METHOD FOR THE ADHESIVE BONDING OF ELECTRICAL SHEETS AND ELECTRICAL SHEET PACKS PRODUCED ACCORDING TO A CORRESPONDING METHOD

Abstract

The invention relates to a method for producing an electrical sheet pack using an anaerobically curing adhesive, to an electrical sheet pack produced or producible by such a method, and to a device for creating an electrical sheet pack of the invention.

Claims

1. A method for producing an electrical sheet pack (11), comprising the steps of: a) providing electrical sheet strip (19) as starting material for the sheet laminations for the electrical sheet pack (11), b) coating the electrical sheet strip (19) with an insulation layer on one or both sides, c) coating the electrical sheet strip (19) on one side with an anaerobically curing adhesive, d) rubbing the electrical sheet strip (19), preferably the other side, with a rubbing device (3) which comprises beryllium-containing and/or transition metal-containing surface structure, preferably copper-containing surface structures, e) separating the individual laminations for the electrical sheet pack (11) from the electrical sheet strip (19), f) contacting the laminations for the electrical sheet pack (11) in each case by contacting one side of one lamination with the other side of the next lamination, and g) curing the adhesive.

2. The method as claimed in claim 1, wherein step g) takes place at least partially and/or step f) takes place in the brake (9) of the punching device.

3. The method as claimed in claim 1, wherein the surface structures comprises beryllium-containing and/or transition metal-containing bristles and bristles which contain neither beryllium nor transition metals.

4. The method as claimed in claim 1, wherein the beryllium-containing and/or transition metal-containing surface structures comprise or consist of material selected from the group consisting of elemental copper, brass, bronze (especially phosphor bronze or gunmetal), iron, steel, manganese, and beryllium.

5. The method as claimed in claim 1, wherein the transition metal-containing surface structures have a diameter of ≤1 mm, preferably 0.005 mm-1 mm and/or a Vickers hardness of 25-200 for the beryllium-containing and/or transition metal-containing surface structures and/or possess a nominal tensile strength of 10 MPa-600 MPa.

6. The method as claimed in claim 1, wherein the device is a brush (3) selected from the group consisting of disk brush, plate brush, strip brush, sword brush, cup brush, cone brush, roller brush, round brushes and spiral brush.

7. The method as claimed in claim 1, wherein the adhesive in step c) is applied over the area or part of the area, preferably in a spraying or printing process.

8. The method as claimed in claim 1, wherein the adhesive in step c) is applied in a layer thickness of 0.3 μm-50 μm, preferably of 0.5 μm-20 μm, and more preferably of 0.5 μm to 6 μm.

9. The method as claimed in claim 1 wherein the region to which the adhesive and/or the region which is rubbed is limited in each case by means of a mask (17).

10. The method as claimed in claim 1, wherein the respective first and last laminations of the electrical sheet pack (11) are not subjected to step c) on their outwardly directed side.

11. The method as claimed in claim 1, wherein before step c) one side undergoes an adhesion-improving pretreatment step, especially by means of an atmospheric pressure plasma.

12. The method as claimed in claim 1, wherein the curing of the adhesive is assisted by supply of energy, preferably in the brake (9) or in other regions of the punching device.

13. The method as claimed in claim 1, wherein after step d) on the brushed area there is a concentration of transition metal, preferably of copper, of ≤2 atom %, especially 0.015 atom % to 2 atom %, preferably ≤0.3 atom %, measured by means of ESCA and based on the total number of atoms detected by ESCA.

14. An electrical sheet pack (11) produced or producible by a method as claimed in claim 1, wherein between the insulating layer and the adhesive or in the adhesive there are beryllium-containing and/or transition metal-containing particles, preferably beryllium-containing and/or transition metal-containing metallic particles.

15. A device for creating an electrical sheet pack as claimed in claim 14, comprising a rubbing device (3), a device (13) for applying an anaerobically curing adhesive, and a reservoir containing an anaerobic adhesive.

