SINGLE-LAYER MULTI-STRAND CABLE HAVING IMPROVED ENERGY AT BREAK AND AN IMPROVED TOTAL ELONGATION

Abstract

A multi-strand cord (50) having a 1×N structure comprises a single layer (52) of N strands (54) wound in a helix about a main axis (A), each strand (54) having one layer (56) of metal filaments (F1) and comprising M>1 metal filaments wound in a helix about an axis (B). The cord (50) has a total elongation Δt>8.10% and the energy-at-break indicator Er of the cord (50), defined by Er=∫.sub.0.sup.Atσ(Ai)×dAi where σ(Ai) is the tensile stress in MPa measured at the elongation Ai and dAi is the elongation such that Er is strictly greater than 52 MJ/m.sup.3.

Claims

1.-15. (canceled)

16. A multi-strand cord (50) having a 1×N structure comprising a single layer (52) of N strands (54) wound in a helix about a main axis (A), each strand (54) having one layer (56) of metal filaments (F1) and comprising M>1 metal filaments wound in a helix about an axis (B), wherein the cord (50) has a total elongation At>8.10% determined by the standard ASTM D2969-04 of 2014, and wherein an energy-at-break indicator Er of the cord (50), defined by Er=∫.sub.0.sup.Atσ(Ai)×dAi, where σ(Ai) is a tensile stress in MPa measured at an elongation Ai and dAi is an elongation such that Er is strictly greater than 52 MJ/m.sup.3.

17. The multi-strand cord (50) according to claim 16, wherein the total elongation At≥8.30%.

18. The multi-strand cord (50) according to claim 16, wherein the energy-at-break indicator Er of the cord (50) is greater than or equal to 55 MJ/m.sup.3.

19. The multi-strand cord (50) according to claim 16, wherein the cord (50) has a structural elongation As determined by the standard ASTM D2969-04 of 2014 such that As>4.30%.

20. The multi-strand cord (50) according to claim 16, wherein the cord (50) has a secant modulus E1 ranging from 3.0 to 10.0 GPa.

21. The multi-strand cord (50) according to claim 16, wherein the cord has a tangent modulus E2 ranging from 50 to 180 GPa.

22. The multi-strand cord (50) according to claim 16, wherein the strands (54) define an internal enclosure (59) of the cord (50) of diameter Dv, each strand (54) having a diameter Dt and a helix radius of curvature Rt defined by Rt=Pe/(π×Sin(2αe)), where Pe is a pitch of each strand expressed in millimeters and αe is a helix angle of each strand (54), Dv, Dt and Rt being expressed in millimeters, the cord (50) satisfying the following relationships: 25≤Rt/Dt≤180 and 0.10≤Dv/Dt≤0.50.

23. The multi-strand cord (50) according to claim 16, wherein the metal filamentary elements (F1) define an internal enclosure (58) of the strand (52) of diameter Dvt, each metal filamentary element (F1) having a diameter Df and having a helix radius of curvature Rf defined by Rf=P/(π×Sin(2α)), where P is the pitch of each metal filamentary element expressed in millimeters and a is a helix angle of each metal filamentary element (F1), Dvt, Df and Rf being expressed in millimeters, the cord satisfying the following relationships: 9≤Rf/Df≤30 and 1.30≤Dvt/Df≤4.50.

24. A cord (50′) extracted from a polymer matrix, the extracted cord (50′) having a 1×N structure comprising a single layer (52) of N strands (54) wound in a helix about a main axis (A), each strand (54) having one layer (56) of metal filaments (F1) and comprising M>1 metal filaments wound in a helix about an axis (B), wherein the extracted cord (50′) has a total elongation At′≥5.00% determined by the standard ASTM D2969-04 of 2014, and wherein an energy-at-break indicator Er′ of the extracted cord (50′), defined by Er′=∫.sub.0.sup.At′σ(Ai)×dAi , where σ(Ai) is a tensile stress in MPa measured at an elongation Ai and dAi is an elongation such that Er′ is strictly greater than 35 MJ/m.sup.3.

25. The extracted cord (50′) according to claim 24, wherein the total elongation At′ is such that At′≥5.20%.

26. The extracted cord (50′) according to claim 24, wherein the energy-at-break indicator Er′ of the cord (50) is greater than or equal to 40 MJ/m.sup.3.

