matrix polymer which have been sintered together so that the particles have coalesced without completely losing their identity, and the conductive filler is preferably present substantially only at or near the boundaries of the coalesced particles. The invention is illustrated by the following Example. Example 1 A PTC powder was prepared as follows. Using a Banbury mixer, 56% by weight high density polyethylene resin (Marlex® 50100, available from Phillips Petrolium) was melt blended with 43% carbon black (Statex G, available from Columbian Chemicals) and 1% anti-oxidant. The resulting compound was irradiated to doses ranging from 100-600kGy (10 to 60 Mrad) in a 1 MeV electron beam, and then was pulverized until all the particles were smaller than 150 µm (150 x 10-6 metres). The PTC powder was tumble-blended with 32.5% by weight of high density polyethylene powder (FA750, available from U.S.I. Chemicals). The blend was extruded to produce a 7.5 x 0.10 cm tape. The modulus (M₁₀₀) was measured at 150°C using pieces of this tape. Resistivity values were calculated for each tape by measuring the resistance through the thickness of the sample (ie. in the direction normal to the extrusion direction) at 100volts. The results were as follows: Irradiation Level in kGy (Mrad) M₁₀₀ (kPa) (kg/cm², psi Resistivity (ohm-cm) 100(10) 241(2.43, 32) 5 x 10⁸ 150(15) 517(5.25, 75) 4 x 10⁸ 200(20) 1241(12.6, 180) 4 x 10⁶ 250(25) 1379(14.0, 200) 2 x 10⁵ 400(40) 2827(28.7, 410) 2 x 10⁴ 600(60) 4826(49.0, 700) 1.2 x 10⁴

Claims: A process for preparing a shaped article composed of an electrically conductive material, the process comprising (1) mixing together (a) a first organic polymer and (b) a first particulate conductive filler which comprises a second organic polymer and a second particulate conductive filler which is distributed in the second organic polymer, and (2) shaping the mixture from step (1), whereby the first organic polymer forms a matrix in which the first particulate conductive filler is distributed, characterised in that the first particulate conductive filler, prior to step (1), has been cross-linked so, that it has a hot modulus of at least 1724 kPa (17.5 kg/cm² 250 psi). A process according to claim 1, wherein the shaping of step (2) is carried out by extrusion, injection moulding or blow moulding, and wherein the first particulate conductive filler preferably has a hot modulus of at least 2413 kPa (24.5 kg/cm², 350 psi), particularly at least 3103 kPa (31.5 kg/cm², 450 psi). A process according to claim 1 or 2, wherein the first particulate conductive filler has been cross-linked by irradiating to a dose of at least 400 kGy (40 MRads) preferably at least 600 kGy (60 MRad). A process according to any preceding claim, wherein the first and second polymers are crystalline thermoplastic polymers which are compatible with each other, and wherein the composition exhibits PTC behaviour and has a resistivity at 23°C of at least 100 ohmcm, preferably 1000 to 100000 ohmcm. A process according to any preceding claim, wherein each of the first and second polymers is an olefin polymer, preferably polyethylene, and the second particulate filler is carbon black. A process according to any of claims 1 - 3, wherein at least one of the first and second polymers is a cross-linked elastomer. A process according to any preceding claim wherein the conductive composition comprises a third particulate conductive filler, preferably carbon black. A process for the preparation of a particulate conductive filler suitable for use in the process of any preceding claim, which comprises (1) preparing an intimate mixture of (a) an organic polymer and (b) a homogeneous particulate conductive filler; (2) cross-linking the mixture so that it has a hot modulus of at least 1724 kPa (17.5 kg/cm², 250 psi); and (3) comminuting the mixture.

Inventors: BARMA PRADEEP, ROSENZWEIG NACHUM, BARMA, PRADEEP, ROSENZWEIG, NACHUM
Applicants: RAYCHEM CORP, RAYCHEM CORPORATION (A DELAWARE CORPORATION)
Application year: 1987
Main classification: H01B1
IPC codes: C08L27, C08L1, C08L23, C08L77, C08K3, H01B1, H01C7, C08L101