Description

DESCRIPTION OF FIGURES

[0103] FIGS. 1-4 each represent schematically a device and a process for providing a sheet pack according to the invention. The reference symbols in these figures have the following meanings:

[0104] 1. Punch [0105] 3. Disk brush [0106] 5. IR emitter [0107] 7. Stamping-out station [0108] 9. Brake [0109] 11. Sheet pack [0110] 13. Spraying device [0111] 15. Spraying chamber [0112] 17. Mask [0113] 19. Electrical sheet strip [0114] 21. Roller brush [0115] 23. Strip brush

[0116] With reference to FIG. 1, an embodiment of the method of the invention is described first of all:

[0117] By means of an advancement unit, the electrical strip 19 provided with insulation coating is advanced by a defined length with each stroke of the punching process. Immediately before the stamping-out of the electrical sheet lamination from the electrical strip and transfer of the lamination into the brake 9, the electrical strip 19 is coated from the underside with the adhesive by means of spraying device 13 in the spraying chamber 15. In order for the adhesive to be applied in a locally defined way, a mask 17 is used. The overspray collects on the walls of the spraying chamber 15 and accordingly is collected on the base of the spraying chamber. Simultaneously or, in the case of a prior progressive punching step, by one of the punches 1, the upper side of the electrical strip 19 is brushed with a rotating brush plate 3 having copper filaments (bristle diameter, for example, 0.06 mm). In this case a mask 17 of thin titanium sheet is used, in which the areas to be bonded have been laser-cut.

[0118] Advantageous in the process is that the masks 17 are brought into contact with the electrical sheet only before the application of the adhesive and/or before brushing. This is enabled by guiding the masks 17 onto the electrical sheet strip 19 in synchronicity with the stroke of the punching tools 1 (downward movement). On upward movement of the punching tools 1, both masks are removed synchronously from the electrical strip 19, and so during the advance of the electrical strip 19 there is no smearing of the adhesive and likewise neither brush 3 nor mask 17 cause scuffing on the electrical strip 19.

[0119] Synchronously with the stroke of the punch, 1. the application of the adhesive is triggered, 2. the rotating brush 3 is brought into contact. In order to take account of the wear of the brush, the contact of the brush 3 with the electrical strip is preferably not travel-controlled, but instead force-controlled, so that the filaments are deformed elastically by the pressing pressure of the brush on the electrical strip.

[0120] In the stamping-out station 7, the electrical sheet lamination is punched out of the electrical strip 19 with a ram (male mold) through the female mold and is transferred directly into a punching brake 9. The ram here presses the freshly punched-out lamination onto the sheet pack 11 which is located in the brake, and so moves the sheet pack 11 further downward by the thickness of the sheet lamination. In this case the adhesive of the freshly punched-out lamination comes into contact with the brushed surface of the lamination already present in the brake 6. This initiates the curing of the anaerobically curing adhesive. Pack separation, being the separation of two sheet packs 11, is ensured either by the adhesive not being applied in one stroke or by the surface of one lamination not being brushed. Ideally there is neither brushing of the leading lamination nor application of adhesive to the following lamination.

[0121] The curing of the anaerobically curing adhesive may be accelerated by supply of heat, not only by the stamping-out ram or in the brake but also by IR emitters 5 in the progressive punching tool. In general the operating temperature of the punching tool is higher than the ambient temperature, and this already accelerates the curing of the adhesive.

[0122] Instead of a disk brush, suitability is likewise possessed by strip brushes 23, cup brushes, sword brushes or roller brushes 21. Instead of the spray application, suitability is likewise possessed by printing processes (pad printing, screen printing, flexographic printing). In addition, nonwovens and sponges with transition metal-containing and/or beryllium-containing filaments present on their surface are suitable.

[0123] In a further embodiment, a preceding step is the pretreatment of the electrical strip 19, which may take place both from the bottom side and from the top side (cf. FIG. 2).

[0124] Additionally, it is also possible for only one side to be activated, and then for the adhesive to be applied to the activated surface. The reaction starts only after exclusion of oxygen, namely when this sheet side is contacted with a further sheet (cf. FIGS. 3 and 4).

Examples

Measuring Example 1: Determination of Striations

[0125] 2 polished sections of at least 2 laminations of an electrical sheet pack are prepared in such a way that [0126] the surface normal of the at least one adhesive layer lies orthogonally to the surface normal of the section (that is, lies in the surface of the section), and [0127] the surface normal of the two section surfaces are in turn orthogonal to one another.

[0128] These sections are analyzed materialographically. If it is difficult to distinguish the individual layers (electrical sheet/insulation layer/adhesive layer) under the optical microscope, the skilled person employs scanning electron microscopy.

[0129] The striations resulting from brushing are manifested in a locally confined reduction in the insulation layer thickness of at least 10%. For both sections, the skilled person then measures the length containing 50 striations. Length 1 and length 2 are generally not the same (only on 45° orientation). The greater of the lengths is called a, the lesser length b.

