CA1302609C - Conductive polymer composition - Google Patents
Conductive polymer compositionInfo
- Publication number
- CA1302609C CA1302609C CA000527178A CA527178A CA1302609C CA 1302609 C CA1302609 C CA 1302609C CA 000527178 A CA000527178 A CA 000527178A CA 527178 A CA527178 A CA 527178A CA 1302609 C CA1302609 C CA 1302609C
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- CA
- Canada
- Prior art keywords
- filler
- polymer
- article according
- matrix
- particulate
- 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.)
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Classifications
-
- 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
Abstract
CONDUCTIVE POLYMER COMPOSITIONS
ABSTRACT OF THE DISCLOSURE
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 (17.5 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 1,000 ohm-cm, with a high degree of reproduceability. Alternatively, the matrix polymer has a hot modulus of at least 250 psi (17.5 kg/cm2) and is sintered.
ABSTRACT OF THE DISCLOSURE
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 (17.5 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 1,000 ohm-cm, with a high degree of reproduceability. Alternatively, the matrix polymer has a hot modulus of at least 250 psi (17.5 kg/cm2) and is sintered.
Description
13~`Z6~
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 a~ 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 coe~ficient), ZTC tzero temperature coefficient) or NTC (negative temperature ^oefficient) behavior. The term "PTC behavior" is used in this specification to denote a composition which, in the operating temperature range, eg. 0 to 200C, has an R14 value of at least 2.5 or an Rloo value o~ 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 14C
temperature range showing the greatest increase in rasistivity, Rloo is the ratio of the resistivities at the end and the beginning of the 100C temperature range showing the greatest increase in resistivity, and R30 is the ratio of the resistivities at the end and the beginning of the 30C 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 0C is at least 2 times, preferably at least 5 times, its resistivity at a higher temperature in the ~ 3(~ZG~
~P1095COMFF
operating range. The term "ZTC behavior" is used in this specification to denote a composition which does not show either PTC behavior or NTC beha~ior; ZTC
compositions can exhibit PTC behavior at temperatures above the operating temperature range of the composition.
Documents describing conductive polymer com-positions 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, 4,1~8,276, 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, 4,314,23~, 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, 4,426,33g, 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 Rubat; 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 ~3~
denote a filler in which each particle has a single phase, eg. carbon black, gr~phite or a conductive inorganic material). This conventional method can be used to mak~ 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 ~raph of the filler concentration against the resistivity of the composition often has a very qteep 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 proces~ed, 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-blonding 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 ~3`~ ~6~
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 Rawashima'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 chanqes 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.
~3~
MPl09SCOMFF
We have discovered that the above disadvanta~e 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 ~iller has a hot modulus of at least 250 psi (17.5 kg/cm2); furthermore we have discovered that the filler polymsr can be any polymer, not merely a thermoplastic, and that the filler need not exhibit PTC behavior, as required by Kawashima. ~lthough 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 in which the composition is subjected to shear forces, the highly cross-linked composite fillers can also be used in conjunction with lS dissimilar matrix polymers and/or in manufacturing processes in which do not make use of shear forces. We have also discovered that if the matrix polymer has a hot modulus of at least 250 psi (17.5 kg/cm2) and the composition i3 shaped by sintering, excellent results are obtained even when the composite filler has not been cross-linked, or has been cross-linked to a relatively low level, eg. by irradiating it to a dosage of up to Mrad, preferably 5-l0 Mrad, or by an equivalent level of chemical cross-linking, prior to mixing it with the matrix polymer.
In its first aspect, the present invention provides a shaped article composed of an electrically conductive composition which comprises ~a) a continuous matrix comprising a first or~anic polymer (often called the matrix polymer in this specification), and 13~JZ~
(b) a first particulate conductive filler (often called the composite filler in this specification) which is distributed in the matrix and which comprises S (i) a second organic polymer (often called the filler polymer in this specification), and (ii) a second particulate conductive filler which is distributed in the second polymer;
wherein the first particulate conductive filler has been crosslinked so that it has a hot modulus of at least 250 psi (17.5 kg/cm2) prior to its mixing with the first organic polymer.
In another aspect, the invention provides a shaped article composed of an electrically conductive composition which comprises (a) a continuous matrix comprising a first organic polymer, and (b) a first particulate conductive filler which is distributed in the matrix and which comprises (i) a second organic polymer, and (ii) a second particulate conductive filler which is distributed in the second polymer;
wherein the matrix is a sintered polymer matrix and the matrix polymer has a hot modulus of at least 250 psi (17.5 kg/cm2).
