WO2019115428A1

WO2019115428A1 - Process for the production of a thermally conductive tape

Info

Publication number: WO2019115428A1
Application number: PCT/EP2018/084106
Authority: WO
Inventors: Satoru Kobayashi; Ryuta Suzuki; Marco Greb; Sabine RENKER-MAACK
Original assignee: Merck Patent Gmbh
Priority date: 2017-12-12
Filing date: 2018-12-10
Publication date: 2019-06-20

Abstract

The present invention relates to a process for the production of a thermally conductive tape, which is advantageously useful for the insulation of electrical machines or devices, to a thermally conductive tape produced by such a process as well as to the use thereof.

Description

P 17300 UH

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Process for the production of a thermally conductive tape

The present invention relates to a process for the production of a thermally conductive tape, which is advantageously useful for the insulation of

5 electrical machines or devices, in particular those where high voltages are used, to a thermally conductive tape produced by such a process, as well as to the use thereof.

Electrical machines and devices, in particular those where high voltages

10 are used, such as electric cable bundles, conductors, coils, generators, rotors, stators, etc., need good insulation against corona discharges.

Besides pure insulating polymers or polymers containing fillers, mica is often used as a matter of choice, frequently in the form of mica tapes, wherein ground mica particles are arranged as a film of overlapping

15 particles and where the mica film is in most cases applied onto a carrier material, for example a woven glass fiber, and eventually covered by a protective layer. Flexible mica tapes of different composition are thus available in the market.

20 Mica tapes of the kind mentioned above exhibit a satisfactory protection against corona discharges because of the good dielectric characteristics of mica. Nevertheless, mica exhibits a poor thermal conductivity. Therefore, heat produced in the interior of the electrical machines and devices is not transferred to the surface of these machines and devices in case they are

25 insulated with mica tapes or different mica containing products. In many applications, better thermal conductivity of electrical insulating coverings of the machines and devices would be of high advantage, since increased thermal conductivity would result in increased power ratings of the ma- chines and devices and the commonly used air cooling of those machines

30 would be more effective. P 17300 UH

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Therefore, there have been made many efforts to provide technical solutions in order to achieve good electrical insulation as well as good thermal conductivity for insulating coverings of electrical machines and devices in the last years.

5

In particular attempts have been made to adjust the properties of mica tapes so that a higher insulation resistance, mechanical stability and/or thermal conductivity, each in combination with a certain flexibility, is achieved.

10

To this end, in EP 406 477 A1 a reinforced mica paper is disclosed, where a base layer is made of mica which is then reinforced by a further layer on at least one surface thereof, the further layer containing a mixture obtained by mixing arbitrary amounts of silicone resin, aluminium hydroxide, alumi-

15 nium silicate, potassium titanate and a soft mica powder. The insulation resistance of such a mica paper is increased in comparison to usual mica papers.

A highly heat conductive tape is disclosed in US 7,425,366 B2. Here, the

20 tape contains a mica containing layer and a lining material, and the mica containing layer contains scaly particles having a heat conductivity of 0.5 w/mK or higher, a size of 1 pm or smaller, and a binder. Although mica tapes of this kind are flexible similar to usual mica tapes, the thermal conductivity thereof is, although higher than in usual mica tapes, not

25 sufficient in order to result in a higher energetic efficiency of the insulated electric machine or device. In addition, due to several layers in the mica tapes, the thickness thereof is relatively high, leading to limitations in flexibility and use.

