WO2019115427A1

WO2019115427A1 - Process for the production of a thermally conductive tape

Info

Publication number: WO2019115427A1
Application number: PCT/EP2018/084105
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

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 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 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 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. 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 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 would be more effective. 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.

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.

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- 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 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 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. In US 2004/0018342 A1 , a thermally conductive sheet is disclosed, which includes a polyolefin elastomer mixed with a thermally conductive filler. The sheet has a high thermal conductivity and is said to generate a low amount of volatile organic gas. The polyolefin is a polyisobutylene exhibiting an allyl group at both terminals. Several thermally conductive fillers may be used. The related production process comprises kneading the binder, the thermally conductive filler and further ingredients such as crosslinking agent, curing agent, retarder and antioxidant, sandwiching the resulting mixture between films and press-molding it. The resultant solid sheet is released from the films. It is said that such sheets might be used even in closed places due to the low generated amount of volatile organic gas. Nevertheless, several types of particular organic ingredients are needed and the polymer which is useful must exhibit a special composition, leading to relative high production cost. In addition, the generation of volatile corrosive gas in the production process and from the resulting sheet may not be avoided completely. Nevertheless, 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, 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 the use of organic solvents may be avoided completely during the production step so that production and use of the resulting thermally conductive tape would be highly environmentally friendly.

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. In addition, a further object of the present invention is to provide the corresponding thermally conductive tape produced by an environmentally friendly process. 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: a) keeping an aqueous suspension of a particulate filler material having an intrinsic thermal conductivity of at least 5 W/mK under stirring while adjusting a pH in the range of from 1 to 3, b) adding to the suspension an aqueous binder solution of a water soluble polymer selected from the group consisting of polyvinyl alcohol and cellulose type polymers, and a plasticizer,

c) adding a water soluble alkali metal salt or water soluble alkaline earth metal salt to the suspension,

d) applying the then resulting suspension onto a backing sheet, thereby resulting in a wet layer containing the particulate filler material on the backing sheet, and

e) 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 the particulate filler material, of a film forming binder of polyvinyl alcohol or of a cellulose type polymer, and of the plasticizer.

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 being composed of a particulate filler material having an intrinsic thermal conductivity of at least 5 W/mK, of a film forming binder and of a plasticizer, on the backing sheet, wherein the binder is of polyvinyl alcohol or of a cellulose type polymer. 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. in the first process step a) according to the present process, an aqueous suspension of a particulate filler material having an intrinsic thermal conductivity of at least 5 W/mK is provided.

Filler materials having an intrinsic thermal conductivity of at least 5 W/mK are known per se and have been used as fillers for thermally conductive coatings or resins already. Usually, when being particular, they exhibit rather small particle sizes of about 1 pm or smaller like in US 7,425,366 B2, of from 0.1 to 15 pm as described in EP 266 602 A1 , or of from 1 pm to 100 pm as disclosed in DE 197 18 385 A1. While the smaller particle ranges might be achieved by grounding appropriate starting materials, particles sizes of larger than 20 pm are seldom available in the market, at least not for each and any of the materials which would fulfil the intrinsic thermal conductivity requirement. The particulate filler material used in the process according to the present invention is composed of filler particles which are chosen from at least one of aluminium oxide, boron nitride, boron carbide, diamond, carbon nitride, aluminium carbide, aluminium nitride, silicon oxide, silicon carbide, silicon nitride, magnesium oxide, zinc oxide, beryllium oxide, or mixtures thereof.

Of these, filler particles of aluminium oxide are preferred. Aluminium oxide, according to the present invention, is preferably used as the main compo- nent of the filler material. This means that preferably more than 50% by weight, based on the weight of the filler, is of aluminium oxide, i.e. of aluminium oxide filler particles. The aluminium oxide filler particles may also be used in combination (e. g. mixture) with filler particles made of one or more compounds, chosen from the compounds mentioned above. Preferred is the embodiment of the invention wherein all of the filler, i. e. all of the filler particles, is of aluminium oxide.

In the latter case, aluminium oxide particles exhibiting a platelet shape are preferably used 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, 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.

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 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 < d95 < 40 pm are most preferred.

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 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 (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 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 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 filler particles 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. Aluminium oxide filler particles containing such a minor amount of titanium oxide will be referred to as aluminium oxide filler particles in the following too, like pure aluminium oxide filler particles. Indeed, aluminium oxide filler 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 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). 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 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 crystallization growth in order to achieve at the right particle size and aspect ratio. In a similar process, pure platelet-shaped aluminium oxide filler particles may 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 filler particles containing minor amounts of titanium dioxide as described above are preferred.