In another aspect, the invention provides a process for making a shaped article as defined above which comprises (1) mixing together ~3~ ~6~:~`3 MP1095COMFF
(a) the first organic polymer, and (b) the first particulate conductive filler, at least one of the first and second organic polymers having a hot modulus of at least 250 psi (17.5 kg/cm2) at the time of mixing; and (2) shaping the mixture from step (1).
In another aspect, the invention provides a process for the preparation of a particulate conductive filler which is suitable for use in the above articles and process, said process comprising (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 250 psi (17.5 kg/cm2); and ~3) comminuting the mixture, preferably after the cross-linking step.
The hot modulus values referred to herein are measured at 150C for polymers which do not have a melting point and at a temperature 20C 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 Mloo Value) is calculated from ¢
13~Z6~
stress to elongate sample by 100%
1~ =
100 initial cross-sectional area of sample or, if the sample breaks before 100% elongation, stress to break sample M
100 ~nitial cross-sectional area of sample 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 ~hat the compositlon 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 materlals, since they will not be changed by the mixing and shaping process. In other circumstances, or if the starting material~ 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 measurlng the hot modulus values of the starting materials and/or the finished products of those test processes.
The composite ~iller used in this invention is preferably made by preparing an intimate mixture of the second polymer and a homogeneous conductive particulate flller, 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 D
13(~Z6~
- 8a - 26775-89 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.
D
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employed. The cross-linking i3 preferably such that the cross-linked composite filler has a hot modulus of at least 250 psi (17.5 kg/cm2), particularly at least 350 psi (24.5 kg/cm2), especially at least 450 psi (31.5 kg/cm2). 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 25 Mrad, preferably at least 30 Mrad, particularly at least 40 Mrad, with higher dosages of at least 60 Mrad, eg. ~0 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 aganist hot modulus, preferably a part having a slope of less than 0.5, particularly l~ss 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 microns, eg. 100 to 425 microns. ~he 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 resitivity of the composite filler (on the vertical axis), preferably a part o~ the graph whose 13~6~ 3 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 te!mperature/resistivity relationship (eg. PTC or ZTC) in the composite ~iller and in the final product. We prefer to use carbon black as the conductive filler, and for PTC compositions we have 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 whsn 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. 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 a filler obtained by comminuting a composition of the 13~Z6{~
invention comprising a matrix filler and a composite conductive filler comprising a homogeneous conductive filler. There can be two or more ~omposite 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 behavior, eg. carbon black or graphite, but can exhibit PTC behavior, eg. a doped barium titanate or another PTC ceramic. The average particle size ~f the further conductive filler is preferably at least lnm, eg. 5 to 100 nm.
The amount of composite filler which i3 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 r31atively 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 along ina 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 ~3i';~
are present in a shPar-processed composition, the content of composite ~iller 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 exhi~iting PTC behavior, a sintered composition which exhibits PTC behavior can be obtained. ~ 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 r polyether ketones, polyaryleneetherketones and polyamides.
In the sintered products, the matrix is composed of particles of the matrix polymer which have been 13~6~:;9 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 Examples.
Example 1 A PTC powder was prepared as follows. Using a Banbury~ mixer, 56% by weight high density polyethylene resin (Marlex~ S0100, available from Phillips Petroleum) was melt-blended with 43% carbon black ~Statex~ G, available from Columbian Chemicals) and 1%
antioxidant. the resulting compound was irradiated to doses ranging from 10 to 60 Mrad in a 1 MeV electron beam, and then was pulverized until all the particles were smaller than 150 microns.
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 through a 0.75 inch Brabender~ extruder to produce a 3.0 x 0.040 inch (7.6 x 0.10 cm) tape. The modulus (Mloo)was measured at 150C using pieces of this tape. Resistivity values were calculated for each tape by measuring the resistance through the thickness of the sample (i.e. in the direction normal to the extrusion direction) at lOOV. The results were as follows:
13~6~ MP1095COMFF
Irradiation Mloo tpsi) Resistivity Level (Mrad) (kg/cm2) (ohm-,,c,m,,?,,, 35 (2.43) 5 x 108 75 (5.25) 4 x 108 180 (12.6) 4 x 106 200 (14.0) 2 x 105 410 (28.7) 2 x 104 700 (49.0) 1.2 x 104 Example 2 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 PTC powder was irradiated to 6 megarads by means of an electron beam.