30 In WO 2006/002010 A2, a surface coating of a lapped insulation tape is disclosed. The surface coating comprises a high thermal conductivity (HTC) material and a resin. As tape substrate, e.g. a paper, a mica tape, a glass P 17300 UH

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- 3 - fiber cloth or a polyethylene glycol terephthalate mat may be used. The HTC material is preferably a nanosized material and encompasses AI₂O3, AIN, Mg0₂, ZnO, BN, SbN₄, SiC and S1O₂. It may be contained in a homogeneous mixture with the resin up to a volume percentage of

5 approximately 60%. In case that mixtures of size and shape regarding the HTC materials are used, the volume content may be as high as 90% with 10 to 30% by volume of resin, based on the volume of the layer. The corresponding resins are e.g. epoxy resins, polyesters, polyurethanes, polyimides and polyesterimides. Although the thermal conductivity of the

10 coated tape seems to be high, the resin content of the HTC layer is still too large since, usually, the resin compound diminishes the thermal

conductivity of the HTC filler. On the other side, a certain amount of resin is needed in order to assure the adhesion of the HTC material in the HTC layer to the tape substrate.

15

In order to provide thermally conductive tapes having a very low content of organic binders, WO 2013/053442 A1 discloses a thermally conductive self- supporting sheet which consists of 70.0 to 99.9% by weight of a particulate HTC filler and of from 0.1 to 30% by weight of a film forming organic

20 compound. In the corresponding production process, large agglomerates of the HTC fillers are formed by a particular surface treatment prior to adding the resinous material to a suspension of the HTC materials and applying the resulting slurry to a filter sheet. Although the self-supporting thermally conductive sheet which is achieved by such a process exhibits a certain

25 mechanical stability as well as a good flexibility and thermal conductivity, it has turned out that the mechanical stability thereof needs to be improved even further and that the HTC agglomerates in the resinous slurry tend to settle down due to their large size, leading to a very short lifetime of a stable to use HTC containing slurry. In addition, the process does not fit to

30 usually used industrial processes for the production of thermally conductive tapes or sheets where backing sheets are used in general. P 17300 UH

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Therefore, it would be of high advantage, if an electrically insulating tape could be provided for insulation purposes, which exhibits sufficient insula- tion against corona discharge, a sufficient thermal conductivity for the heat transfer to the outside of the machine or device due to a maximum content

5 of HTC materials, thereby increasing the energy efficiency of the machine or device, which would exhibit a good flexibility and mechanical stability and may be produced in a standard process using a backing sheet for supporting a thermally conductive layer, wherein a resinous slurry of HTC materials is not supposed to be decomposed during the production

10 process.

Therefore, the object of the present invention is to provide a process for the production of an electrically insulating, flexible, thermally conductive tape having the properties described above.

15

In addition, a further object of the present invention is to provide the corresponding thermally conductive tape produced by a convenient industrially adaptable process.

20 Furthermore, it is another object of the present invention to provide useful applications for such a thermally conductive tape.

The object of the present invention is solved by a process for the production of a thermally conductive tape, comprising the following steps:

25

a) keeping an aqueous suspension of platelet-shaped aluminium oxide particles under stirring,

b) adding to the suspension at most 0.1 % by weight of a film

forming organic compound solution or emulsion, based on the

30 total solids content of the film forming organic compound and the aluminium oxide particles, whereby the film forming organic compound is selected from the group consisting of acrylate P 17300 UH

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- 5 - polymers, butadiene polymers and copolymers containing acrylate and/or butadiene units,

c) applying the then resulting suspension onto a backing sheet, thereby resulting in a wet layer containing the platelet-shaped

5 aluminium oxide particles on the backing sheet, and

d) drying the resulting layer, whereby a solid thermally conductive tape is obtained consisting of the backing sheet and of a solid layer thereon being composed of at least 99.9 % by weight of the platelet-shaped aluminium oxide particles and of at most 0.1 % by

10 weight of the film forming organic compound, based on the solid layer.

Furthermore, the object of the present invention is also solved by a thermally conductive tape, consisting of a backing sheet and of a solid layer 15 being composed of at least 99.9 % by weight of platelet-shaped aluminium oxide particles and of at most 0.1 % by weight of an organic film forming compound, based on the solid layer, on the backing sheet, wherein the film forming organic compound is selected from the group consisting of acrylate polymers, butadiene polymers and copolymers containing acrylate and/or 20 butadiene units.

In addition, the object of the present invention is solved by the use of a thermally conductive tape as described above for the insulation of electrical machines or devices.

