Process For The Production Of A Thermally Conductive Article

Abstract

The present invention relates to a process for the production of a thermally conductive article comprising pleating a thermally anisotropic sheet having a thermal conductivity in plain which is higher than the thermal conductivity perpendicular to the plain, and compacting the pleated structure, to an article obtainable by said process, to the use of said thermally conductive article for production of a device, and to such a device.

Claims

1. A process for the production of a thermally conductive article, said process comprising: pleating at least one thermally anisotropic sheet having a first thermal conductivity in a first plane which is higher than a second thermal conductivity in a second plane that is perpendicular to the first plane to form a pleated structure; and compacting the pleated structure, said pleated structure having an upper surface and a lower surface, wherein the pleated structure comprises a plurality of pleats, said pleats having a first surface and a second surface.

2. The process according to claim 1, further comprising bonding said pleats of at least one of said first and second surfaces to each other.

3. The process according to claim 1, wherein said sheet comprises a plurality of thermally anisotropic expanded polytetrafluoroethylene layers.

4. The process according to claim 1, further comprising bonding a film to at least one of the upper surface and the lower surface of the pleated structure.

5. The process according to claim 1, wherein the sheet is treated with a thermal interphase wax.

6. The process according to claim 1, wherein the pleated structure comprises pleats having a pleat height, and wherein said pleat height in relation to the sheet thickness is from 1000:1 to 2:1.

7. The process according to claim 1, wherein the pleats having a pleat height, and wherein said sheet is pleated so that in the pleated structure comprises pleats with different pleat heights.

8. The process according to claim 1, wherein the thermally anisotropic sheet has a thickness from 1 μm to 500 μm.

9. The process according to claim 1, wherein said pleating of the sheet is performed perpendicular to the direction of maximum thermal conductivity.

10. The process according to claim 1, wherein the thermally anisotropic sheet further comprises a layer of a thermally isotropic conductive material.

11. The process according to claim 1, wherein the thermally anisotropic sheet comprises a thermally anisotropic polymer layer.

12. The process according to claim 11, wherein the thermally anisotropic polymer layer comprises a polyolefin or a fluoropolymer.

13. The process according to claim 12, wherein the thermally anisotropic polymer layer comprises a polyethylene.

14. The process according to claim 12, wherein the thermally anisotropic polymer layer comprises a fluoropolymer selected from polytetrafluoroethylene, a modified polytetrafluoroethylene, a fluorothermoplastic, a fluoroelastomer and combinations thereof.

15. The process according to claim 1, wherein the intrinsic thermal conductivity of the thermally anisotropic sheet is 0.5 W/mK or more in the direction of maximum intrinsic thermal conductivity.

16. A thermally conductive article obtained by the process according to claim 1.

17. A thermally conductive article comprising a sheet having a thickness from 1 μm to 500 μm, wherein the article has a thermal conductivity perpendicular to the plane of 1 W/mK or higher.

18. (canceled)

19. A device comprising the thermally conductive article of claim 16.

20. The process of claim 1, wherein said sheet is a composite sheet comprising a layer of a thermally anisotropic expanded polytetrafluoroethylene attached to a metallic film.

21. The process of claim 20, wherein said composite sheet further comprises an adhesive layer on one or both of an upper side and a lower surface of said composite sheet.

22. A thermally conductive article comprising: a pleated and compacted sheet comprising a plurality of layers of at least one thermally anisotropic material, wherein pleats of said pleated sheet are bonded together on a first surface thereof, and wherein said pleats of said pleated sheet are unbonded on a second surface thereof.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0072] The accompanying drawings are included to provide a further understanding of the disclosure and are incorporated in and constitute a part of this specification, illustrate embodiments, and together with the description serve to explain the principles of the disclosure.