27. The extracted cord (50′) according to claim 24, wherein the strands (54) define an internal enclosure (59) of the extracted cord (50′) of diameter Dv, each strand (54) having a diameter Dt and a helix radius of curvature Rt defined by Rt=Pe/(π×Sin(2αe)), where Pe is a pitch of each strand expressed in millimeters and αe is a helix angle of each strand (54), Dv, Dt and Rt being expressed in millimeters, the extracted cord (50′) satisfying the following relationships: 25≤Rt/Dt≤180 and 0.10≤Dv/Dt≤0.50.

28. The extracted cord (50′) according to claim 24, wherein the metal filamentary elements (F1) define an internal enclosure (58) of the strand (52) of diameter Dvt, each metal filamentary element (F1) having a diameter Df and having a helix radius of curvature Rf defined by Rf=P/(π×Sin(2α)), where P is the pitch of each metal filamentary element expressed in millimeters and a is a helix angle of each metal filamentary element (F1), Dvt, Df and Rf being expressed in millimeters, the cord satisfying the following relationships: 9≤Rf/Df≤30 and 1.30≤Dvt/Df≤4.50.

29. A method for manufacturing the multi-strand cord (50) according to claim 16, the method comprising: a step (200) of manufacturing N strands (54) via: a step (100) of supplying a transitory assembly (22) comprising a layer made up of M′>1 metal filaments (F1) wound in a helix around a transitory core (16); a step (110) of separating the transitory assembly (22) into: a first split assembly (25) comprising a layer (26) made up of M1′>1 metal filaments (F1) wound in a helix, the M1′ metal filaments (F1) originating from the layer made up of M′>1 metal filaments (F1) of the transitory assembly (22), a second split assembly (27) comprising a layer (28) made up of M2′>1 metal filaments (F1) wound in a helix, the M2′ metal filaments (F1) originating from the layer made up of M′>1 metal filaments (F1) of the transitory assembly (22), and the transitory core (16) or one or more ensembles (83) comprising the transitory core (16); and a step (140) of reassembling the first split assembly (25) with the second split assembly (27) to form a strand (52) having one layer of metal filaments (F1) and comprising M>1 metal filaments (F1); and a step (300) of assembling the N strands (54) by cabling to form the cord (50).

30. The method according to claim 29, wherein M ranges from 3 to 18.

31. A reinforced product (R) comprising a polymer matrix (Ma) and at least one extracted cord (50′) according to claim 24.

32. A tire (P) comprising at least one extracted cord (50′) according to claim 24.

33. A tire (P) comprising the reinforced product according to claim 31.

Description

[0116] A better understanding of the invention will be obtained on reading the examples which will follow, given solely by way of non-limiting examples and made with reference to the drawings, in which:

[0117] FIG. 1 is a view in cross section perpendicular to the circumferential direction of a tyre according to the invention;

[0118] FIG. 2 is a detail view of the region II of FIG. 1;

[0119] FIG. 3 is a view in cross section of a reinforced product according to the invention;

[0120] FIG. 4 illustrates part of the stress-elongation curve for a cord (50) according to the invention;

[0121] FIG. 5 is a schematic view in cross section perpendicular to the axis of the cord (which is assumed to be straight and at rest) of a cord (50) according to a first embodiment of the invention;

[0122] FIG. 6 is a view similar to that of FIG. 5 of a cord (60) according to a second embodiment of the invention;

[0123] FIG. 7 is a schematic depiction of the effect of the deformability of the cord (50) of FIG. 5 under light tensile load thanks to the radial clearance of the filaments; and

[0124] FIGS. 8 and 9 are schematic depictions of the method according to the invention allowing the manufacture of the cord (50) of FIG. 5.

EXAMPLE OF A TYRE ACCORDING TO THE INVENTION

[0125] A frame of reference X, Y, Z corresponding to the usual respectively axial (X), radial (Y) and circumferential (Z) orientations of a tyre has been depicted in FIGS. 1 and 2.

[0126] The “median circumferential plane” M of the tyre is the plane that is normal to the axis of rotation of the tyre and that is located equidistantly from the annular reinforcement structures of each bead.

[0127] FIGS. 1 and 2 depict a tyre according to the invention and denoted by the general reference P.

[0128] The tyre P is for a heavy vehicle of construction plant type, for example of “dumper” type. Thus, the tyre P has a dimension of the type 53/80R63.

