Fibrous nonwoven and method for the production thereof

Abstract

The invention relates to a fibrous nonwoven, in particular for a filter medium, having a first layer, wherein at least one single-piece fiber strand of said first layer has a first fiber portion and a second fiber portion in the longitudinal direction, and wherein the fiber strand has a thickening substantially in said second fiber portion.

Claims

1. A fabric (1) for a filter medium, comprising a nonwoven first layer (12), wherein at least one integral fiber strand (2) of the first layer (12) has at least one first fiber section (3) and at least one second fiber section (4) each section having a respective end, the at least one first fiber section and the at least second fiber section being arranged consecutively in a longitudinal direction of the fiber strand extending in a longitudinal direction along an axis, wherein the at least one first fiber section and the at least one second fiber section are of one piece joined to each other at their respective ends, characterized in that the fiber strand (2) comprises a thickening (5) in the at least one second fiber section (4), the thickening provided by the at least one second fiber section itself, wherein the first fiber section (3) consists of a first polymer mixture comprising a first polymer (9), and the second fiber section (4) consists of a second polymer mixture comprising a second polymer (10), wherein the second polymer mixture has a higher viscosity than that of the first polymer mixture, and the second polymer (10) has a higher viscosity than that of the first polymer (9), and wherein the first polymer mixture comprises different polymers from the polymers of the second polymer mixture, wherein the first and second fiber sections extend across the entire cross-section of the integral fiber, wherein granulate clusters of the second polymer or second polymer mixture are homogeneously mixed with granulates of the first polymer or polymer mixture, such that, in the melted state islands of the second polymer or second polymer mixture are formed, making the first polymer or first polymer mixture inhomogeneous, and wherein in a polymer melt which produces the at least one fiber strand (2) provide areas in the polymer melt in which different polymers dominate, such that the first (3) and the second (4) fiber sections are of differing material structures when the polymer melt discharges from a spinning beam in a primary forming process.

2. The fabric (1) for a filter medium according to claim 1, wherein the thickening has a thickness diameter at its widest point and the at least one first fiber section (3) has an average diameter, and wherein the ratio of the thickness diameter at its widest point to the average diameter of the at least one first fiber section is at least 3:1.

3. The fabric (1) for a filter medium according to claim 1, wherein the fiber strand in the first fiber section has an average diameter of 0.5-20 μm.

4. The fabric (1) for a filter medium according to claim 1, wherein the thickening has a thickness diameter at its widest point and the thickening has a longitudinal extent of the thickening, and the ratio of the thickness diameter to the longitudinal extent is 0.5:1 to 4:1.

5. The fabric (1) for a filter medium according to claim 1, wherein the fiber strand includes loops and the thickening (5) comprises entwined loops of the fiber strand.

6. The fabric (1) for a filter medium according to claim 1, wherein the thickening (5) has an area (8) in which the fiber strand (2) rests upon itself and is at least partially fused in said area (8).

7. The fabric (1) for a filter medium according to claim 1, wherein the second polymer mixture includes the first polymer and wherein the percentage of the second polymer (10) in the second polymer mixture amounts to 2-20% by weight of the melt.

8. The fabric (1) for a filter medium according to claim 1, wherein the integral fiber strand (2) comprises a third fiber section (6) which adjoins the second fiber section (4) and has the structure of the first fiber section (3).

9. The fabric (1) for a filter medium according to claim 1, wherein the first layer (12) has a mass distribution of 25-45 g/m.sup.2 and a thickness of 0.4 mm to 0.7 mm.

10. The fabric (1) for a filter medium according to claim 1, wherein the first layer (12) has an average of 2 to 10 thickenings/cm.sup.2.

11. The fabric (1) for a filter medium according to claim 1, wherein the first layer (12) has an air permeability of 5000-7000 l/(m.sup.2s) and/or a filtration efficiency of 10-20%.

12. The fabric (1) for a filter medium according to claim 1 which additionally comprises a second layer (13), the pore size of which is on average smaller than that of the first layer.

13. The fabric (1) for a filter medium according to claim 12, wherein the second layer (13) has a mass distribution of 45-75 g/m.sup.2 and a thickness of 0.5-0.9 mm.

14. The fabric (1) for a filter medium according to claim 12, wherein the second layer (13) has an air permeability of 3000-4000 l/m.sup.2s and/or a filtration efficiency of 10-25%.