77% by volume of Ultra High Molecular Weight Polyethylene (UHMWPE) (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 200C for 20 minutes, and finally cooled under pressure. The product was exposed to 10 megarads of high energy electrons.
The product had a resistivity of about 100 ohm-cm, at 23C, about 100 ohm-cm at 112C, and about 100,000 ohm-cm at about 120C.
t~;
1'~
Example 3 The procedure used in Example 2 was carried out, but the volume fractions of the components were:
UHMWPE 93.8%
PTC Powder 4.2%
STATEX G 2.0%
The product had a resistivity of about 1300 ohm-cm at 23C, about 10,000 ohm-cm at 112C and about 1,000,000 ohm-cm at 120C.
Example 4 The procedure used in Example 2 was carried out, but the volume fractions of the components were UHMWPE 65%
PTC Powder 35%
ST~TEX S; %
The product had a resistivity of about 400 ohm-cm at 23C, about 1,300 ohm-cm at 112C and about 9,000 at 120C.
~ 30
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 a~ 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 coe~ficient), ZTC tzero temperature coefficient) or NTC (negative temperature ^oefficient) behavior. The term "PTC behavior" is used in this specification to denote a composition which, in the operating temperature range, eg. 0 to 200C, has an R14 value of at least 2.5 or an Rloo value o~ 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 14C
temperature range showing the greatest increase in rasistivity, Rloo is the ratio of the resistivities at the end and the beginning of the 100C temperature range showing the greatest increase in resistivity, and R30 is the ratio of the resistivities at the end and the beginning of the 30C 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 0C is at least 2 times, preferably at least 5 times, its resistivity at a higher temperature in the ~ 3(~ZG~
~P1095COMFF
operating range. The term "ZTC behavior" is used in this specification to denote a composition which does not show either PTC behavior or NTC beha~ior; ZTC
compositions can exhibit PTC behavior at temperatures above the operating temperature range of the composition.
Documents describing conductive polymer com-positions 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, 4,1~8,276, 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, 4,314,23~, 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, 4,426,33g, 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 Rubat; 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 ~3~
denote a filler in which each particle has a single phase, eg. carbon black, gr~phite or a conductive inorganic material). This conventional method can be used to mak~ 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 ~raph of the filler concentration against the resistivity of the composition often has a very qteep 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 proces~ed, 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-blonding 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 ~3`~ ~6~
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 Rawashima'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 chanqes 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.
~3~
MPl09SCOMFF
We have discovered that the above disadvanta~e 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 ~iller has a hot modulus of at least 250 psi (17.5 kg/cm2); furthermore we have discovered that the filler polymsr can be any polymer, not merely a thermoplastic, and that the filler need not exhibit PTC behavior, as required by Kawashima. ~lthough 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 in which the composition is subjected to shear forces, the highly cross-linked composite fillers can also be used in conjunction with lS dissimilar matrix polymers and/or in manufacturing processes in which do not make use of shear forces. We have also discovered that if the matrix polymer has a hot modulus of at least 250 psi (17.5 kg/cm2) and the composition i3 shaped by sintering, excellent results are obtained even when the composite filler has not been cross-linked, or has been cross-linked to a relatively low level, eg. by irradiating it to a dosage of up to Mrad, preferably 5-l0 Mrad, or by an equivalent level of chemical cross-linking, prior to mixing it with the matrix polymer.
In its first aspect, the present invention provides a shaped article composed of an electrically conductive composition which comprises ~a) a continuous matrix comprising a first or~anic polymer (often called the matrix polymer in this specification), and 13~JZ~
(b) a first particulate conductive filler (often called the composite filler in this specification) which is distributed in the matrix and which comprises S (i) a second organic polymer (often called the filler polymer in this specification), and (ii) a second particulate conductive filler which is distributed in the second polymer;
wherein the first particulate conductive filler has been crosslinked so that it has a hot modulus of at least 250 psi (17.5 kg/cm2) prior to its mixing with the first organic polymer.
