Conductive filled fluoropolymers

Abstract: Tetrafluoroethylene copolymer compositions containing conductive carbon black, graphite fiber or thermally conductive fillers have significantly increased conductivity when the tetrafluoroethylene copolymer has been treated with a fluorinating agent.

Description: BACKGROUND OF THE INVENTION Conductive polymer compositions prepared by incorporating conductive particles of carbon black or graphite are well known in the art. The polymers so prepared include thermoplastics, elastomers, thermosets and blends thereof. (See e.g., U.S. Patent 4,545,926) Carbon and graphite filled fluorocarbon compositions for electrical applications have also been known for some time. They are primarily used in preference to other conductive polymers when chemically active and/or high temperature environments are to be encountered. Applications in which conductive fluorocarbons containing carbon black and/or graphite are used include current-limiting devices e.g., self-regulating heater cable (U.S. Patent 4,318,881, U.S. Patent 4,624,990 and U.S. Patent 4,545,926), antistatic containers for semiconductor chip processing (J6 1027842A), conductive coating compositions (U.S. Patent 4,482,476, U.S. Patent 4,064,074 and EP 79589), battery electrode constructions (U.S Patent 4,468,362, U.S. Patent 3,676,222, EP 126511A and JA 7016669 R), thermally and electrically conductive caulk (U.S. Patent 4,157,327), antistatic/antifriction sheets for use in tape or film cassettes (U.S. Patent 3,908,570) and conductive filaments (J 58163725A, J 75013957). However, there are difficulties associated with adding carbon black to fluoropolymers to achieve conductivity. One difficulty is the relatively large land rapid rise that occurs in effective melt viscosity of the blend as carbon black is added. This large and rapid viscosity increase results in more difficult and time consuming processing. At low enough levels of carbon black to be of little influence on effective melt viscosity, the electrical conductivity is usually lost or in a range below that desired. Higher conductivity can be obtained with graphite fibers as filler instead of carbon black. For example, U.S. 4,163,742 uses 5-50 weight percent graphite fibers and obtains resistivity in the range 0.03 to 6.8 ohm-cm. It is also known that fillers with high thermal conductivity can be added to give a polymer blend with increased thermal conductivity as compared with the unfilled polymer. Compositions with high thermal conductivity can be used in articles such as heat exchanger tubing and heater cable jackets. SUMMARY OF THE INVENTION The present invention relates generally to melt processible perfluorinated tetrafluoroethylene copolymers and terpolymers which have been treated with fluorinating agent to remove unstable end groups. Thermally conductive fillers blended with these fluorinated fluoropolymers result in a composite that provides surprisingly higher thermal conductivity than a corresponding formulation using a nonfluorinated version of the melt processible tetrafluoroethylene copolymer. DETAILED DESCRIPTION OF THE INVENTION Specifically, the present invention relates to a composition consisting essentially of (a) 50 to 90% by weight of a melt-processible tetrafluoroethylene copolymer comprising recurring units of tetrafluoroethylene comonomer and recurring units of a comonomer represented by the formula wherein R is R f , -R' f -X, or O-R f or -O-R' f -X in which R f is a perfluoroalkyl radical of 1-12 carbon atoms, R' f is -(CF 2 ) n -, where n = 1-12, or the same diradical containing ether oxygen; and X is H or Cl; and wherein said copolymer has been fluorinated by subjecting it to a fluorinating agent until the number of unstable end groups is less than 70 per 10 6 carbon atoms; and (b) a thermoconductive filler, but excluding carbon black. The tetrafluoroethylene copolymers useful herein are those that are melt processible and are referred to in the art as perfluorinated. The ethylenically unsaturated comonomers useful herein that are copolymerizable with tetrafluoroethylene are represented bv the formula: wherein R 1 