15. The fabric (1) for a filter medium according to claim 12, which additionally comprises a third layer (14), the pore size of which is on average smaller than that of the second layer and which third layer is disposed on a side of the first layer opposing the second layer so the first layer is disposed between the second and third layers.

16. The fabric (1) for a filter medium according to claim 15, wherein the third layer (14) has a mass distribution of 35-60 g/m.sup.2 and a thickness of 0.4-0.7 mm.

17. The fabric (1) for a filter medium according to claim 15, wherein the third layer (14) has an air permeability of 800-1300 l/(m.sup.2s) and/or a filtration efficiency of 40-80%.

18. The fabric (1) for a filter medium according to claim 15, wherein the three layers (12, 13, 14) together have a mass distribution of 105-180 g/m.sup.2 and a thickness of 1.2-2.5 mm.

19. The fabric (1) for a filter medium according to claim 15, wherein the three layers (12, 13, 14) together have an air permeability of 500-1300 l/(m.sup.2s).

20. A filter having a fabric (1) according to claim 1, wherein the fabric (1) is pleated.

21. A method for producing the fabric of claim 1, comprising the following procedural steps: providing a first polymer melt; producing the first layer from the first polymer melt in a melt-blown or spun-bond process; varying, periodically or oscillatorily, at least one process parameter in the melt-blown or spun-bond process, the process parameter including one or more of the process temperature, the composition of the polymer melt, the polymer flow, and the process airspeed, to produce the at least one integral fiber strand from two fiber sections having different structures; and depositing the fiber strand on a substrate.

22. The method according to claim 21 comprising mixing (16) at least two polymers or polymer mixtures (9, 10) to produce the first polymer melt (9, 10) in the manner such that the polymers or polymer mixtures (9, 10) are inhomogeneously distributed in the first polymer melt (9, 10).

23. The method according to claim 21 comprising providing (21) a second polymer melt and/or a third polymer melt; producing (22) a second layer (13) from the second polymer melt and/or third layer (14) from the third polymer melt, wherein the first layer (12) and the second layer (13) and/or the third layer (14) are simultaneously produced in a single primary forming process which process is one or more of the melt-blown or spun-bond process.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)

(1) Reference will be made to the drawings in the following description of preferential embodiments in specifying the above as well as further advantages, features and possible applications of the present invention. The drawings depict the following:

(2) FIG. 1 is a partial schematic representation of two fiber strands having a nonwoven fabric thickening in accordance with the present invention;

(3) FIG. 2a is an electron microscope image of a nonwoven fabric according to the present invention;

(4) FIG. 2b is a partial schematic representation of the electron microscope image according to FIG. 2a;

(5) FIG. 3a is a further electron microscope image of a nonwoven fabric according to the present invention;

(6) FIG. 3b is a partial schematic representation of the electron microscope image according to FIG. 3a;

(7) FIG. 4a is a further electron microscope image of a nonwoven fabric according to the present invention;

(8) FIG. 4b is a partial schematic representation of the electron microscope image according to FIG. 4a;

(9) FIG. 5a is a further electron microscope image of a nonwoven fabric according to the present invention;

(10) FIG. 5b is a partial schematic representation of the electron microscope image according to FIG. 5a;

(11) FIG. 6 is a partial schematic representation of a nonwoven fabric having a plurality of thickenings in accordance with the present invention;

(12) FIG. 7 is a partial schematic representation of the manufacturing process according to a first embodiment of the present invention;

(13) FIG. 8 is a partial schematic representation of a manufacturing process according to a second embodiment of the present invention;

(14) FIG. 9 is a partial schematic representation of a polymer melt for the manufacturing process in accordance with the second embodiment of the invention according to FIG. 8;

(15) FIG. 10 is a partial schematic representation of an inventive nonwoven fabric having three layers;

(16) FIG. 11 is a partial schematic representation of a manufacturing process for an inventive nonwoven fabric having three layers;

(17) FIG. 12 is a block diagram partly schematically depicting a manufacturing process for an inventive nonwoven fabric having three layers.