In another aspect, the invention provides a shaped article composed of an electrically conductive composition which comprises (a) a continuous matrix comprising a first organic polymer, and (b) a first particulate conductive filler which is distributed in the matrix and which comprises (i) a second organic polymer, and (ii) a second particulate conductive filler which is distributed in the second polymer;
wherein the matrix is a sintered polymer matrix and the matrix polymer has a hot modulus of at least 250 psi (17.5 kg/cm2).
In another aspect, the invention provides a process for making a shaped article as defined above which comprises (1) mixing together ~3~ ~6~:~`3 MP1095COMFF
(a) the first organic polymer, and (b) the first particulate conductive filler, at least one of the first and second organic polymers having a hot modulus of at least 250 psi (17.5 kg/cm2) at the time of mixing; and (2) shaping the mixture from step (1).
In another aspect, the invention provides a process for the preparation of a particulate conductive filler which is suitable for use in the above articles and process, said process comprising (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 250 psi (17.5 kg/cm2); and ~3) comminuting the mixture, preferably after the cross-linking step.
The hot modulus values referred to herein are measured at 150C for polymers which do not have a melting point and at a temperature 20C 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 Mloo Value) is calculated from ¢
13~Z6~
stress to elongate sample by 100%
1~ =
100 initial cross-sectional area of sample or, if the sample breaks before 100% elongation, stress to break sample M
100 ~nitial cross-sectional area of sample 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 ~hat the compositlon 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 materlals, since they will not be changed by the mixing and shaping process. In other circumstances, or if the starting material~ 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 measurlng the hot modulus values of the starting materials and/or the finished products of those test processes.
The composite ~iller used in this invention is preferably made by preparing an intimate mixture of the second polymer and a homogeneous conductive particulate flller, 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 D
13(~Z6~
- 8a - 26775-89 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.
D
~3i~z~
employed. The cross-linking i3 preferably such that the cross-linked composite filler has a hot modulus of at least 250 psi (17.5 kg/cm2), particularly at least 350 psi (24.5 kg/cm2), especially at least 450 psi (31.5 kg/cm2). 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 25 Mrad, preferably at least 30 Mrad, particularly at least 40 Mrad, with higher dosages of at least 60 Mrad, eg. ~0 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 aganist hot modulus, preferably a part having a slope of less than 0.5, particularly l~ss 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 microns, eg. 100 to 425 microns. ~he 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 resitivity of the composite filler (on the vertical axis), preferably a part o~ the graph whose 13~6~ 3 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 te!mperature/resistivity relationship (eg. PTC or ZTC) in the composite ~iller and in the final product. We prefer to use carbon black as the conductive filler, and for PTC compositions we have 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 whsn 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. 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 a filler obtained by comminuting a composition of the 13~Z6{~
invention comprising a matrix filler and a composite conductive filler comprising a homogeneous conductive filler. There can be two or more ~omposite 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 behavior, eg. carbon black or graphite, but can exhibit PTC behavior, eg. a doped barium titanate or another PTC ceramic. The average particle size ~f the further conductive filler is preferably at least lnm, eg. 5 to 100 nm.
The amount of composite filler which i3 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 r31atively 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 along ina 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 ~3i';~
are present in a shPar-processed composition, the content of composite ~iller 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 exhi~iting PTC behavior, a sintered composition which exhibits PTC behavior can be obtained. ~ 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 r polyether ketones, polyaryleneetherketones and polyamides.
In the sintered products, the matrix is composed of particles of the matrix polymer which have been 13~6~:;9 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 Examples.
Example 1 A PTC powder was prepared as follows. Using a Banbury~ mixer, 56% by weight high density polyethylene resin (Marlex~ S0100, available from Phillips Petroleum) was melt-blended with 43% carbon black ~Statex~ G, available from Columbian Chemicals) and 1%
antioxidant. the resulting compound was irradiated to doses ranging from 10 to 60 Mrad in a 1 MeV electron beam, and then was pulverized until all the particles were smaller than 150 microns.
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 through a 0.75 inch Brabender~ extruder to produce a 3.0 x 0.040 inch (7.6 x 0.10 cm) tape. The modulus (Mloo)was measured at 150C using pieces of this tape. Resistivity values were calculated for each tape by measuring the resistance through the thickness of the sample (i.e. in the direction normal to the extrusion direction) at lOOV. The results were as follows:
13~6~ MP1095COMFF
Irradiation Mloo tpsi) Resistivity Level (Mrad) (kg/cm2) (ohm-,,c,m,,?,,, 35 (2.43) 5 x 108 75 (5.25) 4 x 108 180 (12.6) 4 x 106 200 (14.0) 2 x 105 410 (28.7) 2 x 104 700 (49.0) 1.2 x 104 Example 2 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 PTC powder was irradiated to 6 megarads by means of an electron beam.
