Use of an Anisotropic Fluoropolymer for the Conduction of Heat

Abstract

The present invention relates to the use of an anisotropic fluoropolymer having a different intrinsic thermal conductivity in at least two directions as a heat conducting material in a thermally conductive article, to a thermally conductive article comprising said anisotropic fluoropolymer and to a process for the production of said anisotropic fluoropolymer.

Claims

1-15. (canceled)

16. An anisotropic membrane comprising: an anisotropic fluoropolymer membrane including a maximum intrinsic thermal conductivity in one dimension, wherein the anisotropic fluoropolymer membrane does not comprise thermally conductive fillers, and wherein the anisotropic fluoropolymer membrane has a dielectric constant below that of the fluoropolymer resin from which the membrane is formed.

17. The anisotropic membrane of claim 16, wherein the anisotropic fluoropolymer membrane is a porous anisotropic fluoropolymer membrane.

18. The anisotropic membrane of claim 16, wherein the anisotropic fluoropolymer membrane is a microporous fluoropolymer membrane, and wherein the microporous fluoropolymer membrane has a pore size of voids between 0.01 and 15 micrometers.

19. The anisotropic membrane of claim 16, wherein the anisotropic fluoropolymer membrane is a dense anisotropic fluoropolymer membrane.

20. The anisotropic membrane of claim 16, wherein the anisotropic fluoropolymer membrane has a thermal conductivity in the direction of maximum intrinsic heat conduction of 0.5 W/mK or more in the direction of maximum intrinsic thermal conductivity.

21. The anisotropic membrane of claim 20, wherein the anisotropic fluoropolymer membrane has an axial thermal diffusivity in the direction of maximum intrinsic heat conduction does not exceed 40 W/mK.

22. The anisotropic membrane of claim 16, wherein the anisotropic fluoropolymer membrane has an axial thermal diffusivity in the direction of maximum intrinsic heat conduction of more than 0.2 mm.sup.2/s.

23. The anisotropic membrane of claim 22, wherein the anisotropic fluoropolymer membrane axial thermal diffusivity does not exceed 22 mm.sup.2/s.

24. The anisotropic membrane of claim 16, wherein the anisotropic fluoropolymer membrane has surface resistivity of 10.sup.10 Ohm/square or more or more at 20° C. and 42% relative humidity.

25. The anisotropic membrane of claim 16, wherein the anisotropic fluoropolymer membrane conducts heat from a heat source to a heat sink in the direction of maximum intrinsic thermal conductivity.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0072] In FIG. 1, a fiber resin composite is shown in which axially thermally conductive PTFE fiber or fiber bundles (1) are arranged by a compliant matrix (2), with the direction of heat conduction being parallel to the fiber bundles.

[0073] In FIG. 2, a woven fiber mat with in-plane thermal conductivity (3) and through-plain thermal insulation (4) is shown.

[0074] In FIG. 3, heat transfer from a processor (7) to a heat sink (5) is shown with a thermal interface composite (6) which may e.g. be that shown in FIG. 1.

[0075] The present invention will be further illustrated by the examples described below.

METHODS AND EXAMPLES

[0076] Fibers were prepared from sheetlike materials, e.g. tapes by slitting into narrow strips and strechting them as described in example 1. It is assumed that the measured thermal properties of fibers also apply to samples with other aspect ratios, e.g. highly stretched sheets or membranes, if they were strechted under the same conditions.

[0077] In addition, a membrane has been prepared from the precursor tape as described in Example 2.

a) Measurement of Thermal Properties

[0078] Fiber samples for thermal analysis were prepared from continous fibers as described in Example 1. Bundles of parallel fibers of 8 cm length were aligned within a shrink hose made of Polyvinylidenfuoride (PVF). The shrink hose was shrinked at 180° C. for 15 minutes in order to produce cylinders comprising a bundle of parallel fibers held together by the shrink hose. The diameter of the cylindrical fiber bundles, which can be described as fiber-air composites, was between 15 mm and 20 mm. Thermal properties were measured according to ISO 22007-2 using a Hot Disk TPS 2500S thermal constants analyser.

[0079] The thermal properties of the single fiber (see Table 2 and 3) were calculated from the apparent thermal properties of the fiber bundle, which can be described as a fiber-air composite. From the apparent density of the fiber bundle and the apparent density of the fibers, the actual volume fraction of the fibers in the fiber-air composite was determined. Using this and a thermal conductivity of air of 0.0262 W K.sup.−1 m.sup.−1, the actual thermal properties of the fibers were calculated as described in Fujishiro, H.; Ikebe, M.; Kashima, T. & Yamanaka, A. (1997), “Thermal Conductivity and Diffusivity of High-Strength Polymer Fibers”, Japanese Journal of Applied Physics 36 (Part 1, No. 9A), 5633-5637, equations 1 and 2.

[0080] The anisotropy ratio, which is defined as the ratio of the axial to the radial thermal conductivity of a fiber, has been estimated using the literature value of ca. 0.3 W K.sup.−1 m.sup.−1 and 0.33 W K.sup.−1 m.sup.−1, as reported for PTFE in Blumm, J.; Lindemann, A.; Meyer, M. & Strasser, C. (2010), ‘Characterization of PTFE Using Advanced Thermal Analysis Techniques’, International Journal of Thermophysics 31, 1919-1927.

[0081] The determination of the radial thermal properties of the single fibers from the measured apparent values of the fiber bundle suffers from the unknown thermal contact resistance between the bundled fibers, however, there is strong experimental evidence that the reported literature values fit the actual radial thermal properties of the fibers within an acceptable range.

[0082] An average relative error of about 10% was estimated for thermal properties from data scatter.

b) Measurement of Mechanical Properties

[0083] Fiber samples for tensile testing were prepared from continous fibers as described in Example 1.