DETAILED DESCRIPTION OF THE INVENTION

(18) FIG. 1 shows two fiber strands 2 of a nonwoven fabric 1 according to the invention, each exhibiting a thickening 5. Each fiber strand 2 can be divided into three fiber sections 3, 4 and 6. A first fiber section 3 extends from the one end of the fiber strand to the area of the thickening 5. A second fiber section 4 comprises the thickening 5 as well as preferably the area right in front and right in back of the thickening 5. Preferably, the second fiber section 4 can also be formed solely by a thickening 5. The third fiber section 6, which has a similar or even identical material structure as the first fiber section 3, adjoins the second fiber section 4. A second fiber section 4 having a further thickening 5 can in turn also adjoin the third fiber section 6, whereby the length of the fiber strand 2 is theoretically unlimited.

(19) As FIG. 1 indicates, the thickening 5 can exhibit entwined loops in the fiber strand 2 or also constitute solely a thickening of the fiber strand 2. Preferably, the thickening 5 is formed by a crimping of the fiber strand 2, further preferable is for sections of a fiber strand to abut in the area of the thickening, wherein these areas are at least partially fused. It is further preferable for the thickenings 5 to comprise fiber bundles 7 of a plurality of adjoining fiber strands 2. Such a fiber bundle 7 can, however, preferably also consist of just one fiber strand 2, whereby the fiber Strand 2 forms substantially parallel loops. It is further preferable for the first fiber section 3 and the second fiber section 4 to consist of different polymers. Preferably, the second polymer has a higher viscosity than the first polymer. Various polymers are hereby further preferable, also two polymers with the same structural formula but different characteristic molecular chain length distributions. This supports the forming of a thickening in the second section, as is described in detail with reference to FIG. 5.

(20) FIGS. 2a and 2b show an electron microscope image of a thickening 5 arranged in a nonwoven fabric 1. It is clearly recognizable that the fiber strand 2 is conjoined in the area of the thickening 5, partly forming fiber bundle 7 and partly twisted into loops. Regions adjacent to fiber strand 2 in the area of the thickening 5 are in part fused together.

(21) FIGS. 3a and 3b show a further electron microscope image of a thickening 5 arranged in a nonwoven fabric 1. Clearly recognizable is the increased fiber thickness or fiber diameter respectively of the fiber strand identified as “2” compared to the lower right edge of the image where the fiber strand 2 continues on.

(22) FIGS. 4a and 4b show a further electron microscope image of a thickening 5 arranged in a nonwoven fabric 1. It is clearly recognizable that the individual fibers of the fiber strand 2 are fused together in the thickening 5 in the area of the latter.

(23) FIGS. 5a and 5b show a further electron microscope image of a thickening 5 arranged in a nonwoven fabric 1 and which is in particular formed by fiber bundles 7.

(24) FIG. 6 is a schematic depiction of a nonwoven fabric 1 in accordance with the present invention. The nonwoven fabric 1 consists of a plurality of fiber strands 2. Some of the fiber strands 2 have thickenings formed by fiber bundles 7, by an increased fiber thickness, by crimping and/or by loops. Preferably, the thickenings exhibit a combination of thickenings, fiber bundles, crimps and/or loops.

(25) As a rule, fiber strands 2 do not end in thickenings 5; instead, a fiber strand 2 with a thickening 5 continues on in a first fiber section 3 and a third fiber section 6 on both sides of the second fiber section 4. This differentiates the thickenings particularly from so-called shots which form in a nonwoven fabric when the fiber strand 2 tears off during the manufacturing process. Since the thickenings 5 are part of the fiber strands 2, good filtration efficiency to the nonwoven fabric 1 is maintained despite the accumulation of fiber strand-forming polymer at the site of a thickening 5.

(26) This is attributable to the fact that the surface area of the thickenings 5 can also absorb particles to be filtered.

(27) A nonwoven fabric 1 in accordance with the invention achieves better filtration efficiency than a material reinforced with a second fiber or a bicomponent fiber having similar rigidity values as the inventive nonwoven fabric 1.

(28) FIG. 7 schematically depicts a method for manufacturing an inventive nonwoven fabric 1 in a melt-blown or spun-bond process. The spinning beam 12 is hereby respectively depicted on a nozzle for discharging a polymer at different times which proceed chronologically consecutively from left to right.