77% by volume of Ultra High Molecular Weight Polyethylene (UHMWPE) (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 200C for 20 minutes, and finally cooled under pressure. The product was exposed to 10 megarads of high energy electrons.
The product had a resistivity of about 100 ohm-cm, at 23C, about 100 ohm-cm at 112C, and about 100,000 ohm-cm at about 120C.
t~;
1'~
Example 3 The procedure used in Example 2 was carried out, but the volume fractions of the components were:
UHMWPE 93.8%
PTC Powder 4.2%
STATEX G 2.0%
The product had a resistivity of about 1300 ohm-cm at 23C, about 10,000 ohm-cm at 112C and about 1,000,000 ohm-cm at 120C.
Example 4 The procedure used in Example 2 was carried out, but the volume fractions of the components were UHMWPE 65%
PTC Powder 35%
ST~TEX S; %
The product had a resistivity of about 400 ohm-cm at 23C, about 1,300 ohm-cm at 112C and about 9,000 at 120C.
~ 30
Claims (17)
1. A shaped article composed of an electrically conductive composition which comprises (a) a continuous matrix comprising a first organic polymer, and (b) a first particulate conductive filler which is distributed in the matrix and which comprises (i) a second organic polymer, and (ii) a second particulate conductive filler which is distributed in the second polymer;
wherein the first particulate conductive filler has been crosslinked so that it has a hot modulus of at least 250 psi (17.5 kg/cm2) prior to its mixing with the first organic polymer.
wherein the first particulate conductive filler has been crosslinked so that it has a hot modulus of at least 250 psi (17.5 kg/cm2) prior to its mixing with the first organic polymer.
2. An article according to claim 1 which has been shaped by extrusion, injection molding or blow-molding and in which the first filler has a hot modulus of at least 250 psi (17.5 kg/cm2).
3. An article according to claim 2 wherein the first filler has a hot modulus of at least 350 psi (24.5 kg/cm2).
4. An article according to claim 2 wherein the first filler has a hot modulus of at least 450 psi (31.5 kg/cm2).
5. An article according to claim 1 wherein the first filler has been cross-linked by irradiating it to a dose of at least 40 Mrad.
6. An article according to claim 5 wherein the first filler has been cross-linked by irradiating it to a dose of at least 60 Mrad.
7. An article according to claim 1, 2, or 5 wherein the first and second polymers are crystalline thermoplastic polymers which are compatible with each other, and the composition exhibits PTC behavior and has a resistivity at 23°C of at least 1000 ohm-cm.
8. An article according to claim 1, 2, or 5 wherein each of the first and second polymers is an olefin polymer, and the second particulate filler is carbon black.
9. An article according to claim 1, 2, or 5 wherein each of the first and second polymers is polyethylene and the second particulate filler is carbon black.
10. An article according to claim 1, 2, or 5 wherein at least one of the first and second polymers is a cross-linked elastomer.
11. A shaped article composed of an electrically conductive composition which comprises (a) a continuous matrix comprising a first organic polymer, and (b) a first particulate conductive filler which is distributed in the matrix and which comprises (i) a second organic polymer, and (ii) a second particulate conductive filler which is distributed in the second polymer;
wherein the matrix is a sintered polymer matrix and the matrix polymer has a hot modulus of at least 250 psi (17.5 kg/cm2).
wherein the matrix is a sintered polymer matrix and the matrix polymer has a hot modulus of at least 250 psi (17.5 kg/cm2).
12. An article according to claim 11 wherein the matrix is composed of particles of ultra high molecular weight polyethylene which have been sintered together so that the particles have coalesced without completely losing their identity, and the first filler is present substantially only at or near the boundaries of the coalesced particles.
13. An article according to claim 1, 2, or 11 wherein the conductive composition comprises a third particulate conductive filler.
14. An article according to claim 1, 2, or 11 wherein the conductive composition comprises a third particulate conductive filler which is carbon black.
15. An article according to claim 11 wherein each of the first and second polymers is an olefin polymer and the second particulate filler is carbon black.