Platelet shaped filler particles having a particle size within the size range of 5-40 pm, made of boron nitride, boron carbide, diamond, carbon nitride, aluminium carbide, aluminium nitride, silicon oxide, silicon carbide, silicon nitride, magnesium oxide, zinc oxide, beryllium oxide or mixtures thereof, are available in the market.

The thermally conductive particulate filler material is kept in the aqueous suspension under stirring and a pH in the range of from 1 to 3 is adjusted by adding a useful acid to the aqueous suspension. Acids which may be used are in particular strong mineral acids 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 strong acidic pH achieved by such an addition of a mineral acid alters the 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. In the following process step b), an aqueous binder solution is added to the acidic aqueous suspension of the thermally conductive particulate filler material. As a binder, water soluble polymers are used which are selected from the group consisting of polyvinyl alcohol and cellulose type polymers. As cellulose type polymers, methyl cellulose, carboxymethyl cellulose, ethyl cellulose or hydroxyethyl cellulose may be used. Preferably, the water soluble binder is polyvinyl alcohol.

In the present process, the binder is used as an aqueous solution thereof. Regarding the polyvinyl alcohol which is preferably used, partially or completely saponified types may be used, or mixtures of both types. The degree of hydrolysis of the partially saponified (hydrolysed) type is between 70 and 95, preferably between 72 and 92 mol%, whereas the degree of saponification of the fully saponified grades is usually between 95 and 100 mol%, preferably between 97 and 100 mol%. If mixtures of partially and fully saponified grades are used, the ratio thereof can vary continuously in the range of from 0.1 : 99.9 to 99.9 :0.1. The degree of polymerization Pw of the polyvinyl alcohol is usually between 300 and 3000, while preferred products exhibit a Pw in the range of from 600 to 2500.

Typically, polyvinyl alcohols are described by their viscosity in water (as 4% by weight aqueous solution, see DIN 53015). According to the present process, particularly useful polyvinyl alcohols exhibit a viscosity ranging from 2 to 70 mPas, especially from 5 to 60 mPas, determined according to DIN 53015.

Polyvinyl alcohols which comprise additional functional groups such as silanol groups, amino groups, carboxyl groups and/or ethylene groups may also be used.

The plasticizer shall be an alcohol compatible plasticizer or a mixture of alcohol compatible plasticizers and is selected from the group consisting of alcohols and their derivatives, in particular of polyhydric alcohols and their derivatives. Examples are glycols, e.g. glycol, diglycol, triglycol,

polyethylene glycol, polypropylene glycol, glycerol, diols and/or triols. Preferably, polyethylene glycol is used as plasticizer. Depending on the molecular mass thereof, polyethylene glycol may be added either in solid or in liquid form. The addition of a plasticizer is in particular of advantage if the binder component is a crystalline thermoplastic such as polyvinyl alcohol. The plasticizer serves for increasing the flexibility of the binder component and the adhesion thereof to the backing sheet and, thus, increases the flexibility and mechanical stability of the resulting thermally conductive tape.

With respect to the products useful for the present process, in principle all kinds of products available in the market may be used. For the respective examples, the present inventors used a polyvinyl alcohol binder compound (Poval 15-99, product of Kuraray) and as plasticizer polyethylene glycol (PEG200, product of Sanyo Kasei Kogyo).

In order to achieve at a resulting thermally conductive tape having the desired mechanical stability and flexibility, both compounds, i.e. the film forming binder as well as the plasticizer, are indispensable for the present process. Neither the one nor the other compound alone will lead to a thermally conductive tape exhibiting the desired mechanical characteristics. To this end, both components may be used in different weight ratios without harming the desired mechanical characteristics of the end product.

The useful weight ratio of binder : plasticizer may vary in a broad range and is preferably in the range of from 10:1 to 1 :1.

The total content of film forming binder and plasticizer in the solid layer formed in the present process is preferably in the range of from 0.5 to 10% by weight, based on the weight of the layer. Correspondingly, the content of the particulate filler material in the solid layer is preferably in the range of from 90 to 99.5% by weight, based on the weight of the layer. In each case, the total sum of the contents of film forming binder, plasticizer and particulate filler material 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 binder, the plasticizer and the particulate filler material is chosen accordingly. Since a low content of organic compounds (binder and plasticizer) in the thermally conductive sheet according to the present invention is of advantage, the amount of the organic compounds in the present process should be chosen as low as possible. Regarding the process of the present invention, no organic volatile compounds are necessary to form the corresponding thermally conductive layer on the backing sheet. Rather, aqueous solutions of all ingredients are used in case these are not in a liquid state anyway. Therefore, no volatile organic gas may escape during the production of the thermally conductive tape or during its technical application.