is -R f , -R' f -X, or -O-R f or -O-R' f -X in which R f is a perfluoroalkyl radical of 1-12 carbon atoms, R' f is -(CF 2 ) n -, where n = 1-12, or the same diradical containing ether oxygen, and X is H or Cl; Specific copolymerizable fluorinated ethylenically unsaturated comonomers include hexafluoropropylene; perfluoro(methyl vinyl ether); perfluoro(n-propyl vinyl ether); perfluoro(n-heptyl vinyl ether); 3,3,4,4,5,5,6,6,6-nonafluorohexene-1; 3-hydrohexafluoropropyl vinyl ether; or mixtures thereof, such as a mixture of hexafluoropropylene and perfluoro(propyl vinyl ether). Preferably the comonomers are selected from perfluoro(alkyl vinyl ethers) of the formula R f -O-CF=CF 2 or from perfluoroolefins of 3-6 carbons. Comonomer content can range from 0.5 mole percent up to about 20 mole percent, and more than one comonomer can be present. Longer comonomer chains do not require as much mole % incorporation to provide melt processibility as smaller comonomer chains. During the polymerization of these monomers, a variety of chemically unstable end groups form on the polymer chains. These can be carboxamides, carboxylic acid, acid fluorides, hydrides, CH 2 OH, etc. They result from the use of initiators and chain transfer agents in polymerization and from competing side reactions. The end groups are quantitatively determined using infrared analysis. It has been known for some time (GB 1210794, Ref. 3) that by contacting high molecular weight solid fluorocarbon polymers in the absence of oxygen with a source of fluorine radicals such as fluorine gas, that the radicals react with unstable end groups of the main polymer chain and convert them to a more chemically stable form. It has now been discovered that these copolymers treated with a fluorinating agent and filled with a thermally conductive filler are more thermally conductive than their nonfluorinated counterparts. The degree of fluorination has been found to govern the degree of the improvement herein. For a high degree of effectiveness; i.e., conductivity and high melt flow, fluorination can be carried out until the unstable end group content is reduced to less than 6 end groups of -CH 2 OH, -CO 2 H, -CO 2 CH 3 , and -COF per 10 6 carbon atoms in the copolymer chain. Generally speaking, the benefits of this invention are seen even when as many as 70 such end groups per 10 6 carbon atoms are present. Procedures for fluorination and tests for determining the number of unstable end groups are set forth in U.S. Patent No. 4,743,658 to Imbalzano et al. Ordinarily fluorination is carried out by using fluorine gas mixed with nitrogen. A wide variety of fluorinating agents is useful to prepare the compositions claimed herein. Various fluorinating agents may be suitable, depending on the end groups present. Some fluorinating agents are listed in Kirk-Othmer Encyclopedia of Chemical Technology, Volume 10, Third Edition, pp. 832-835, and in Sheppard and Sharts, Organic Fluorine Chemistry , W. A. Benjamin, N. Y., 1969, pp. 56-59, 74-77, 124-127, and 163-179. While metal fluorides are fluorinating agents under suitable conditions, their use is for some purposes less attractive because they may leave inorganic residues. Volatile agents such as ClF, HF/antimony fluorides, OF 2 , SF 4 , and fluorine are preferred, and fluorine is most preferred. The fillers which assist with increasing thermal conductivity are, e.g., metal powders and granules, metal oxides such as iron oxide, zinc oxide, aluminum oxide and magnesium oxide, and silicon carbide. The amount of copolymer in the blend is 50 to 90% by weight, preferably 60 to 85%, and most preferably 70 to 80%. Test Procedures Melt flow number (MFN) is based on the melt flow rate from a melt viscometer using a 5000 g total mass piston and weight after 5 minutes residence time at 372 ± 1°C. The orifice is 0.0825 inch ± 0.002 inch (0.21 ± 0.005 cm)diameter with a 0.315 inch ± 0.005 inch (0.8 ±0.013 cm) land length. A polymer charge of about 5 grams is rapidly charged into the viscometer bore with the orifice in place and the sample is compacted with the unloaded piston. Just before 5 minutes (270 seconds) after