16. A process for the preparation of a shaped article as claimed in claim 1, 2, or 11 which comprises (1) mixing together (a) the first organic polymer, and (b) the first particulate conductive filler, at least one of the first and second organic polymers having a hot modulus of at least 250 psi (17.5 kg/cm2) at the time of mixing; and (2) shaping the mixture from step (1).
17. A process for the preparation of a particulate conductive filler 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 250 psi (17.5 kg/cm2); and (3) comminuting the mixture.
(2) cross-linking the mixture so that it has a hot modulus of at least 250 psi (17.5 kg/cm2); and (3) comminuting the mixture.
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| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US81884686A | 1986-01-14 | 1986-01-14 | |
| US81884586A | 1986-01-14 | 1986-01-14 | |
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| Publication Number | Publication Date |
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| CA1302609C true CA1302609C (en) | 1992-06-02 |
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ID=27124310
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| Application Number | Title | Priority Date | Filing Date |
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| CA000527178A Expired - Fee Related CA1302609C (en) | 1986-01-14 | 1987-01-13 | Conductive polymer composition |
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| Country | Link |
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| US (1) | US5106540A (en) |
| EP (1) | EP0231068B1 (en) |
| JP (1) | JP2513659B2 (en) |
| AT (1) | ATE103095T1 (en) |
| CA (1) | CA1302609C (en) |
| DE (1) | DE3789325T2 (en) |
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| JPS59122524A (en) * | 1982-12-28 | 1984-07-16 | Matsushita Electric Works Ltd | Composition having positive temperature characteristics of resistance |
| DE3478163D1 (en) * | 1983-09-22 | 1989-06-15 | Raychem Corp | Electrical devices comprising conductive polymers exhibiting ptc characteristics |
| US4514620A (en) * | 1983-09-22 | 1985-04-30 | Raychem Corporation | Conductive polymers exhibiting PTC characteristics |
| US4518552A (en) * | 1983-11-09 | 1985-05-21 | Mitsuboshi Belting Ltd. | Method of producing accurately sized material of ultra high molecular weight polyethylene |
| JPS60115678A (en) * | 1983-11-29 | 1985-06-22 | Matsushita Electric Ind Co Ltd | Anisotropically conductive adhesive and production thereof |
| DE3440617C1 (en) * | 1984-11-07 | 1986-06-26 | Zipperling Kessler & Co (Gmbh & Co), 2070 Ahrensburg | Antistatic or electrically semiconducting thermoplastic polymer blends, processes for their production and their use |
| JPS61123655A (en) * | 1984-11-19 | 1986-06-11 | Hitachi Chem Co Ltd | Epoxy forming material for sealing electronic parts |
| DE3502077A1 (en) * | 1985-01-23 | 1986-07-24 | Basf Ag, 6700 Ludwigshafen | METHOD FOR PRODUCING MOLDED ARTICLES FROM CONDUCTIVE THERMOPLASTICS AND THE USE THEREOF IN THE ELECTROINDUSTRY |
| JPH0685361B2 (en) * | 1985-08-12 | 1994-10-26 | 松下電器産業株式会社 | Positive resistance temperature coefficient Method for producing heating element resin composition |
-
1987
- 1987-01-12 AT AT87300231T patent/ATE103095T1/en not_active IP Right Cessation
- 1987-01-12 DE DE3789325T patent/DE3789325T2/en not_active Expired - Fee Related
- 1987-01-12 EP EP87300231A patent/EP0231068B1/en not_active Expired - Lifetime
- 1987-01-13 JP JP62005861A patent/JP2513659B2/en not_active Expired - Lifetime
- 1987-01-13 CA CA000527178A patent/CA1302609C/en not_active Expired - Fee Related
- 1987-07-21 US US07/075,929 patent/US5106540A/en not_active Expired - Fee Related
Also Published As
| Publication number | Publication date |
|---|---|
| DE3789325D1 (en) | 1994-04-21 |
| EP0231068A2 (en) | 1987-08-05 |
| ATE103095T1 (en) | 1994-04-15 |
| JP2513659B2 (en) | 1996-07-03 |
| DE3789325T2 (en) | 1994-10-27 |
| US5106540A (en) | 1992-04-21 |
| JPS62167358A (en) | 1987-07-23 |
| EP0231068B1 (en) | 1994-03-16 |
| EP0231068A3 (en) | 1987-09-16 |
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