[0073] FIG. 1 are schematic depictions of an embodiment of the process of the invention; and

[0074] FIG. 2 is a schematic depiction of two embodiments of the sheet before the pleating step in the process of the present invention; and

[0075] FIG. 3 shows counter-rotating pleating rolls (FIG. 3a) and an enlarged view of the tooth design of the pleating rolls (FIG. 3b) for an embodiment of the pleating step in the process of the present invention; and

[0076] FIG. 4 is a schematic view of one embodiment of the step of compacting the pleated structure in the process of the present invention; and

[0077] FIG. 5a is a picture of a thermally conductive article in the form of a cooling ribbon obtainable by the process according to the invention; and

[0078] FIG. 5b is an enlarged view of the pleated structure of the cooling ribbon in FIG. 5a; and

[0079] FIG. 6 is a picture of a thermally conductive article in the form of a partially pleated structure obtainable by a process according to the invention; and

[0080] FIG. 7 are SEM views of a pleated PTFE film (thickness: 10 micrometer) with a pleat height of 450 micrometer formed according to Example 1; and

[0081] FIG. 8 are SEM views of a pleated PTFE film (thickness: 40 micrometer) with a pleat height of 450 micrometer formed according to Example 2; and

[0082] FIG. 9 is a SEM view of a stack of a PTFE film with a thickness of 40 micrometer, with a total stack height of 450 micrometer, produced as described in Example 3; and

[0083] FIG. 10 are SEM views of a pleated polyethylene film formed according to Example 4.

DETAILED DESCRIPTION

[0084] Persons skilled in the art will readily appreciate that various aspects of the present disclosure can be realized by any number of methods and apparatus configured to perform the intended functions. It should also be noted that the accompanying drawing figures referred to herein are not necessarily drawn to scale, but may be exaggerated to illustrate various aspects of the present disclosure, and in that regard, the drawing figures should not be construed as limiting.

[0085] A schematic illustration of one embodiment of the process of the present invention is shown in FIG. 1. In FIG. 1a an “endless” sheet 10 is shown in a view perpendicular to the plain of the sheet. The sheet 10 comprises an first surface 11 and a second surface 12 and two opposing edges 13. The untreated sheet 10 is subjected to a first pleating step (section A in FIG. 1) in which bends are created perpendicular to the machine direction of the sheet so that pleats 18 are formed. This step is followed by a second compacting step (section B in FIG. 1), in which the formed pleats 18 are brought into contact with each other. This results in a pleated and compacted structure 20. In a following optional bonding step the structure 20 may be fixed, e.g. as shown in FIG. 1 in section C by bonding the surfaces of the pleats 18 to each other. The compacting and bonding of the pleated structure can be carried out in one step.

[0086] In order to stabilize the pleated and compacted structure 20 in one embodiment a stabilizing film 24 can be attached to one or more of the surfaces of the structure 20. In section D in FIG. 1 a stabilization film 24 is bonded to the upper surface 19 of the structure 20.

[0087] FIG. 1b is a perspective view of those steps as shown in FIG. 1a.

[0088] The sheet 10 can be made of at least one single layer of sheet material. In another embodiment, the sheet 10 comprises several layers of sheet material.

[0089] The use of a sheet using several layers is another form to stabilize the pleated and compacted structure 20. Pleats formed by a multilayer sheet are self-standing and show own stability.

[0090] FIG. 2 shows embodiments of sheet 10.

[0091] In FIG. 2a the sheet 10 comprises several layers 14 of the same material arranged in a stack of one layer on top of the other, e.g. several layers of an thermally anisotropic expanded PTFE.

[0092] In FIG. 2b sheet 10 comprises one layer 14 of thermally anisotropic expanded PTFE attached to a metallic film 15, for example made of copper, aluminum or silver. The metallic film 15 can be attached to layer 14 by using a thin adhesive layer of FEP or by vaporizing. The composite can further comprise at least one layer of FEP 16, forming either the upper or lower surface or both surfaces 11, 12.