25

In the first process step a) according to the present process, an aqueous suspension of platelet-shaped aluminium oxide particles is provided and kept in movement, preferably by stirring it.

30 Aluminium oxide particles are known to exhibit a high intrinsic thermal

conductivity and have been used as fillers for thermally conductive coatings or resins already. P 17300 UH

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The aluminium oxide particles which are used in the process according to the present invention exhibit a platelet shape which means that they exhibit a platy, flat structure and an aspect ratio [ratio of mean longest axis (length or width) to mean shortest axis (thickness) of the particles] of at least 20,

5 preferably of at least 50, and most preferred of at least 80. The platelet shaped form of the primary filler particles allows slight overlaps of the single particles in the resulting tape and good orientation of the primary filler particles along the largest surfaces of the thermally conductive tape which is formed.

10

In general, aluminium oxide platelets exhibiting a volume based particle size dso in the range of from 1 < dso <50 pm and a volume based particle size dgs in the range of from 2 < dgs < 100 pm may be used. Preferably, aluminium oxide platelets exhibiting a volume based particle size dso in the

15 range of from 5< dso <30 pm and a volume based particle size dgs in the range of from 10 < dgs < 50 pm are useful, and aluminium oxide platelets exhibiting a volume based particle size dso in the range of from 10 < dso <20 pm and a volume based particle size dgs in the range of from 30 < dgs < 40 pm are most preferred.

20

For the purpose of the present invention, the particle size as such is regarded as being the length of the longest axis of the particulate filler material. The particle size of these particles can in principle be determined using any method for particle-size determination that is familiar to the

25 person skilled in the art. The particle-size determination can be carried out in a simple manner, depending on the size of the particles, for example by direct observation and measurement of a number of individual particles or agglomerates in high-resolution light microscopes, such as the scanning electron microscope (SEM) or the high-resolution electron microscope

30 (HRTEM), but also in the atomic force microscope (AFM), the latter in each case with appropriate image analysis software. The determination of the particle size can advantageously also be carried out using measuring P 17300 UH

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- 7 - instruments (for example Malvern Mastersizer 3000, APA300, Malvern Instruments Ltd., UK), which operate on the principle of laser diffraction. Using these measuring instruments, both the particle size and also the particle-size distribution in the volume as disclosed above (dso, dgs) can be

5 determined from a particle suspension in a standard method (SOP). The last-mentioned measurement method is preferred in accordance with the present invention.

In addition, the aluminium oxide for the platelet aluminium oxide particles

10 may also be doped with a minor amount of titanium dioxide. About 0.1 to 5% by weight, based on the total weight of aluminium oxide and titanium oxide, may be of titanium dioxide. Platelet-shaped aluminium oxide particles containing such a minor amount of titanium oxide will be referred to as platelet-shaped aluminium oxide particles in the following too, like

15 pure aluminium oxide platelets. Indeed, platelet-shaped aluminium oxide particles containing such minor amounts of titanium oxide are especially preferred according to the present invention.

Platelet shaped filler particles of aluminium oxide who’s particle size and

20 aspect ratio is within the ranges described above can be prepared

according to the patent mentioned below. Preferred are aluminium oxide platelets, which are usually used as substrates for the production of effect pigments such as interference pigments (e.g. interference pigments which are traded under the name Xirallic® by Merck KGaA, Darmstadt, Germany).

25 Platelet shaped aluminium oxide pigments of this type may be produced by particular crystallization processes leading to single crystals and may contain a minor amount (up to about 5% by weight) of foreign metal oxides such as titanium dioxide. They may be produced in a process similar to the substrate forming steps as described in EP 763573 B1 , by varying the

30 amount of titanium dioxide within the limits given in the a.m. patent, by

varying the temperature of the final heat treatment and the time for P 17300 UH

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8 crystallization growth in order to achieve at the right particle size and aspect ratio.