[0130] The mean spacing d of the striations is then obtained as follows:

d=cos(arctan(a/b))×a/50

[0131] Typical spacings between two striations are around 100 μm for a brush filament diameter of 250 μm. The striation width is around 10 to 30 μm.

Measuring example 2a: Determination of Beryllium Content or Transition Metal Content, Especially Cu Content, by ESCA (XPS)

[0132] ESCA analyses were carried out in order to determine the copper content after brushing. The content reported in each case is that based on the total number of atoms determinable by ESCA.

[0133] The ESCA analyses were carried out using the KRATOS AXIS Ultra spectrometer from Kratos Analytical. The analysis chamber was equipped with an X-ray source for monochromatic Al Kα radiation and with an electron source as neutralizer. Moreover, the unit possessed a magnetic lens, which focused the photoelectrons via an entry slit into a hemispherical analyzer.

[0134] Through calibration, the aliphatic component of the C 1s peak was set at 284.5 eV. During the measurement, the surface normal pointed to the entry slit of the hemispherical analyzer.

[0135] In the determination of the amount-of-substance ratios, the pass energy in each case was 80 eV and the step width 0.5 eV. The corresponding spectra are termed overview spectra. In the determination of the peak parameters, the pass energy was in each case 20 eV and the step width 0.05 eV.

Measuring Example 2b: Determination of the Beryllium Content or Transition Metal Content, Especially Cu Content, by ESCA (XPS) with Improved Detection Limit for Copper

[0136] The XPS analyses took place using a Thermo K-Alpha K1102 system with upstream argon glovebox for the handling of samples sensitive to air. Parameters: Take-off angle of the photoelectrons at 0°, monochromatic Al Kα excitation, Constant Analyzer Energy mode (CAE) with 150 eV pass energy in overview spectra (step width 0.5 eV, 2 scans with a recording duration of 9 min 4.2 sec) and also in the energetically high-resolution Cu2p (step width 0.05 eV, 10 scans with a recording duration of 12 min 21 sec). The high-resolution Cu2p spectrum is employed in order to quantify the copper.

[0137] Area under analysis: 0.40 mm ø. Electrically nonconducting samples are neutralized through a combination of low-energy electrons and low-energy argon ions. To compensate for charging effects, the C1s principal photoemission line to be assigned to the C—C/C—H species in the analysis is fixed at 285 eV, with a corresponding consequent shift in the positions of the other photo lines.

[0138] Quantification takes place on the basis of documented relative sensitivity factors of the elements, taking account of the specific analyzer transmission function, based on the assumption of a homogeneous distribution of the elements within the XPS information depth (around 10 nm).

[0139] The detection limit of the method is element-specific and lies at around 0.1 at %. On the basis of the measuring conditions and of the sensitivity factor of copper, the detection limit of copper in the measurements is 0.005 at %.

[0140] The stated measuring conditions are preferred in order to make the results largely independent of the type of spectrometer.

Measuring Example 3: Determination of Beryllium Particles or Transition Metal Particles, Especially Copper Particles

[0141] Samples which have not yet been bonded can be analyzed directly after the rubbing procedure.

[0142] Laminations of sheet laminates that have already been bonded are parted from one another by means of peeling stress. It is in each case the outermost lamination that is parted from the sheet pack. In order to apply the peeling stress, the skilled person employs a wedge, which is driven into the adhesive layer. After the sheet lamination has been parted, both fracture faces are analyzed by microscope. In general there is a mixed fracture pattern with adhesive components. The particles are preferably located with embedment in the adhesive layer, and are therefore “floating in the adhesive”. Measuring examples 4-6 are suitable for the actual, closer determination.

Measuring Example 4: FIB Section and EDX Measurement

[0143] These particles can likewise be measured by SEM/EDX, following surface preparation at the location of one particle by means of fast ion bombardment (FIB).

[0144] For the preparation of the cross section using the FEI Helios 600 Dualbeam, following location of the particles, ion beam-induced deposition (IBID) was first used to deposit an approximately 30×2×1 μm platinum/carbon protective layer directly over the particle. Then a cross section was prepared by means of Ga+ ions (30 kV, 21 nA) and this cross section surface was finally polished (30 kV, 2.8 nA). The cross section thus produced was imaged in situ in the FEI Helios 600 DualBeam using secondary electrons. The secondary electron images were recorded at 5 kV and 0.17 nA. In addition, the cross section surface was investigated for its elemental composition by means of energy-dispersive X-ray analysis (EDX). The parameters selected for this investigation (10 kV, 1.4 nA or 0.69 nA) enable all of the elements occurring to be detected with the maximum possible lateral resolution.