(29) A polymer is first discharged from the spinning beam 12 under pressure. The dispersed polymer is drawn out by means of a process airflow circulating above the spinning beam 12, indicated in FIG. 7 by an arrow, and conveyed to the substrate 11 on which the resulting fiber strand 2 deposits. In a next step, the intensity of the process airflow is reduced. The fiber strand 2 thickens at the nozzle of the spinning beam 12 since the polymer is only being removed or dispersed more slowly (weaker process airflow indicated by the small arrow). A first section 3 of substantially homogeneous structure and a second section 4 gradually forming a thickening 5 is thus produced on the fiber strand 2. The intensity of the airflow is then increased again at a specific time, whereby the removal of the polymer sprayed or extruded through the spinning beam 12 increases again. A third section 6 of material structure resembling or even identical to the first section 3 is thereby formed in fiber strand 2.

(30) Lastly, as depicted on the far right in FIG. 7, the thickening 5 deposits on the substrate 11. The substrate 11 is preferably in motion during the entire process such that the fiber strand 2 is deposited successively on the substrate 11. Preferably, the fiber strand 2 does not separate from the spinning beam 12 or the respective spray nozzle for the duration of forming a thickening 5 so that a continuous fiber strand 2 will result.

(31) FIG. 8 depicts the manufacturing process for an inventive nonwoven fabric 1 according to a second embodiment. The chronological sequence of actions at a nozzle of a spinning beam 12 is again depicted from left to right. The manufacturing method according to FIG. 8 essentially differs from the manufacturing method according to FIG. 7 by the fact of the process airflow remaining constant while the viscosity of the polymer, however, changes. Preferably, different polymers or polymer mixtures are introduced into the spinning beam 12 in succession to form areas of a first polymer 9 and areas of a second polymer 10 in the spinning beam. The first polymer 9 thereby forms a first fiber section 3 or a third fiber section 6 of a fiber strand 2 when sprayed out of the spinning beam 12. The second polymer 10 forms a second section 4 of a fiber strand 2 when sprayed out of the nozzle in which thickenings 5 form in a first layer 12 of the nonwoven fabric when deposited on the substrate.

(32) The thickenings 5 form due to the fact of the second polymer 10 having a higher viscosity than the first polymer 9, which is why the process airflow cannot draw it out so easily. It therefore remains longer in the area of a spinneret of the spinning beam 12 and accumulates there into a structure corresponding to a third section 4, which eventually forms a thickening 5.

(33) Alternatively or additionally to different polymers 9, 10, the process temperature and/or process air speed at the spinning beam 12 or the spinnerets of the spinning beam 12 respectively can preferably also be varied so as to affect the viscosity of a single polymer in order to respectively form the first section 3, second section 4 and third section 6.

(34) FIG. 9 shows a container of polymer melt used in producing an inventive nonwoven fabric 2 in accordance with the embodiment of FIG. 5. Areas having the second polymer and/or polymer mixture 10 are thereby introduced into the melt of a first polymer and/or polymer mixture 9. The individual granules of the second polymer/polymer mixture 10 being introduced into the polymer granulates of the first polymer/polymer mixture 9 thus enables the polymers to be mixed in a solid state. Further preferably, such a mixing can be achieved by polymer granulates of the second polymer 10 being introduced into a melt of the first polymer 9. Further preferably, different polymers 9, 10 can also be introduced into the respective nozzle 9 of a spinning beam 12 through different feed lines.

(35) The percentage of the higher-viscosity polymer in the melt typically amounts to 2-20% by weight (weight percent), preferentially 5-15% by weight and most preferentially 10% by weight.

(36) Applicable polymers for producing a nonwoven fabric 1 in accordance with the invention include for the first polymer 9 preferably synthetic polymers, particularly polyesters, selected from among the group consisting of polyethylene (PE), polyethylene terephthalate (PET), polycarbonate (PC), polyamide (PA), polypropylene (PP), polyvinylchloride (PVC), polybutylene terephthalate (PBT) and mixtures thereof. Applicable with respect to the second polymer 10 are preferably synthetic polymers selected from among the group consisting of polyethylene (PE), polyethylene terephthalate (PET), polycarbonate (PC), polyamide (PA), polypropylene terephthalate (PPT), polypropylene (PP), polyester, polyvinyl chloride (PVC), polybutylene terephthalate (PBT) and mixtures thereof. Any combination of the first polymer 9 and the second polymer 10 can be used. Furthermore, mixtures of different polymers as above or others can preferably be used for the first polymer 9 as well as for the second polymer 10.