In a similar process, pure platelet-shaped aluminium oxide particles may

5 also be produced simply by omitting the titanium dioxide. For the purpose of the present invention, the platy shape, the size and the thickness of those aluminium oxide particles would be of sufficient quality. Nevertheless, platelet-shaped aluminium oxide particles containing minor amounts of titanium dioxide as described above are preferred.

10

Although not necessary, it is preferred that the pH of the aqueous

suspension of the platelet-shaped aluminium oxide particles is adjusted in the range of from pH 1 to 5 by adding a useful acid to the aqueous suspension. Acids which may be used are in particular strong mineral acids

15 such as HCI, H₂S0₄ or HNO3 which are added to the suspension in an

appropriate amount in order to achieve the desired pH range. HCI is particularly preferred.

The acidic pH achieved by such an addition of a mineral acid alters the

20 surface charge of the particular filler material so that the particulate filler material is capable to form strong bonds to the binder material which is added in the next production step as well as to enable aggregation of the filler particles with each other. Nevertheless, the formation of aggregates must be stopped at a certain degree in order to avoid settlement of these

25 aggregates in the aqueous slurry containing the aluminium oxide particles and the film forming organic compound which is provided in the next production step. Therefore, it seems to be of advantage if a time limit for the formation of the aqueous suspension of platelet-shaped aluminium oxide particles is set. Thus, it is recommended to set the time limit for production

30 step a) of the present process to a period of from 5 to 20 minutes,

especially from 5 to 15 minutes, in particular if the pH of the aqueous suspension is in the preferred range of from 1 to 5. P 17300 UH

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In the following process step b), a film forming organic compound is added to the aqueous suspension of the aluminium oxide platelets in form of a solution or emulsion thereof. Preferably used are aqueous solutions or emulsions. The film forming organic compound is selected from the group

5 consisting of acrylate polymers, butadiene polymers and copolymers

containing acrylate and/or butadiene units. These polymers exhibit excellent film forming characteristics even when used in tiny amounts relative to the particulate filler material. Especially preferred are acrylate polymers and copolymers containing acrylate and butadiene units, in

10 particular those of the latex type, such as acrylonitrile butadiene latex.

Examples for acrylate polymers are polypropylacrylate, poly-n-butylacrylate, poly-iso-butylacrylate, poly-t-butylacrylate, polypentylacrylate, polyhexyl- acrylate, polyheptylacrylate, polyoctylacrylate, poly-2ethylhexylacrylate,

15 polynonylacrylate and polydecylacrylate. Useful polymeric diacrylates are polymers of e.g. iso-propylen diacrylate and tetramethylen diacrylate. In addition, methacrylate polymers such as polymers of butylmethacrylate, pentylmethacrylate, hexylmethacrylate, heptylmethacrylate, octylmeth- acrylate, nonylmethacrylate, decylmethacrylate and ethylenglycol

20 dimethacrylate may be used.

Copolymers may be used which are formed by acrylate polymers in combination with e.g. acrylonitrile, acrylic acid, methacrylic acid, maleic acid, styrene, alpha-methylstyrene, chlorostyrene, acrylamide,

vinylbenzylalcohol, styrenesulfinic acid, styrenesulfonic acid or

25 divinylbenzene. Copolymers containing acrylate units are preferably used.

Polymers containing butadiene units are for example polybutadiene and acrylonitrile butadiene latex, the latter containing acrylate units as well as butadiene units in the molecule. The acrylonitrile butadiene latex is most

30 preferably used in the present process due to its excellent film forming

characteristics. P 17300 UH

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The content of the film forming organic compound which acts as a binder for the platelet-shaped aluminium oxide particles to each other and to the backing sheet in the solid layer on the backing sheet which is formed in the present process is preferably in the range of from 0.001 to 0.05 % by

5 weight, based on the weight of the layer. Especially preferred, the content of the film forming compound is as low as 0.01 to 0.045 % by weight, based on the weight of the solid layer. Correspondingly, the content of the platelet- shaped aluminium oxide particles in the solid layer is preferably in the range of from 99.95 to 99.999 % by weight, and most preferred of from

10 99.955 to 99.99 % by weigt, based on the weight of the layer. In each case, the total sum of the contents of film forming organic compound and the platelet-shaped aluminium oxide particles constitutes 100% of the weight of the solid layer. Therefore, based on the solids content of the starting materials, the amount of the different starting materials for the film forming

15 organic compound and the platelet-shaped aluminium oxide particles is chosen accordingly. Since a low content of organic compounds (binder) in the thermally conductive tape according to the present invention is of advantage, the amount of the film forming organic compounds in the present process should be chosen as low as possible.