[0129] The tyre P has a crown 12 reinforced by a crown reinforcement 14, two sidewalls 16 and two beads 18, each of these beads 18 being reinforced with an annular structure, in this instance a bead wire 20. The crown reinforcement 14 is surmounted radially by a tread 22 and connected to the beads 18 by the sidewalls 16. A carcass reinforcement 24 is anchored in the two beads 18 and is in this instance wound around the two bead wires 20 and comprises a turnup 26 positioned towards the outside of the tyre 20, which is shown here fitted onto a wheel rim 28. The carcass reinforcement 24 is surmounted radially by the crown reinforcement 14.

[0130] The carcass reinforcement 24 comprises at least one carcass ply 30 reinforced by radial carcass cords (not depicted). The carcass cords are positioned substantially parallel to one another and extend from one bead 18 to the other so as to form an angle comprised between 80° and 90° with the median circumferential plane M (plane perpendicular to the axis of rotation of the tyre which is situated midway between the two beads 18 and passes through the middle of the crown reinforcement 14).

[0131] The tyre P also comprises a sealing ply 32 made up of an elastomer (commonly known as “inner liner”) which defines the radially internal face 34 of the tyre P and which is intended to protect the carcass ply 30 from the diffusion of air coming from the space inside the tyre P.

[0132] The crown reinforcement 14 comprises, radially from the outside towards the inside of the tyre P, a protective reinforcement 36 arranged radially on the inside of the tread 22, a working reinforcement 38 arranged radially on the inside of the protective reinforcement 36 and an additional reinforcement 40 arranged radially on the inside of the working reinforcement 38. The protective reinforcement 36 is thus radially interposed between the tread 22 and the working reinforcement 38. The working reinforcement 38 is interposed radially between the protective reinforcement 36 and the additional reinforcement 40.

[0133] The protective reinforcement 36 comprises first and second protective plies 42, 44 comprising protective metal cords, the first ply 42 being arranged radially on the inside of the second ply 44. Optionally, the protective metal cords make an angle at least equal to 10°, preferably in the range from 10° to 35° and preferentially from 15° to 30°, with the circumferential direction Z of the tyre.

[0134] The working reinforcement 38 comprises first and second working plies 46, 48, the first ply 46 being arranged radially on the inside of the second ply 48. Each ply 46, 48 comprises at least one cord 50. Optionally, the working metal cords 50 are crossed from one working ply to the other and make an angle at most equal to 60°, preferably in the range from 15° to 40°, with the circumferential direction Z of the tyre.

[0135] The additional reinforcement 40, also referred to as a limiting block, the purpose of which is to absorb in part the mechanical stresses of inflation, comprises, for example and as known per se, additional metal reinforcing elements, for example as described in FR 2 419 181 or FR 2 419 182, making an angle at most equal to 10°, preferably in the range from 5° to 10°, with the circumferential direction Z of the tyre P.

[0136] Example of a Reinforced Product According to the Invention

[0137] FIG. 3 depicts a reinforced product according to the invention and denoted by the general reference R. The reinforced product R comprises at least one cord 50′, in this instance several cords 50′, embedded in the polymer matrix Ma.

[0138] FIG. 3 depicts the polymer matrix Ma, the cords 50′ in a frame of reference X, Y, Z, in which the direction Y is the radial direction and the directions X and Z are the axial and circumferential directions. In FIG. 3, the reinforced product R comprises several cords 50′ arranged side-by-side in the main direction X and extending parallel to one another within the reinforced product R and collectively embedded in the polymer matrix Ma. In this instance, the polymer matrix Ma is an elastomer matrix based on an elastomer compound.

[0139] Cord According to a First Embodiment of the Invention

[0140] FIG. 5 depicts the cord 50 according to a first embodiment of the invention.

[0141] Each protective reinforcing element 43, 45 and each hoop reinforcing element 53, 55 is formed, once it has been extracted from the tyre 10, of an extracted cord 50′ as described below. The cord 50 is obtained by embedding in a polymer matrix, in this instance in a polymer matrix respectively forming each polymer matrix of each protective ply 42, 44 and of each hoop layer 52, 54 in which the protective reinforcing elements 43, 45 and the hoop reinforcing elements 53, 55 are respectively embedded.

[0142] The cord 50 and the extracted cord 50′ are made of metal having a single layer.