(37) Preferable suitable combinations of a first polymer 9 and second polymer 10 are as follows: PET and PC, PET and PA, PET and PP, PET and PE, PET and PVC, PET and PBT, PC and PA, PC and PP, PC and PE, PC and PVC, PC and PBT, PA and PP, PA and PE, PA and PVC, PA and PBT, PP and PE, PP and PVC, PP and PBT, PE and PVC, PE and PBT, or PVC and PBT.

(38) Further preferably suitable combinations are as follows: PBT and PVC, PBT and PE, PBT and PP, PBT and PA, PBT and PC, PBT and PET, PVC and PE, PVC and PP, PVC and PA, PVC and PC, PVC and PET, PE and PP, PE and PA, PE and PC, PE and PET, PP and PA, PP and PC, PP and PET, PA and PC, PA and PET, or PC and PET.

(39) Two polymers having the same structural formula but different characteristic distribution in molecular chain length are also particularly suitable as the first polymer 9 and second polymer 10. Polymer compositions of the above-cited polymers are likewise suitable as the first polymer 9 and/or the second polymer 10.

(40) Lastly, as illustrated with reference to the preceding figures, the nonwoven fabric 1 can also be produced from just one single polymer and/or just one single characteristic molecular chain length from this group.

(41) It is also possible to combine the manufacturing processes 8 presented in FIGS. 7 and 8. Further preferably, the different sections 3, 4 and 6 of a fiber strand 2 can be formed by varying the polymer flow rate through the nozzles of the spinning beam 12, for example by changing the spinning beam 12 spray pressure. This procedure can also be combined with the previous procedures.

(42) A first layer 12 of a nonwoven fabric 5 produced in this manner preferably has a mass distribution of approximately 25-45 g/m.sup.2, preferentially approximately 30-40 g/m.sup.2 and most preferentially approximately 35 g/m.sup.2, and further preferably a thickness of approximately 0.4 mm to approximately 0.7 mm, preferentially approximately 0.5 mm to approximately 0.6 mm and most preferentially approximately 0.55 mm. Said first layer 12 further preferably has an average of approximately 2 to 10 thickenings/cm.sup.2, preferentially approximately 4 to 8 thickenings/cm.sup.2, particularly preferentially approximately 5 to 7 thickenings/cm.sup.2 and most preferentially approximately 6 thickenings/cm.sup.2. The air permeability of said first layer 12 preferably amounts to approximately 5000-7000 l/(m.sup.2s), preferentially approximately 5500-6500 l/(m.sup.2s) and most preferentially approximately 6000 l/(m.sup.2s). The filtration efficiency preferably amounts to approximately 10-20% and preferentially approximately 15%. The inventive effect can also be achieved at the peripheries of the respective range specifications. Additionally, all the cited parameter values are average values which can deviate significantly at individual locations within the first layer 12.

(43) The first layer 12 preferably has an average pore size pursuant to the bubble point test of greater than 80-100 μm. The first fiber section 3 and/or the third fiber section 6 further preferably have an average thickness of 10-40 μm, preferentially 20-30 μm and most preferentially 25 μm. The second fiber section 4 further preferably has an average thickness of 100-1000 μm, preferentially 200-900 μm, preferentially 300-800 μm, more preferentially 400-700 μm and most preferentially 500 μm in the area of a thickening 5.

(44) It was surprisingly discovered that a nonwoven fabric 1 having thickenings 5 disposed therein increases the rigidity of said nonwoven fabric 1. This is in particular due to the fact that when the nonwoven fabric 1 is being produced, the thickenings 5 in the second fiber sections cool more slowly than the fiber strand 2 in the first fiber sections 3 and the third fiber sections 6. Particularly the accumulation of polymer in the second fiber sections 4 of the fiber strand 1 thereby results in the slower cooling.

(45) On the one hand, the cooled thickenings 5 form a reinforcement in the nonwoven fabric 1, on the other hand, further parts of the fiber strand 2 and/or other fiber strands 2 are also at least partly fused with the thickening 5 so that the entire area around a thickening 5 results in a reinforcement of the material. At the same time, however, the thickening 5 remains a part of the fiber strand 2 with similar surface structure and can thus absorb particles to be filtered.