20

Regarding the time frame for production steps a) and b), an overall time consumption of more than 30 minutes should be avoided. The preferred time limit for exhibiting production steps a) and b) is, therefore, in the range of from 10 to 30 minutes in order to avoid settling of the aluminium oxide

25 particles in the resulting slurry and, thus, decomposition thereof.

The aqueous suspension resulting from process steps a) and b) is then applied to a backing sheet in production step c), resulting in a wet layer containing the platelet-shaped aluminium oxide particles as well as the film

30 forming organic compound on the backing sheet. P 17300 UH

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Although in principle every backing sheet known in the art for the produc- tion of thermally conductive tapes may be used, backing sheets made of glass fiber cloths or polymer films are preferred, since these materials are most useful for the present process in order to avoid mica containing

5 materials and in order to be as close to the standard production methods for thermally conductive tapes as possible. In particular, backing sheets made of glass fiber cloths are preferred. These backing sheets exhibit pores which allow the liquid compounds of the applied compositions to run through while the solids remain on the upper surface of the backing sheet.

10 According to the present invention, standard glass cloths having a pore size in the range of from 300 to 500 pm may advantageously be used, although the particle size of the starting platelet-shaped aluminium oxide particles is much smaller than the pore size of the backing sheet material.

Nevertheless, the preparation method according to steps a) to c) allows the

15 formation of a layer on the surface of the backing sheet being composed of a wet composition containing the platelet-shaped aluminium oxide particles particulate filler material as well as the film forming organic compound which is selected from the group consisting of acrylate polymers, butadiene polymers and copolymers containing acrylate and/or butadiene units.

20

In production step d) which follows, the wet layer on the backing sheet achieved in step c) is dried, whereby a solid thermally conductive tape is obtained consisting of the backing sheet and of a solid layer thereon being composed of at least 99.9 % by weight of the platelet-shaped aluminium

25 oxide particles and of at most 0.1 % by weight of the film forming organic compound, based on the solid layer.

The drying temperature is chosen in the range of from about 50°C to about 250°C and is applied for about 5 minutes to 24 hours. A shorter drying time

30 is of economic advantage. As well, the drying temperature should be

chosen as low as possible in order to avoid the formation of micro cavities in the resulting solid layer on the backing sheet. P 17300 UH

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It might be of advantage if the dried thermally conductive tape resulting from the process according to the present invention is allowed to be stored at a temperature in the range of from 1 to 10°C after drying in order to strengthen the hardness and mechanical stability thereof. Such a cooling

5 procedure does astonishingly not influence the flexibility of the thermally conductive tape in a negative way.

The resulting thermally conductive tape according to the present process is of an excellent mechanical strength and exhibits a high flexibility as well as

10 a high thermal conductivity due to the very high relative content of the

thermally conductive platelet-shaped aluminium oxide particles in the thermally conductive layer on the backing sheet.

The object of the present invention is also achieved by a thermally

15 conductive tape, consisting of a backing sheet and of a solid layer being composed of at least 99.9 % by weight of platelet-shaped aluminium oxide particles and of at most 0.1 % by weight of an organic film forming compound, based on the solid layer, on the backing sheet, wherein the film forming organic compound is selected from the group consisting of acrylate

20 polymers, butadiene polymers and copolymers containing acrylate and/or butadiene units.