[0143] The cord 50 or the cord 50′ comprises a layer of 1×N structure comprising a single layer 52 of N=3 strands 54 wound in a helix about a main axis (A), each strand 54 having one layer 56 of metal filaments F1 and comprising M>1 metal filaments wound in a helix about an axis (B), with in this instance M=5.

[0144] As described above, the value At is determined by plotting a force-elongation curve for the cord 50, by applying the standard ASTM D2969-04 of 2014.

[0145] The cord 50 has a total elongation At>8.10%, preferably At≥8.30% and more preferentially At≥8.50% and the total elongation At≤20.00% and preferably At≤16.00%, in this instance At=13.4%.

[0146] As described hereinabove, from this stress-elongation curve, the area under this curve is deduced. FIG. 4 depicts the rectangle method for determining the energy-at-break indicator for the cord 50.

[0147] The energy-at-break indicator Er for the cord 50 is such that Er=∫.sub.0.sup.Atσ(Ai)×dAi which is substantially equal to Σ.sub.0%.sup.13.4% ½(σ(Ai)+σ(Ai+1))×0.025%=89 MJ/m.sup.3, which is strictly greater than 52 MJ/m.sup.3, preferably greater than or equal to 55 MJ/m.sup.3 and less than or equal to 200 MJ/m.sup.3 and preferably less than or equal to 150 MJ/m.sup.3.

[0148] The cord 50 has a structural elongation As such that As>4.30%, preferably As≥4.50% and more preferentially As≥4.60% and such that As≤10.0% and preferably As≤9.50%. In this instance As=9.3%.

[0149] The cord 50 has a secant modulus E1 ranging from 3.0 to 10.0 GPa and preferably ranging from 3.5 to 8.5 GPa. In this instance E1=4.0 GPa.

[0150] The cord 50 has a tangent modulus E2 ranging from 50 to 180 GPa and preferably from 55 to 150 GPa. In this instance, E2=73 GPa.

[0151] The extracted cord 50′ has a total elongation At′>5.00% and preferably At′≥5.20%. In this instance At′=10%.

[0152] The energy-at-break indicator Er′ for the extracted cord 50′ is such that Er′=∫.sub.0.sup.At′σ(Ai)×dAi which is substantially equal to Σ.sub.0%.sup.10.0%½(σ(Ai)+σ(Ai+1))×0.025%=82 MJ/m.sup.3, which is strictly greater than 35 MJ/m.sup.3, preferably greater than or equal to 40 MJ/m.sup.3.

[0153] The strands 54 define an internal enclosure 59 of the cords 50; 50′ of diameter Dv, each strand 54 having a diameter Dt and having a helix radius of curvature Rt defined by Rt=Pe/(π×Sin(2σe))=80/(π×sin(2×5.3×π/180)=138 mm.

[0154] Rt/Dt=138/2.03=68≤180 and 68≥25.

[0155] Dv/Dt=0.32/2.03=0.16≤0.50 and 0.16≥0.10.

[0156] The metal filamentary elements F1 of each strand 52 define an internal enclosure 58 of the strand 52 of diameter Dvt, each metal filamentary element F1 has a diameter Df and has a helix radius of curvature Rf defined by Rf=P/(π×Sin(2α))=10.4/(π×sin(2×25.8×π/180)=4.2 mm.

[0157] Rf/Df=4.2/0.46=9≤30.

[0158] Dvt/Df=1.12/0.46=2.46≤4.50 and 2.46≥1.30.

[0159] Method for Manufacturing the Cord According to the Invention

[0160] An example of a method for the manufacture of the multi-strand cord 50 as depicted in FIGS. 8 and 9 will now be described.

[0161] First of all, the filamentary elements F1 and the transitory core 16 are unwound from the supply means.

[0162] Next, the method comprises a step 100 of supplying the transitory assembly 22 comprising, on the one hand, a step of assembly by twisting the M′ metal filamentary elements F1 in a single layer of M′ metal filamentary elements F1 around the transitory core 16 and, on the other hand, a step of balancing the transitory assembly 22 carried out by means of a twister.

[0163] The method comprises a step 110 of separating the transitory assembly 22 into the first split assembly 25, the second split assembly 27 and the transitory core 16 or one or more ensembles comprising the transitory core 16, in this case the transitory core 16.