intial charging, the weight is placed on the piston. Total weight of piston and weight is 5000 grams. At exactly 5 minutes (300 seconds) the extrudate is cut and discarded. At time equal to 360 seconds the extrudate is cut and weighed. The weight of the extrudate is determined, multiplied by ten, and the calculated weight per 10 minutes is recorded as the melt flow number. The MFN was run on the compositions as prepared in a batch mixer. End group analysis for tetrafluoroethylene/perfluoro(propyl vinyl) ether (TFE/PPVE) copolymer is done on thin (0.25-0.30 mm) films molded at 350°C using a heated platen press. For TFE/HFP copolymers, the sample film should be prepared by cold pressing of powder instead of melt pressing. The films are scanned on a Nicolet Model 5DX Fourier Transform infrared spectrometer. All operational settings used were those provided as default settings in the Nicolet control software, except for the number of scans collected before the transform is performed (40 scans vs. 10 scans in default mode). Similarly, a film of a reference material known to have none of the end groups to be analyzed is molded and scanned. The reference absorbance spectrum is subtracted from the sample absorbance, using the interactive subtraction mode of the software. The -CF 2 overtone band at 4.25 micrometers is used to compensate for the thickness differences between sample and reference during this interactive substraction. The difference spectrum, in two ranges, 5.13 to 5.88 micrometers (1950-1700 wave numbers) and 2.7 to 3.45 micrometers (3700-2900 wave numbers), represents the absorbances due to the reactive end groups. Calibration factors to allow calculation of end groups per million carbon atoms are determined from the absorbance of model compounds. The table below presents wavelengths and factors for determining end groups via the equation: Ends/10 6 carbons = absorbance x CF/film thickness in mils End Group Wavelength (Micrometers) Calibration Factor (CF) -COF 5.31 15,000 -CH 2 OH 2.75 88,300 -CO 2 H 5.52 10,900 -CO 2 CH 3 5.57 14,500 The sensitivity of the analysis for -COF groups in TFE/PPVE copolymers is 2-3 groups/10 6 carbon atoms. This means that a measurement of zero groups/10 6 carbon atoms means less than 3 groups/10 6 carbon atoms. Thermal Conductivity Measurement Thermal conductivity was determined using a DYNATECH Model C-Matic testing apparatus. Samples were 5 cm (2 inch) diameter disks nominally 1 mm (40 mils) thick. Results were obtained at 50, 100, and 200°C. Composition Preparation Electrically conductive fluoropolymer compositions were prepared in a laboratory batch mixer. The mixer was a Rheomix 3000 manufactured by Haake Bucheler Corp. controlled and driven with Haake's Rheocord 40 microprocessor controlled torque rheometer. The unit consists of a closed mixing chamber in which dual counter-rotating rotors turn at different gear ratios to provide intensive mixing. Temperature and rotor speed of the mixer are controlled by adjustment within the Rheocord and torque and melt temperature are monitored by the unit. The rotors used were the roller type. The product removed from the mixer-after melt blending was in relatively large chunks, which were cut into small pieces suitable for molding and in some cases these small pieces were ground into a powder before molding. An alternative method of preparing the fluoropolymer composition is that of U.S. 4,163,742. THERMAL CONDUCTIVITY EXAMPLES Control Example 1 About 250 g of a 285 g charge of unfluorinated TFE/PPVE copolymer A (3 weight percent PPVE, MFN = 13.6; for end group analysis see Table 2) pellets was melted in the mixer with control temperature set at 350°C and rotor speed at 15 rpm. When the polymer was molten 95 g of powdered aluminum nitride was added to the mixing bowl and the plunger lowered to force the powder into the melt. After the aluminum nitride appeared to be well distributed in the melt, the remainder of the polymer was added. The rotor speed was 15 rpm until the melt temperature reached 337°C. Rotor speed was then increased to 60 rpm for 5 minutes at which time the temperature was 347°C. The rotor