After the film forming binder and the plasticizer are added in step b), it might be of advantage, though not necessary, to adjust the pH of the resulting suspension to a value in the range of from 6.5 to 7.5. To this end, a strong basic compound, especially a strong mineral base, may be added to the suspension. Strong mineral bases such as NaOH, KOH or NH₄OH in an appropriate amount and concentration are especially useful.

In production step c) which follows, a water soluble alkali metal salt or a water soluble alkaline earth metal salt is added to the suspension.

Preferably, the alkali metal salt or the alkaline earth metal salt is used in an aqueous solution. In particular NaCI, KCI, Na₂S0₄, MgCh, Mg(N03)₂ or MgS0₄ are useful for the purpose of the present invention. Particular preference is given to Na₂S0₄. The alkali metal salt or alkaline earth metal salt urges the precipitation of the binder compound on the backing sheet and thus enables the formation of a layer of the film forming binder, the plasticizer and the particulate filler on the backing sheet. Since the alkali metal salt or alkaline earth metal salt is water soluble, it is not contained in the resulting solid layer on the backing sheet.

The aqueous suspension resulting from process steps a) to c) is then applied to a backing sheet in production step d), resulting in a wet layer containing the particulate filler material as well as the film forming binder and the plasticizer, on the backing sheet.

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 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 solid compounds remain on the upper surface of the backing sheet. 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 particulate filler material is much smaller than the pore size of the backing sheet material. Nevertheless, the preparation method according to steps a) to d) allows the formation of a layer on the surface of the backing sheet being composed of a wet composition containing the particulate filler material as well as the film forming binder materials polyvinyl alcohol and polyethylene glycol.

In production step e) which follows, the wet layer on the backing sheet achieved in step d) is dried, whereby a solid thermally conductive tape is obtained which is composed of the backing sheet and of a solid layer thereon being composed of the particulate filler material, of a film forming binder of polyvinyl alcohol or of a cellulose type polymer and of a plasticizer. 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 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.

The resulting thermally conductive tape is of a sufficient mechanical strength and exhibits a high flexibility as well as a high thermal conductivity due to the high relative content of the thermally conductive particulate filler in the thermally conductive layer on the backing sheet.

The object of the present invention is also achieved by a thermally conductive tape, consisting of a backing sheet and of a solid layer being composed of a particulate filler material having an intrinsic thermal conductivity of at least 5 w/mK , of a film forming binder and of a plasticizer, on the backing sheet, wherein the binder is of polyvinyl alcohol or of a cellulose type polymer.

As already described above, the particulate filler material is chosen from the group consisting of aluminium oxide, boron nitride, boron carbide, diamond, carbon nitride, aluminium carbide, aluminium nitride, silicon oxide, silicon carbide, silicon nitride, magnesium oxide, zinc oxide, beryllium oxide, and mixtures thereof. Particularly preferred is a particulate filler material of aluminium oxide, especially of aluminium oxide platelets.

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 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 miti and a volume based particle size dgs in the range of from 30 < d95 < 40 pm are most preferred.

In the thermally conductive tape according to the present invention, the content of the particulate filler material in the solid layer is preferably in the range of from 90 to 99.5% by weight, based on the weight of the solid layer. In addition, the total content of the film forming binder and the plasticizer is preferably 0.5 to 10% by weight, based on the weight of the solid layer, while the sum of particulate filler and organic compounds (binder and plasticizer), in each case, is 100% by weight, based on the weight of the solid layer.

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 glass fiber cloths or are polymeric films, and glass fiber cloths 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 conductive layer. The thermally conductive tape of the present invention has a thickness in 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 particulate filler and 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. 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.

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, the thermally conductive and, at the same time, electrical insulating tape of the present invention may be advantageously 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 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 a high extent, preferably to more than 90% by weight, of materials having a high intrinsic thermal conductivity per se. Therefore, they may be advantageously used instead of mica tapes for insulation purposes 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 and, even better, since the generation of volatile organic gas is avoided, may be regarded environmentally friendly in their production process as well as in their technical application. Figure 1 : relates to photographs 1a and 1 b which show the thermally conductive tapes of examples 1 and 2 according to the invention after testing the flexibility thereof Figure 2: relates to photographs 2a, 2b and 2c which show the thermally conductive tapes of comparative examples 1 to 3 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.