[0164] Downstream of the supply means 11, the step 110 of separating the transitory assembly 22 into the first split assembly 25, the second split assembly 27 and the transitory core 16 comprises a step 120 of separating the transitory assembly 22 into the precursor ensemble, the second split assembly 27 and finally the transitory core 16.

[0165] Downstream of the separation step 122, the step 120 of separating the transitory assembly into the precursor ensemble and the split ensemble comprises a step 124 of separating the split ensemble into the second split assembly 27 and the transitory core 16. In this case, the separation step 124 comprises a step of splitting the split ensemble into the second split assembly 27, the transitory core 16 and the complementary ensemble.

[0166] Downstream of the supply step 100, the step 110 of separating the transitory assembly into the first split assembly 25, the second split assembly 27 and the transitory core 16 comprises a step 130 of separating the precursor ensemble into the first split assembly 25 and the complementary ensemble.

[0167] Downstream of the separation steps 110, 120, 124 and 130, the method comprises a step 140 of reassembling the first split assembly 25 with the second split assembly 27 to form the strand 54. In this embodiment, the reassembly step 140 is a step of reassembling the first split assembly 25 with the second split assembly 27 to form the strand 54 and comprising M>1 metal filaments F1, where M ranges from 3 to 18 and preferably from 4 to 15, and here M=5.

[0168] In this embodiment, the supply step 100, the separation step 110 and the reassembly step 140 are carried out so that all the M′ metal filamentary elements F1 have the same diameter Dfi, are helically wound at the same pitch P and have the same helix radius of curvature Rf that are described above.

[0169] In this embodiment allowing a partial reassembly of the M′ metal filamentary elements, the separation step 110 and the reassembly step 140 are carried out so that M1′+M2′<M′. Here, M1′=1 and M2′=4: M1′+M2′=5<8. It will finally be noted that M1′≤0.70×M′=0.70×8=5.6 and M2′≤0.70×M′=0.70×8=5.6.

[0170] A final balancing step is performed.

[0171] Finally, the strand 54 is stored on a storage spool. N strands 54 are manufactured in the same way.

[0172] As regards the transitory core 16, the method comprises a step of recycling the transitory core 16. During this recycling step, the transitory core 16 is recovered downstream of the separation step 110, in this case downstream of the separation step 124, and the transitory core 16 previously recovered is introduced upstream of the assembly step. This recycling step is continuous.

[0173] It will be noted that the method thus described does not have steps of individually preforming each of the metal filamentary elements F1.

[0174] An assembly step 300 is performed that involves assembling the N strands 54 by cabling to form the cord 50. In this instance N=3.

[0175] It will be noted that the method thus described does not have steps of individually preforming each of the strands 54.

[0176] Cord According to a Second Embodiment of the Invention

[0177] FIG. 6 depicts the cord 60 according to a second embodiment of the invention.

[0178] Unlike in the first embodiment described hereinabove, the cord 60 according to the second embodiment is such that N=4.

[0179] The characteristics of the various cords 50, 50′, 60, 60′, 51, 52, 53, 53′, 54 according to the invention and of the cords of the prior art EDT1, EDT1′, EDT2 and EDT2′ are summarized in Tables 1, 2 and 3 below.

[0180] Comparative Tests

[0181] Evaluation of the Total Elongation and of the Enemy-at-Break Indicator for the Cords

[0182] The stress-elongation curves for the cords were plotted by applying the standard ASTM D2969-04 of 2014, and the total elongation and the energy-at-break indicator for the various cords 50, 50′, 60, 60′, 51, 52, 53, 53′, 54 according to the invention and for the cords EDT1, EDT1′, EDT2 and EDT2′ of the prior art were calculated.

[0183] In Table 3, “NA” signifies that the parameter has not been measured.