[0145] In rare cases, especially with (high layer thicknesses not preferred), it may be appropriate to part the adhesive layer from the substrate entirely. For that purpose, a scalpel is used to part the residues of adhesive remaining on the fracture face from the sheet, and these residues of adhesive are analyzed by microscope, in this case preferably using EDX element mapping. The skilled person selects the acceleration voltage such that the information depth of the measurement is the thickness of the layer of adhesive.

Measuring Example 5: Optical Microcopy

[0146] The instrument used is a Keyence VHX 600 digital microscope with VH-Z 100 lens and OP-72404 ring light source, magnification 700×.

[0147] The particles are evident in the plan view, with Cu particles in particular being readily apparent in this way.

[0148] If the contrast between particle and sheet substrate is too low, measurement takes place with SEM and EDX element mapping.

Measuring Example 6: SEM-EDX:

[0149] A primary electron beam is generated using an electrode cathode and acceleration toward the anode, and, using subsequent electromagnetic lenses, is focused with maximum precision onto the surface of the sample under analysis. In the sample, in an interaction volume which is dependent on the acceleration voltage and on the composition of the material, secondary electrons (SE), backscattered electrons (BSE) and X-radiation are generated. The energy of the X-radiation is dependent on the atomic number of the emitting atom and is therefore “characteristic” of the element in question. All of these signals can be recorded using corresponding detectors. Corresponding topographic, material and/or elemental contrasts can be imaged in this way.

[0150] EDX is a method for the spatially resolved elemental analysis of solids. Energy-dispersive X-ray microanalysis makes it possible to ascertain the elemental composition on a surface imaged by SEM. In addition to surface and spot measurement, element mappings can also be recorded.

[0151] The acceleration voltage is selected by the skilled person so as to reliably capture the particles as a function of the thickness of the layer of adhesive, and is typically 15 keV.

[0152] Instrument used: High-resolution analytical Leo 1530 Gemini FE-SEM with EDX (Oxford INCA with Si and germanium detector).

Measuring Example 7: Determination of the Insulation Effect

[0153] Either the surface resistance is carried out using a Franklin tester according to IEC 60404-11, or, preferably, the following procedure is adopted (measuring protocol IFAM):

[0154] An electrical sheet of 100 mm×25 mm is brushed on one side. This sample is clamped between two copper rams with a diameter of 19.8 mm and a length of 30 mm and is subjected to a force of 650N, producing a pressing pressure of 2.1 MPa. The ram surfaces which come into contact with the electrical sheet have optically bright polishing.

[0155] The two Cu rams are connected by two cables to a Keithley Multimeter 2001 resistance meter. The resulting resistance is measured by means of two-point measurement. This measurement is repeated on 10 samples and the arithmetic mean is formed. Subtracted from this resistance is the resistance which comes about without the steel sheet (lead resistance and ram resistance).

Measuring Example 8: Determination of the Roughnesses

[0156] The roughnesses of the sample surfaces are detected using the plμ Neox instrument from Sensofar, by recording linear profiles orthogonally to the brushing direction. The linear profiles thus detected are analyzed according to DIN EN ISO 4288. Evaluations are made of the characteristic roughness values R.sub.a and R.sub.z.

Working Examples

Working Example 1

[0157] An Isovac 270-35 A electrical sheet with a C5-grade insulation varnish (insulation varnish contains no transition metals and no beryllium) (manufacturer: VoestAlpine) in the size of a lap shear sheet (100 mm×25 mm) is brushed using a brass brush (bristle diameter 0.06 μm, material CuZn36, condition soft) in the region of the adhesive area (10 mm).

[0158] A further electrical sheet is coated, using a doctor blade, with the anaerobically curing adhesive DELO ML 5327, with a nominal layer thickness of 4 μm.

[0159] The brushed side of the first sheet is contacted with the adhesive-coated side of the second sheet and the assembly is fixed with a clamp (anaerobic conditions). After 2 minutes, the strength of the bond is sufficient to allow the bonds to move (handling strength). After 6 minutes at 40° C. (typical temperature of the punching tool in operation), the bond already exhibits lap shear values of 2.5+−0.7 MPa. After 24 h at room temperature, lap shear strengths of 4.5+−0.8 MPa are attained.