[0093] In FIG. 3, a preferred embodiment for the device used for the pleating of the sheet is shown. FIG. 3a shows the device comprising two counter-rotating pleating rolls 30 with teeth 35 through which the unpleated sheet is passed. The distance between the rolls 30 is selected so that the sheet is not disrupted or adversely affected in any way. FIG. 3b shows an enlarged view of the area A of FIG. 2a. This Figure shows an embodiment of the tooth design of the pleating rolls. The tooth high in this example is about 450 micrometer and determines the high of the pleats formed upon passing through the pleating rolls. The end of the tooth is rounded to prevent a disruption of the sheet.

[0094] FIG. 4 shows a schematic depiction of a possible embodiment for the compacting of the pleated structure. In FIG. 4a the pleated structure 16 is inserted between two plates, e.g. glass plates. In one example the pleats 18 are arranged substantially vertically between a top plate 42 and a bottom plate 44. The distance between the plates 42, 44 is selected so that it corresponds closely to the height of the (highest) pleats of the pleated structure 16. As depicted in FIGS. 4b and 4c, steel blades 46 with a thickness of approximately the distance between the plates 42, 44 are inserted at the ends of the pleated structure 16 and a pressure is exerted on at least one of the steel blades, so that the pleats 18 are compacted and the surface of the formed pleats is brought in contact with each other. In one example, the blades have a thickness of about 400 micrometer.

[0095] FIG. 5a and FIG. 5b show an example of a thermally conductive article obtainable by the process according to this invention.

[0096] In that embodiment the pleated and compacted structure 20 comprises a sheet 10 made of multiple layers of a thermally anisotropic material. The pleats 18 are only bonded on its second surface 12, the first and outer surface 11 of the pleats is un-bonded and therefore available for a heat transfer to the outside. Such a structure of “cooling ribbons” creates additional forced convective heat loss and increased surface area.

[0097] The pleated and compacted structure 20 is arranged around objects like for example around the outer surface of a metallic thermally conductive stick 32. Heat in the stick can move in the pleats 18 of the structure 20 and be render to the surrounding environment via the first surface 11 of the sheet 10. Such an arrangement forms a cooling ribbon device for heat conducting articles.

[0098] FIG. 6 shows another example for a thermally conductive article in the form of a partially pleated structure obtainable by a process according to the invention. In this example a sheet made of multiple layers of a thermally anisotropic material has been used. The pleating and compacting step has been carried out only for a part of the sheet. As indicated by the arrows in FIG. 6, the main transport of heat in the non-pleated section is parallel to the main axis and in the folded area the main direction of heat transport is rotated by 90°.

[0099] The thermal conductivity was measured according to ISO 22007-2 using a Hot Disk TPS 2500S thermal constants analyser at 40° C.

EXAMPLES

Example 1

[0100] A thermally anisotropic expanded PTFE film with a thickness of 10 μm was produced according the following procedure.

[0101] Following the procedures disclosed in U.S. Pat. Nos. 3,953,566, 3,962,153, and 4,064,214 a tape was prepared in the following manner: 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 then dried at 185° C. to remove the mineral spirit. The dry tape had a final thickness of 0.415 mm. The tape was stretched over hot plates at 350° C. to 370° C., at a total stretch ratio of 78:1. After stretching, tape was not subjected to any further treatment at elevated temperature.

[0102] The tape has a thermal conductivity in machine direction of 7.82 W/mK, a thermal conductivity in transverse direction of 1.12 W/mK and a thermal conductivity in the direction perpendicular to the plain of 0.05 W/mK.

[0103] The tape is subjected to the process of the present invention starting with the first pleating step in using 2 counter rotating gear wheels with a tooth height of 450 μm and a pleated structure of said tape with a corresponding pleat height of around 450 μm is obtained.

[0104] The pleated structure is inserted between two plates, e.g. glass plates, for the second compressing step. The pleats are arranged substantially vertically between a top plate and a bottom plate according to the process as shown in FIG. 4. The distance between the plates is selected so that it corresponds closely to the height of the (highest) pleats of the pleated structure. Steel blades with a thickness of approximately the distance between the plates are inserted at the ends of the pleated structure and a pressure is exerted on at least one of the steel blades by hand for about 3 seconds, so that the pleats are compacted and the surface of the formed pleats is brought in contact with each other. The blades have a thickness of about 400 micrometer.