[0084] The apparent density of the fibers is the mass per unit volume including voids inherent in the material as tested. It was measured by a liquid displacement method using water containing 0.05 vol % TRITON X-100 wetting agent to lower the surface tension of the water.

[0085] The specific density is the mass per unit lenth including voids inherent in the material as tested. It was measured according to ISO 2060. Values are reported in dtex.

[0086] The specific tensile strength was measured according to ISO 2062. The specific tensile modulus was determined from the steepest slope within the elastic regime of the force-deflection curve measured according to ISO 2062.

[0087] Shrinkage was measured from free end shrinking of 1 m long fibers at 250° C. for 15 minutes according to ASTM D4974-01. Values are determined from the ratio of the change in length by shrinking to the initial enshrined length. [0088] c) Measurement of Electrical Properties

[0089] Surface resistivity was measured according ASTM D 257 set up between two parallel electrodes with a square configuration with a Keithley Model 617 Electrometer. The temperature was set to 21° C. with relative humidity of 42%.

d) Measurement of Water Uptake

[0090] Fiber samples for water uptake measurements were prepared from continous fibers as described in example 1.

[0091] The water uptake was measured according to ISO 4611. The mass of the conditioned specimen conditioned at 23° C. and 50% relative humidity for 86 h was determined by weighing. After the conditioning step, the specimen were subjected to a constant climate at 40° C. and 90% relative humidity for 24 h, and weighted again. Water uptake was determined by the relative difference in mass per surface area of samples equilibrated at the two conditions described above.

EXAMPLE 1

[0092] Following the procedures disclosed in U.S. Pat. Nos. 3,953,566, 3,962,153, and 4,064,214 a precursor fiber was prepared in the following manner:

[0093] A fine powder PTFE resin was mixed with mineral spirit (22.6 wt % Isopar K™) to form a paste and extruded through a die to form a wet tape of 0.980 mm thickness. Subsequently, the wet tape was rolled down, stretched at a ratio of 1 to 0.75 and than dried at 185° C. to remove the mineral spirit. The dry tape had a final thickness of 0.415 mm and was slit to 4.31 mm widths by passing it between a set of gapped blades to serve as precursor fibers.

[0094] The precursor fibers were stretched over hot plates at 350° C. to 370° C., at a total stretch ratio as shown in Table 1 and a stretch rate exceeding 75%/s to form a fiber.

[0095] After stretching, the fibers were not subjected to any further treatment at elevated temperature.

[0096] The precursor fiber (sample ID F0) and stretched specimen (sample ID F1-F3) were measured to determine mechanical properties and thermal properties by the methods described herein. The results are shown in Tables 1-3.

EXAMPLE 2

[0097] Following the procedures disclosed in U.S. Pat. Nos. 3,953,566, 3,962,153, and 4,064,214 a precursor fiber was prepared in the following manner:

[0098] A fine powder PTFE resin was mixed with mineral spirit (20.9 wt % Isopar K™ ) to form a paste and extruded through a die to form a wet tape of 0.980 mm thickness. Subsequently, the wet tape was rolled down, stretched at a ratio of 1 to 0.71 and than dried at 185° C. to remove the mineral spirit. The dry tape had a final thickness of 0.352 mm.

[0099] The dry tape was stretched over hot plates at 300° C., at a total stretch ratio as shown in Table 1 and a stretch rate exceeding 10%/s. After stretching, the tape was not subjected to any further treatment at elevated temperature.

[0100] The tape (sample ID M1) was measured to determine mechanical properties, thermal properties, and electrical properties by the methods described herein. The results are shown in Tables 1-2.

TABLE-US-00001 TABLE 1 Mechanical Properties Axial Axial specific specific Specific tensile tensile Axial Axial Total Apparent density strength modulus tensile tensile Sample stretch density ρ.sub.in σ.sub.t E.sub.t strength modulus Shrinkage ID ratio λ [g/cm.sup.3] [dtex] [cN/dtex] [cN/dtex] [GPa] [GPa] [%] F0 1 1.49 21127 0.14 0.9 0.02 0.13 2.2 F1 40 1.25 1317 4.97 185.9 0.62 23.24 4.6 F2 80 1.72 661 5.41 299.1 0.93 51.45 8.2 F3 140 1.68 380 5.37 410.5 0.90 69.96 6.9 M1 40 1.21 n.a.

TABLE-US-00002 TABLE 2 Thermal Properties at 40° C. Axial thermal Axial thermal Estimated Sample conductivity κ.sub.a diffusivity α.sub.a Anisotropy ratio ID [W K.sup.−1 m.sup.−1] [mm.sup.2/s] κ.sub.a/κ.sub.r F0 0.9 0.6 3 F1 7.8 5.9 26 F2 10.5 7.3 35 F3 14.3 9.5 48 M1 4.6 3.6 15

TABLE-US-00003 TABLE 3 Thermal Properties Axial thermal Axial thermal Sample Temperature conductivity κ.sub.a diffusivity α.sub.a Anisotropy ID [° C.] [W K.sup.−1 m.sup.−1] [W K.sup.−1 m.sup.−1] ratio κ.sub.a/κ.sub.r F3 0 17 12.8 57 F3 60 13.5 8.9 45 F3 120 12.9 8.0 43 F3 180 10.2 5.9 34

TABLE-US-00004 TABLE 4 Electrical Properties and Water Uptake Electrical Surface Sample Resistivity Water uptake ID [Ohm/square] [g/m.sup.2] F0 n.a. <0.05% F1 n.a. <0.05% F2 n.a. <0.05% F3 n.a. <0.05% M1 >200 * 10.sup.9 n.a.