[0160] After 150 h of aging in transmission oil (FEBI Bilstein 39071) at 150° C., the lap shear strength was still 4.2+−0.5 MPa. After 1000 h of aging at 85° C. and 85 rel % humidity, the lap shear strength was 4.1+−0.2 MPa.

Working Example 2

[0161] The insulation quality of the electrical sheet stated in example 1 is determined by means of DIN EN 60404-11:2013-12; VDE 0354-11:2013-12.

[0162] Title: Magnetic materials—Part 11: Method of test for the determination of surface insulation resistance of magnetic sheet and strip (IEC 60404-11:1991+A1:1998+A2:2012); EN 60404-11:2013

[0163] In deviation from example 1: [0164] a) no brush, [0165] b) a brush with 125 μm bristles (same bristle material as in working example 1), and [0166] c) a brush with 250 μm bristles (same bristle material as in working example 1) are used.

[0167] Within the bounds of measurement accuracy (−+10%), no reduction in the surface resistance of the electrical sheet as a result of the brushing is observable: [0168] a) rhoSURF=178+−13 ohm/cm2 [0169] b) rhoSURF=169+−15 ohm/cm2 (after brushing with 125 μm) [0170] c) rhoSURF=171+−8 ohm/cm2 (after brushing with 250 μm)

Comparative Test (Prior Art)

[0171] For comparative testing, the activator DELO Quick 5004 was applied using a 4 μm doctor blade to the sheets of example 1. No brushing was performed. The subsequent steps were as follows: [0172] application of the activator DELO Quick 5004 with 4 μm doctor blade [0173] 10 min evaporation of the activator [0174] application of DELO ML 5327 with 4 μm doctor blade to activated surface [0175] curing in analogy to working example 1.

[0176] It emerged that the strengths of the bonds and also the rate of the curing were no different from the results of working example 1, within measurement accuracies.

Working Example 3

[0177] Determination of the copper content in the samples from working example 2

[0178] For the samples from working example 2, the copper content was determined in accordance with measuring example 2a. The result was that no copper could be found for the non-brushed variant, whereas for the brushed variant there was indeed a portion of copper, but it was close to the detection limit (measured according to measuring example 2a, 0.1 atom %). Accordingly, the degree of surface coverage with copper is around 0.1% or around 0.2% on the assumption that copper oxide is present.

[0179] It is also noted that in no case was the electrical sheet strip exposed, since iron was not detectable.

Working Example 4

[0180] The samples from working example 1 and from working example 2 and also from the comparative example were then analyzed for copper particles in accordance with measuring example 3.

[0181] Here it emerged that in all three of the brushed examples (from working example 1 and also the brushed examples from working example 2), copper particles were detectable even under the optical microscope. The particles have sizes (maximum distance between the edges in plan view) of between 1 μm to 15 μm. The degree of surface coverage by measurement and counting of the particles on 10 microscope images according to measuring example 3 gave degrees of surface coverage of between 0.02 and 0.1%. Conversely, in the two other examples (not brushed or copper application in accordance with the prior art), there were no particles present.

[0182] From this it can be inferred that the copper particles (which are still detectable even after curing) give a distinct indication of the method of the invention.

Working Example 5

[0183] An electrical sheet M310-50A with a C5 insulation varnish EB5308 (manufacturer: Arcelor Mittal) in the size of a lap shear sheet (100 mm×25 mm) is brushed using various brushes (different bristles) in the region of the adhesive area (10 mm).

[0184] The brushes have a treatment width of 30 mm and consist of a series of brushes closely packed to one another; for example, for a brush diameter of 125 μm, there are 240 bristles adjacent to one another and in parallel alignment in a row. The length of the bristles varies.

[0185] The angle between bristle direction and movement vector of the electrical sheet is 45°. The bristle ends form a straight line. This line is parallel to the steel surface and oriented orthogonally to the movement vector of the electrical sheet. The pressing pressure is set by positioning the bristle mount of the brush, after contact of the bristle ends with the steel sheet, by a further 3 mm in the direction of the steel surface. In this case there is elastic deformation of the bristles.

[0186] The electrical sheet is then guided through under the brush with a velocity of 10 m/min. The number of brushing operations is varied.