TABLE-US-00001 TABLE 1 Cords 50 50′ 60 60′ N/direction of cord 3/S 3/S 4/Z 4/Z direction of strand 54 S S S S M′ 8 8 8 8 M 5 5 5 5 Rf (mm) 4.2 4.2 4.2 4.2 P (mm) 10.4 10.4 10.4 10.4 α (°) 25.8 25.8 25.8 25.8 Df (mm) 0.46 0.46 0.46 0.46 Dvt (mm) 1.12 1.12 1.12 1.12 Rf/Df 9 9 9 9 Dvt/Df 2.46 2.46 2.46 2.46 Rt (mm) 138 138 113 113 Pe (mm) 80 80 80 80 αe (°) 5.3 5.3 6.5 6.5 Dt (mm) 2.03 2.03 2.03 2.03 Dv (mm) 0.32 0.32 0.86 0.86 Rt/Dt 68 68 56 56 Dv/Dt 0.16 0.16 0.42 0.42 E1 (GPa) 4.0 — 3.8 — ML (g/m) 21.3 21.3 28.5 28.5 E2 (GPa) 73 39 59 33 At % 13.4 — 13.8 — At′ % — 10.0 — 10.1 Er (MJ/m.sup.3) 89 — 88 — Er′ (MJ/m.sup.3) — 82 — 83 As % 9.3 — 9.4 — D (mm) 4.38 4.38 4.92 4.92

TABLE-US-00002 TABLE 2 Cords 51 52 53 53′ 54 N/direction of cord 3/Z 3/Z 3/Z 3/Z 3/Z Direction of strand 54 S S S S S M′ 8 8 8 8 7 M 6 7 8 8 5 Rf(mm) 4.2 4.2 4.2 4.2 4.8 P (mm) 10.4 10.4 10.4 10.4 10.4 α (°) 25.8 25.8 25.8 25.8 21.8 Df(mm) 0.46 0.46 0.46 0.46 0.46 Dvt(mm) 1.12 1.12 1.12 1.12 0.84 Rf/Df 9 9 9 9 10 Dvt/Df 2.46 2.46 2.46 2.46 1.85 Rt(mm) 138 138 138 138 159 Pe (mm) 80 80 80 80 80 αe (°) 5.3 5.3 5.3 5.3 4.6 Dt(mm) 2.03 2.03 2.03 2.03 1.75 Dv(mm) 0.32 0.32 0.32 0.32 0.28 Rt/Dt 68 68 68 68 91 Dv/Dt 0.16 0.16 0.16 0.16 0.16 E1 (GPa) 3.9 4.3 8.0 — 7.1 ML (g/m) 25.5 29.4 33.4 33.4 20.4 E2 (GPa) 85 94 106 53 95 At % 11.9 8.9 8.5 — 9.0 At′ % — — — 5.9 — Er (MJ/m.sup.3) 82 63 56 — 72 Er′ (MJ/m.sup.3) — — — 48 — As % 7.8 6.0 4.6 — 5.6 D (mm) 4.38 4.38 4.38 4.38 3.78

TABLE-US-00003 TABLE 3 Cords EDT1 EDT1′ EDT2 EDT2′ N/direction of cord 4/S 4/S 4/S 4/S Direction of strands S S S S M′ — — — — M 3 3 4 4 V 8 8 9 9 Rf(mm) NA NA NA NA P1 (mm) 6.7 6.7 5.1 5.1 P2 (mm) 10 10 7.5 7.5 α (°) NA NA NA NA Df(mm) 0.35 0.35 0.26 0.26 Dvt(mm) NA NA NA NA Rf/Df NA NA NA NA Dvt/Df NA NA NA NA Rt(mm) 9 9 6.3 6.3 Pe (mm) 20 20 15 15 αe (°) 22.5 22.5 24.8 24.8 Dt(mm) 1.48 1.48 1.15 1.15 Dv(mm) 0.84 0.84 0.80 0.80 Rt/Dt 6.1 6.1 5.4 5.4 Dv/Dt 0.57 0.57 0.70 0.70 E1 (GPa) 1.0 — 1.0 — ML (g/m) 35.8 35.8 23.1 23.1 E2 (GPa) 104 81 81 68 At % 6.0 — 8.1 — At′ % — 3.4 — 4.7 Er (MJ/m.sup.3) 44 — 52 — Er′ (MJ/m.sup.3) — 30 — 31 As % 2.8 — 4.3 — D (mm) 3.80 3.80 3.10 3.10

[0184] Tables 1, 2 and 3 demonstrate that the cords 50, 50′, 60, 60′, 51, 52, 53, 53′, 54 according to the invention have both an improved energy-at-break indicator and have better deformability in comparison with the cords of the prior art EDT1, EDT1′, EDT2 and EDT2′.

[0185] Thus, the cords according to the invention are able to solve the problems mentioned in the preamble.

[0186] The invention is not limited to the above-described embodiments.