The aluminium oxide platelets useful in the present thermally conductive tape generally exhibit a volume based particle size dso in the range of from

25 1 < dso <50 pm and a volume based particle size dgs in the range of from 2 < dgs < 100 pm prior to their application in the respective process. Preferably, aluminium oxide platelets exhibiting a volume based particle size dso in the range of from 5< dso <30 pm and a volume based particle size dgs in the range of from 10 < dgs < 50 pm are useful, and aluminium oxide platelets

30 exhibiting a volume based particle size dso in the range of from 10 < dso <20 pm and a volume based particle size dgs in the range of from 30 < dgs < 40 pm are most preferred. P 17300 UH

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In the thermally conductive tape according to the present invention, the content of the platelet-shaped aluminium oxide particles in the solid layer is preferably in the range of from 99.95 to 99.999% by weight, based on the weight of the solid layer. In addition, the content of the film forming organic

5 binder is preferably 0.001 to 0.05 % by weight, based on the weight of the solid layer, while the sum of platelet-shaped aluminium oxide particles and binder, in each case, is 100% by weight, based on the weight of the solid layer.

10 As described above, the backing sheet might be any backing sheet which is usually used for the production of thermally conductive tapes and may even be a mica sheet, but since mica due to its low thermal conductive characteristics should be avoided if possible, preferred backing sheets are composed of glass fiber cloths or are polymeric films, and glass fiber cloths

15 being most preferred.

In case it would be requested or desired, the thermally conductive tape according to the present invention may also be provided with an additional covering layer, in particular as a protective layer, on top of the thermally

20 conductive layer. In addition, it might be advantageous to further treat the resulting thermally conductive tape as a whole during application thereof, e.g. by applying resinous compounds thereon.

The thermally conductive tape of the present invention has a thickness in

25 the range of from 0.01 to 50 mm which may be varied according to the

thickness of the backing sheet and of the amount of platelet-shaped aluminium oxide particles and film forming organic binder components applied thereon. The thickness of the sheet may be measured by any instrument being able to measure length in the range of micrometers.

30

In order to achieve a high value of an overall thermal conductivity of the tape, the layer thickness of the layer containing the thermally conductive P 17300 UH

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- 14 - platelet-shaped aluminium oxide particles shall be adjusted in such a way that the volume content of the platelet-shaped aluminium oxide particles in the thermally conductive tape is at least 70% by volume, based on the volume of the thermally conductive tape (i.e. based on the total volume of

5 thermally conductive layer and backing sheet). In particular, the volume content of the platelet-shaped aluminium oxide particles shall be in the range of from 70 to 90 % by volume, in particular of from 75 to 85% by volume, based on the volume of the thermally conductive tape as a whole.

10 Furthermore, the object of the present invention is also achieved by the use of the thermally conductive tape according to the present invention for the insulation of machines and devices. Examples for machines and devices are electrical facilities such as electric cable bundles, conductors, coils, generators, rotors, stators, etc.

15

Machines and devices using or generating high voltages are subject to exhibit corona discharges if not insulated good enough. Therefore, in order to avoid such corona discharges and in order to allow a good cooling beha- viour and, combined therewith, increased power ratings, of such facilities,

20 the thermally conductive and, at the same time, electrical insulating tape of the present invention may advantageously be used for such purposes. The thermally conductive tapes according to the present invention exhibit a dielectrical behavior, but also flexibility and mechanical stability, while showing a high thermal conductivity at the same time. They may be rolled

25 up in a web like form and may, thus, be used in versatile application modes, since the insulation made therewith may be lapped or wrapped around a device or facility which exhibits any form or size. Contrary to mica tapes, they exhibit a high thermal conductivity which is due to the fact that the thermally conductive layer of the tapes is composed to an extremely high

30 extent, preferably to more than 99.9% by weight, of a platelet-shaped

material having a high intrinsic thermal conductivity per se. Therefore, they may be advantageously used instead of mica tapes for insulation purposes P 17300 UH

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15 - when a high thermal conductivity of the insulation material is appropriate. In addition, they may be easily produced in an established production process which is usually used for the production of thermally conductive tapes.