[0187] The resulting surface resistances are measured, the elemental composition of the surface is determined by XPS in accordance with measuring example 2b, and the roughness is determined (see above).

[0188] Sample Matrix Table

TABLE-US-00001 No Brushing Brushing brushing operation operation Brush operation 10 times 100 times No brush Ref Cu 99.9% soft-annealed Cu-50-10  Diameter 50 μm Bristle length 12.5 mm Normal force with 3 mm pressing: 60 +− 5 mN Cu 99.9% soft-annealed Cu-125-10 Cu-125-100 Diameter 125 μm Bristle length 12.5 mm Normal force with 3 mm pressing: 740 +− 20 mN Cu 99.9% Cu-500-10 Tensile strength: 260 +− 15 MPa (soft-annealed) Diameter 500 μm Bristle length 30 mm Normal force with 3 mm pressing: 3920 +− 82 mN Brass CuZn36  M-400-10 M-400-100 Tensile strength: 814 +− 47 MPa (spring-hard) Diameter 400 μm Bristle length 30 mm Normal force with 3 mm pressing: 2551 +− 74 mN

[0189] A further electrical sheet in each case is coated, using a doctor blade, with the anaerobically curing adhesive DELO ML 5327, with a nominal layer thickness of 4 μm.

[0190] The brushed sides of the samples, and the activator-coated reference sample, are contacted with the adhesive-coated side of the second sheet and the assembly is fixed with a clamp (anaerobic conditions). The pressing pressure is 100 kPa. After 5 minutes at room temperature, testing takes place to determine whether the strength of the bond is sufficient to enable movement of the bonds (handling strength).

TABLE-US-00002 Resistance Handling Cu Fe measurement strength Strengths concentration concentration (measuring (crosslinking after at surface at surface protocol Roughnesses after 24 h (metallic) (oxidic) IFAM) R.sub.a/R.sub.z Sample 5 min) [MPa] [at %] [at %] [ohms] [μm] Ref No <detection 0.7 5.3 +− 1.9 0.48 ± 0.05/ limit 4.83 ± 0.62 Ref + 1.33 +− 0.61 DELO Quick 5004 Cu-50-10 No  1.5 +− 0.48 0.01 0.1 4.6 +− 1.5 Cu-125-10 Yes 1.57 +− 0.22 0.04 0.1 4.7 +− 1.5 0.46 ± 0.01/ 4.75 ± 0.40 Cu-125-100 Yes (1.97) 0.17 0.2 5.1 +− 1.0 0.48 ± 0.05/ 4.83 ± 0.62 Cu-500-10 Yes 2.0 +− 0.1 0.03 0.3 4.4 +− 1.6 0.48 ± 0.05/ 5.28 ± 0.60 M-400-10 No 1.82 +− 0.19 0.01 0.2 4.4 +− 1.9 0.48 ± 0.06/ 5.04 ± 0.41 M-400-100 Yes (2.0) 0.15 0.2 1.0 +− 0.5 0.49 ± 0.05/ 5.62 ± 0.66 Polished 0.054 +− 0.01  0.59 ± 0.07/ sheet metal 5.47 ± 0.40

[0191] First it may be noted that for all of the samples according to the invention, in contrast to the reference, there was curing of the adhesive. However, in the case of the variants which ultimately possessed a copper concentration on the surface of 0.01 atom %, the process of adhesive curing was relatively slow. Accordingly it is preferred, in the method of the invention, for the resulting copper concentration at the surface, measured by XPS, to be 0.015 atom %, preferably 0.02 atom %, based on the atoms measured by XPS.

[0192] From the Fe concentration at the surface it is evident that even in the case of the M-400-100 sample, the insulating layer was not rubbed off, otherwise the iron concentration would be much higher. It emerged, however, that the resistance values are markedly poorer.

[0193] Without being tied to a theory, this can be explained by excessive abrasion of the insulating layer, with the necessary assumption that the insulating layer was not completely removed. Nevertheless it was found that the rubbing process can be controlled in such a way that the electrical sheet retains resistance values which are suitable even for relatively high-grade requirements. In the sense of the present invention, therefore, it is preferred for the rubbing process to be carried out in such a way that after the rubbing the electrical sheet with insulating layer retains a resistance of ≤2.0 ohms, preferably ≤2.5 ohms, more preferably ≤3 ohms, very preferably ≤4 ohms.