(46) FIG. 10 shows an inventive nonwoven fabric 1 according to a further embodiment of the invention. Here, the nonwoven fabric 1 has three layers 12, 13, 14. These three layers 12, 13, 14 are preferably produced in a collective primary forming process as per FIG. 11. In said process, three spinning beams 15a, 15b, 15c are preferably arranged in succession over a substrate 11 and simultaneously deposit fiber strands 2 on the substrate 11, preferably in a melt-blown or a spun-bond process, which is moving toward the right in FIG. 11. A second layer 13 is produced by the left spinning beam 15a, a first layer 12 by the middle spinning beam 15b, as described above, and a third layer 14 by the right spinning beam 15c. The sequence with which the three layers 12, 13, 14 are deposited is thereby not limited to the present embodiment as described, but rather can accordingly occur in any given order. Preferably, the inventive nonwoven fabric can also only have two layers, the first layer 12 and the second layer 13 or the first layer 13 and the third layer 14, whereby each of the respective two layers can be deposited first during the manufacturing method. Further preferably, production can also be realized with just one spinning beam, its nozzles able to extrude different respective polymers.

(47) The second layer 13 and the third layer 14 can thereby be produced by means of the same method as the first layer 12, preferably, however, different methods are used to produce layers having different properties. Preferably, the second layer 13 is a relatively coarse nonwoven fabric 1 in order to further stabilize the nonwoven fabric 1 and/or be able to prefilter coarse particles. The third layer 14 is preferably an electret filter, same increasing the particulate matter storage capacity as well as the filtration efficiency by electrostatic binding of particles.

(48) The second layer 13 preferably has a mass distribution of approximately 45-75 g/m.sup.2, preferentially approximately 50-70 g/m.sup.2, particularly preferentially approximately 55-65 g/m.sup.2 and most preferentially approximately 60 g/m.sup.2, and further preferably a thickness of approximately 0.5-0.9 mm, preferentially approximately 0.6-0.8 mm and most preferentially approximately 0.7 mm. Further preferably, the second layer 13 has an air permeability of approximately 3000-4000 l/(m.sup.2s), preferentially approximately 3250-3750 l/(m.sup.2s) and most preferentially approximately 3500 l/(m.sup.2s), and further preferably a filtration efficiency of approximately 10-25%, preferentially approximately 15-20% and most preferentially approximately 17.5%. The second layer 13 further preferably has an average pore size pursuant to the bubble point test of greater than 65-120 μm, preferentially 70-90 μm and most preferentially 80 μm.

(49) The third layer 14 preferably has a mass distribution of approximately 35-60 g/m.sup.2, preferentially approximately 40-55 g/m.sup.2, particularly preferentially approximately 45-50 g/m.sup.2 and most preferentially approximately 47.5 g/m.sup.2, and preferably a thickness of approximately 0.4-0.7 mm, preferentially approximately 0.5-0.6 mm and most preferentially approximately 0.55 mm. The third layer 14 further preferably has an air permeability of approximately 800-1300 l/(m.sup.2s), preferentially approximately 900-1200 l/(m.sup.2s) and most preferentially approximately 1000 l/(m.sup.2s), and/or a filtration efficiency of approximately 40-80%, preferentially approximately 50-70% and most preferentially approximately 60%. The third layer 14 further preferably has an average pore size pursuant to the bubble point test of greater than 10-60 μm, preferentially 20-50 μm, preferentially 30-40 μm and most preferentially 35 μm.

(50) Together, the three layers 12, 13, 14 preferably have a mass distribution of approximately 105-180 g/m.sup.2, preferentially approximately 120-160 g/m.sup.2 and most preferentially approximately 140 g/m.sup.2, and further preferably a thickness of approximately 1.2-2.5 mm, preferentially approximately 1.3-2.3 mm, particularly preferentially approximately 1.5-2.1 mm, preferentially approximately 1.7-1.9 mm and most preferentially approximately 1.8 mm. The collective air permeability amounts to approximately 500-1300 l/(m.sup.2s), preferentially approximately 600-1200 l/(m.sup.2s), particularly preferentially approximately 800-1000 l/(m.sup.2s) and most preferentially approximately 900 l/(m.sup.2s).

(51) The average pore diameter of the three layers amounts preferably to 15-25 μm.

(52) The inventive effect can also be achieved at the peripheries of the respective specifications of range. Additionally, all of the cited parameter values are average values which can deviate significantly at individual locations within the respective layers.