5 Figure 1 : relates to photographs 1 a through 1 e which show the

thermally conductive tapes of examples 1 to 5 according to the invention after testing the flexibility thereof

Figure 2: relates to photographs 2a and 2b which show the thermally

10 conductive tapes of comparative examples 1 and 2 after testing the

flexibility thereof

The present invention shall be explained to some detail by the following examples, but shall not be limited to these examples.

15

Example 1 :

A slurry of alumina flake particles having a mean diameter of about 20 pm is prepared in a concentration of 10Og/l. The pH of the slurry is adjusted to 1.0 by adding a 32% HCI solution and the slurry is stirred for 10 minutes.

20 2.32 g of the resulting alumina flake slurry are mixed with 0.48 g of a

0.045% solution of LX874 (acrylonitrile butadiene latex, product of Nihon Zeon Corp., Japan) and 7.20 g of deionized water and mixed for 10 minutes.

25 The suspension described above is poured onto a glass mesh sheet

substrate having a pore size of 400 pm. The coated substrate is dried at 100°C for about 10 minutes. The resulting thermally conductive tape is evaluated as disclosed in table 1.

30 Example 2:

Example 1 is repeated except that 0.048 g of the LX874 solution is used instead of 0.48 g in example 1. P 17300 UH

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Example 3:

Example 1 is repeated except that 0.0048 g of the LX874 solution is used instead of 0.48 g in example 1.

5 Example 4:

Example 1 is repeated except that the pH is adjusted to 5.

Example 5:

Example 2 is repeated except that the pH is adjusted to 5.

10

Comparative example 1 :

A thermally conductive tape is produced according to example 1 , but neither the pH is adjusted nor is any film forming organic compound added.

15 Comparative example 2:

A thermally conductive tape is produced according to example 1 , except that 0.00048 g of the LX874 solution is used instead of 0.48 g in example 1.

Evaluation of the thermally conductive tapes:

20

The passage of the aluminium oxide flakes through the glass mesh substrate in all examples is evaluated, as well as the flexibility of the thermally conductive tape and the adhesion of the thermally conductive layer on the backing sheet.

25 The results are disclosed in table 1 , wherein“5” means very good and“1” means poor.

Passage of alumina flakes (the lower the better):

5: no passage is observed

30 4: passage is less than 0.01 % by weight

3: passage is less than 0.1 % by weight

2: passage is less than 1 % by weight P 17300 UH

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17 -

1 : passage is more than 1 % by weight

Adhesion of the thermally conductive layer on the glass substrate:

5: no change of the surface of the alumina containing layer is observed, no 5 peeling occurs when the surface of the layer is touched by a finger

2-4: some destruction of the surface, little to some peeling 1 : the alumina containing layer is completely destroyed or peeled at the point where it is touched by a finger

10 Flexibility of the thermally conductive tape:

The alumina containing thermally conductive sheet is put on a cylinder surface having a diameter of 40 mm for 1 minute, then removed and evaluated on a flat desk whether or not cracks have occurred 5: no change on the surface can be observed

15 2-4: there is little to some cracks in the surface observable

1 : the alumina containing layer on the backing sheet is completely cracked and is partly peeled off the backing sheet

Table 1 :

20

Regarding comparative examples 1 and 2, the adhesion of the alumina containing layer on the glass fiber backing sheet is weak and can be easily peeled off by touching it with a finger. Contrary to this, the adhesion of the alumina containing layer in examples 1 through 5 varies from excellent to P 17300 UH

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- 18 - sufficient. In addition, the flexibility of the thermally conductive tape in examples 1 , 2 and 4 is also much better than that of comparative examples 1 and 2. Although the passage behaviour of the alumina particles in comparative example 2 is excellent, the content of the film forming

5 compound of merely 0.0001 % by weight of the thermically conductive layer is too low in order to achieve sufficient adhesion of the thermally conductive layer to the backsheet.

Photographs of the resulting thermally conductive tapes after evaluating the 10 flexibility thereof are shown in Figures 1 (1 a = example 1 , 1 b = example 2, 1 c = example 3, 1 d = example 4, 1 e = example 5) and 2 (2a = comparative example 1 , 2b = comparative example 2).