was stopped and the blend removed from the mixer. Invention Example 1 About 250 g of a 285 g charge of fluorinated TFE/PPVE copolymer B (3 weight percent PPVE, MFN = 17.2; for end group analysis see Table 2) pellets was melted in the mixer with control temperature set at 350°C and rotor speed at 15 rpm. When the polymer was molten 95 g of powdered aluminum nitride was added to the mixing bowl and the plunger lowered to force the powder into the melt. After the aluminum nitride appeared to be well distributed in the melt, the remainder of the polymer was added. The rotor speed was 15 rpm until the melt temperature reached 338°C. Rotor speed was then increased to 60 rpm for 5 minutes at which time the temperature was 347°C. The rotor was stopped and the blend removed from the mixer. Control Example 2 About 250 g of a 285 g charge of unfluorinated TFE/PPVE copolymer A (3 weight percent PPVE, MFN = 13.6) pellets was melted in the mixer with control temperature set at 350°C and rotor speed at 15rpm. When the polymer was molten 95 g of powdered boron nitride was added to the mixing bowl and the plunger lowered to force the powder into the melt. After the boron nitride appeared to be well distributed in the melt, the remainder of the polymer was added. The rotor speed was 15 rpm until the melt temperature reached 337°C. Rotor speed was then increased to 60 rpm for 5 minutes at which time the temperature was 348°C. The rotor was stopped and the blend removed from the mixer. Invention Example 2 About 250 g of a 285 g charge of fluorinated TFE/PPVE copolymer B (3 weight percent PPVE, MFN = 17.2) pellets was melted in the mixer with control temperature set at 350°C and rotor speed at 15 rpm. When the polymer was molten 95 g of powdered boron nitride was added to the mixing bowl and the plunger lowered to force the powder into the melt. After the boron nitride appeared to be well distributed in the melt, the remainder of the polymer was added. The rotor speed was 15 rpm until the melt temperature reached 341°C. Rotor speed was then increased to 60 rpm for 5 minutes at which time the temperature was 346°C. The rotor was stopped and the blend removed from the mixer. DATA ANALYSIS Data for therma conductivity of the examples 1 and 2 are in Table 1. The results show that, surprisingly, the fluorinated compositions give higher thermal conductivity over the entire temperature range explored than the nonfluorinated compositions. Table 2 shows the end group analysis for the copolymers used as feedstocks for thermal conductivity compositions.

Claims: A composition consisting essentially of (a) 50 to 90% by weight of a melt-processible tetrafluoroethylene copolymer comprising recurring units of tetrafluoroethylene comonomer and recurring units of a comonomer represented by the formula wherein R is R f , -R' f -X, or O-R f or -O-R' f -X in which R f is a perfluoroalkyl radical of 1-12 carbon atoms, R' f is -(CF 2 ) n -, where n = 1-12, or the same diradical containing ether oxygen; and X is H or Cl; and wherein said copolymer has been fluorinated by subjecting it to a fluorinating agent until the number of unstable end groups is less than 70 per 10 6 carbon atoms; and (b) a thermoconductive filler, but excluding carbon black. The composition according to Claim 1 wherein the comonomer is hexafluoropropylene. The composition of Claim 1 wherein the comonomer is perfluoro(propyl vinyl ether). The composition of Claim 1 wherein the comonomer is a mixture of hexafluoropropylene and perfluoro(alkyl vinyl ether). The composition according to Claim 1 wherein the fluorinating agent is fluorine gas. The composition of Claim 1 wherein the comonomer is a mixture of hexafluoropropylene and perfluoro(propyl vinyl ether). The composition of Claim 1 wherein the thermo-conductive filler is a metal oxide such as iron oxide, zinc oxide, aluminum oxide and magnesium oxide, silicon carbide, aluminum nitride and boron nitride.

Inventors: KOLOUCH ROBERT JOSEPH, KOLOUCH, ROBERT JOSEPH
Applicants: DU PONT, E.I. DU PONT DE NEMOURS AND COMPANY
Application year: 1991
Main classification: C08L27
IPC codes: C08L27, C08K7, H01B1, C08F8, C08K3