[0105] The process was carried out once without, and once with the application of a thermal interface wax (Crayotherm® KU-CR of KUNZE Folien GmbH).

[0106] The pleated film is shown in an enlarged view in FIGS. 7a and 7b. The results as regards the thermal conductivity in the direction perpendicular to the pleats, with and without an additional thermal interphase wax is given in Table 1.

Example 2

[0107] A thermally anisotropic expanded PTFE film with a thickness of 40 μm was manufactured according to the procedure as described in example 1. In order to manufacture a film with a thickness of 40 μm the dry tape had a final thickness of 0,653 mm. Said film is subjected to the pleating and compressing process of the present invention as described in example 1 and a pleated structure of said film with a pleat height of 450 μm is obtained. The process was carried out once without, and once with the application of a thermal interface wax.

[0108] The pleated film is shown in an enlarged view in FIGS. 8a and 8b. The results as regards the thermal conductivity in the direction perpendicular to the pleats, with and without an additional thermal interphase wax is given in Table 1.

Example 3 (Comparative)

[0109] A thermally anisotropic expanded PTFE film with a thickness of 40 μm was manufactured according to the procedure in example 1. In order to manufacture a film with a thickness of 40 μm the dry tape had a final thickness of 0,653 mm. Eight (8) layers of said film were put one on top of the other forming a stack. The stack was slightly compressed and is shown in FIG. 9. The stack was made once without, and once with application of a thermal interface wax.

[0110] The results as regards the thermal conductivity in the direction perpendicular to the film plain are given in Table 1.

Example 4

[0111] A highly oriented UHMW polyethylene film (commercially available by the company ENDUMAX of Teijin Ltd, Japan) with a thickness of 64 μm has been subjected to the following process:

1) The film was pleated with two counter rotating saving 2 mm tooth height
2) Primer 94 (commercially available by the company 3M Deutschland GmbH, Germany) was applied with a brush on top of the pleat tips and then dried for 3 hours at room temperature to remove the solvent
3) The pleated structure was prior to compacting, arranged in a channel construction made of aluminum. The bottom plate of the channel construction has a width of 80 mm and the side walls of the channel construction have a height of 2 mm. After arranging the pleated structure in the channel construction, a top plate was arranged on top of the construction to close the channel. Two plates, each with a thickness of about 2 mm, were put at the respective ends of the pleated structure and pressed against each other with a pressure by hand. From one side additional pressure was applied with a hammer by hand to compact the material even more. Then the material was put in an oven at 80° C. for 5 min. After the end of the heating step and a 30 min cooling period the top plate was removed and the compacted pleated structure was taken off the channel construction

[0112] FIG. 10 shows enlarged cross-section views of the pleated and compressed structure.

[0113] The results as regards the thermal conductivity in the direction perpendicular to the film plain are given in Table 1.

TABLE-US-00001 TABLE 1 Thermal Properties Thermal conductivity in z-direction (perpendicular to pleats/plain of sheet(s)) Example 1 Unpleated PTFE film 0.05 W/mK Pleated PTFE film without 4 W/mK thermal interface wax Pleated PTFE film with thermal 5.5 W/mK interface wax Example 2 Unpleated PTFE film 0.05 W/mK Pleated PTFE film without 2 W/mK thermal interface wax Pleated PTFE film with thermal 7 W/mK interface wax Example 3 (comparative) PTFE film stack without 0.06 W/mK thermal interface wax PTFE film stack with thermal 0.06 W/mK interface wax Example 4 Unpleated PE film 0.093 W/mK Pleated/compacted/bonded PE >10 W/mk film

[0114] The invention of this application has been described above both generically and with regard to specific embodiments. It will be apparent to those skilled in the art that various modifications and variations can be made in the embodiments without departing from the scope of the disclosure. Thus, it is intended that the embodiments cover the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.