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30

Claims

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Claims

1. Process for the production of a thermally conductive tape, comprising the following steps :

5 a) keeping an aqueous suspension of platelet-shaped aluminium oxide particles under stirring,

b) adding to the suspension at most 0.1 % by weight of a film forming organic compound solution or emulsion, based on the total solids content of the film forming organic compound and

10 the aluminium oxide particles, whereby the film forming

organic compound is selected from the group consisting of acrylate polymers, butadiene polymers and copolymers containing acrylate and/or butadiene units,

c) applying the then resulting suspension onto a backing sheet,

15 thereby resulting in a wet layer containing the platelet-shaped aluminium oxide particles on the backing sheet, and

d) drying the resulting layer, whereby a solid thermally conductive tape is obtained consisting of the backing sheet and of a solid layer thereon being composed of at least 99.9 % by weight of

20 the platelet-shaped aluminium oxide particles and of at most

0.1 % by weight of the film forming organic compound, based on the solid layer.

2. Process according to claim 1 , wherein the particulate filler material is

25 contained in the solid layer in an amount corresponding to 99.95 to

99.999% by weight, based on the layer.

3. Process according to claim 1 or 2, wherein the film forming organic compound is contained in the solid layer in an amount corresponding

30 to 0.001 to 0.05% by weight, based on the layer. P 17300 UH

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4. Process according to one or more of claims 1 to 3, wherein the

aqueous suspension of the platelet-shaped aluminium oxide particles in step a) is kept under a pH in the range of from 1 to 5 prior to step b).

5

5. Process according to one or more of claims 1 to 4, wherein the film forming organic compound is an acrylate polymer or a copolymer containing acrylate and butadiene units.

10 6. Process according to one or more of claims 1 to 5, wherein process steps a) and b) are accomplished in an overall time limit in the range of from 10 to 30 minutes.

7. Process according to one or more of claims 1 to 6, wherein the 15 platelet-shaped aluminium oxide particles exhibit wherein the alumina platelets exhibit a volume based particle size dso in the range of from 1 < d₅o <50 pm and a volume based particle size dgs in the range of from 2< dgs <100 pm.

20 8. Process according to claim 7, wherein the platelet-shaped aluminium oxide particles exhibit an aspect ratio of at least 20.

9. Process according to one or more of claims 1 to 8, wherein the

backing sheet is selected from glass cloth and polymer film.

25

10. Process according to claim 9, wherein the glass cloth has open pores exhibiting a pore size in the range of from 300 to 500 pm.

11. Thermally conductive tape, consisting of a backing sheet and of a

30 solid layer being composed of at least 99.9 % by weight of platelet- shaped aluminium oxide particles and of at most 0.1 % by weight of an organic film forming compound, based on the solid layer, on the P 17300 UH

WO 2019/115428 PCT/EP2018/084106

- 21 backing sheet, wherein the film forming organic compound is selected from the group consisting of acrylate polymers, butadiene polymers and copolymers containing acrylate and/or butadiene units, obtained by a method according to one or more of claims 1 to 10.

5

12. Thermally conductive tape according to claim 11 , wherein the platelet- shaped aluminium oxide particles are contained in the solid layer in an amount corresponding to 99.95 to 99.999 % by weight, based on the solid layer.

10

13. Thermally conductive tape according to claim 11 or 12, wherein the platelet-shaped aluminium oxide particles are contained in the thermally conductive tape in an amount which corresponds to at least 70% by volume, based on the thermally conductive tape.

15

14. Use of a thermally conductive tape according to one or more of claims 11 to 13 for the insulation of machines and devices.

15. Use according to claim 14, wherein the machine or device is an

20 electric cable bundle, a conductor, a coil, a generator, a rotor or a stator.

16. Electric cable bundle, conductor, coil, generator, rotor or stator, being provided with a thermally conductive tape according to one or more of

25 claims 11 to 13.

30