Example 1 :

A 10% by weight solution of PVA (Poval 15-99, Product of Kuraray) is prepared by dissolving a corresponding amount of PVA in hot water.

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. 107.5 g of the resulting alumina flake slurry are mixed with 5.0 g of the pre- prepared 10 wt.% PVA solution and with 0.50 g of PEG 200 (product of Sanyo Kasei Kogyo) and stirred for 10 minutes. In order to elevate the pH to 7.0, 32% NaOFI is added. Eventually, 4.00 g of Na₂S0₄ are added to the resulting mixture as well.

10 g of the suspension described above are 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. Example 2:

Example 1 is repeated except that 0.5 g of the 10% by weight PVA solution and 0.05 g of PEG 200 are added.

Comparative example 1 :

A thermally conductive tape is produced according to example 1 , but neither the pH is adjusted nor is PVA, PEG or Na₂S0₄ added.

Comparative example 2:

A thermally conductive tape is produced according to example 1 , but PEG and Na2S0₄ is not added.

Comparative example 3:

A thermally conductive tape is produced according to example 1 , but PEG is not added.

Evaluation of the thermally conductive tapes:

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.

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

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

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 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

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

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 :

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 peeld of by touching it with a finger. Contrary to this, the adhesion of the alumina containing layer in examples 1 and 2 is good and obviously much better than in comparative examples 1 and 2. In addition, the flexibility of the thermally conductive tape as a whole is also much better than that of comparative examples 1 and 2. Although the adhesion of the alumina containing layer to the backing sheet in comparative example 3 is sufficient, the flexibility of the resulting thermally conductive tape is very poor.

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

Claims

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

a) keeping an aqueous suspension of a particulate filler material having an intrinsic thermal conductivity of at least 5 W/mK under stirring while adjusting a pH in the range of from 1 to 3, b) adding to the suspension an aqueous binder solution of a water soluble polymer selected from the group consisting of polyvinyl alcohol and cellulose type polymers, and a plasticizer,

2. Process according to claim 1 , wherein the particulate filler material is contained in the solid layer in an amount corresponding to 90 to 99.5% by weight, based on the layer.

3. Process according to claim 1 or 2, wherein the film forming binder and the plasticizer is contained in the solid layer in a total amount corresponding to 0.5 to 10% by weight, based on the layer.

4. Process according to one or more of claims 1 to 3, wherein the film forming binder and the plasticizer are added in a weight ratio in the range of from 10:1 to 1 :1.

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

particulate filler material is composed of at least one of aluminium oxide, boron nitride, boron carbide, diamond, carbon nitride, aluminium carbide, aluminium nitride, silicon carbide, silicon nitride, magnesium oxide or beryllium oxide.

6. Process according to one or more of claims 1 to 5, wherein the

particulate filler material is alumina platelets.

7. Process according to claim 6, wherein the alumina platelets exhibit 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.

8. Process according to claim 6 or 7, wherein the alumina platelets

exhibit an aspect ratio of at least 20.

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

plasticizer is polyethylene glycol.

10. Process according to one or more of claims 1 to 9, wherein the film forming binder is polyvinyl alcohol.

11. Process according to one or more of claims 1 to 10, wherein the

backing sheet is selected from glass cloth and polymer film.

12. Process according to claim 11 , wherein the glass cloth has open

pores exhibiting a pore size in the range of from 300 to 500 pm.

13. Thermally conductive tape, consisting of a backing sheet and of a solid layer being composed of a particulate filler material having an intrinsic thermal conductivity of at least 5 W/mK, of a film forming binder and of a plasticizer, on the backing sheet, wherein the film forming binder is of polyvinyl alcohol or of a cellulose type polymer, obtained by a method according to one or more of claims 1 to 12.

14. Thermally conductive tape according to claim 13, wherein the

particulate filler material is contained in the solid layer in an amount corresponding to 90 to 99.5% by weight, based on the solid layer.

15. Thermally conductive tape according to claim 13 or 14, wherein the particulate filler material is alumina platelets.

16. Use of a thermally conductive tape according to one or more of claims

13 to 15 for the insulation of machines and devices.

17. Use according to claim 16, wherein the machine or device is an

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

18. Electric cable bundle, conductor, coil, generator, rotor or stator, being provided with a thermally conductive tape according to one or more of claims 13 to 15.