Manufacturing method of rectangular wire, fluororesin paint and rectangular wire

Abstract

To provide a flat wire having an insulation film excellent in uniformity of a film thickness.SOLUTION: A method for manufacturing a flat wire having a conductor having a substantially square cross section and an insulation film provided on a surface of the conductor, comprises: (1) a step of manufacturing a conductor of which circumference is coated with a fluororesin coating by passing a coating die, after circumference is coated with a fluororesin coating containing fluororesin powder and having viscosity at 25°C of 2.4 Pa s or more; and (2) a step of irradiating ionizing radiation on the conductor of which circumference is coated with the fluororesin coating, under conditions of oxygen-free, and above a crystal melting point of the fluororesin.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a method for producing a rectangular wire, a fluororesin coating material, and a rectangular wire. [Background technology]

[0002] As a method for manufacturing a rectangular wire having an insulating coating on the surface of a conductor with a roughly rectangular cross section, for example, a method has been proposed in which an insulating varnish is applied to the surface of the conductor, the conductor is passed through a coating die to remove excess insulating varnish, and then the conductor is dried and baked to form an insulating coating on the conductor surface (see, for example, Patent Document 1). Cited Document 1 lists polyamideimide resin, polyimide resin, polyesterimide resin, polyamide resin, polyethersulfone resin, polyetheretherketone resin, polysulfone resin, etc. as examples of the resin component of the insulating varnish. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] International Publication No. 2013 / 073397 [Patent Document 2] International Publication No. 2017 / 043372 Summary of the Invention [Problem to be solved by the invention]

[0004] The present inventors have investigated the use of fluororesin as the resin material for the insulating film. Fluororesin is a low dielectric constant material, and its use as the resin material for the insulating film is expected to ensure high voltage resistance while reducing the thickness of the insulating film. On the other hand, when a rectangular wire is manufactured using fluororesin as the resin material for the insulating coating, the thickness of the insulating coating can become uneven. In particular, the thickness of the insulating coating covering the corners of the conductor tends to be thinner than the thickness of the insulating coating covering the flat parts of the conductor. [Means for solving the problem]

[0005] Under these circumstances, the present inventors have made repeated investigations to provide a rectangular wire having an insulating coating with excellent thickness uniformity, which uses a fluororesin as a resin component. As a result, the inventors surprisingly found that, contrary to the findings disclosed in Cited Document 1, when an insulating coating is formed using a fluororesin paint as the material for the insulating coating and using a coating die, an insulating coating with excellent thickness uniformity can be formed not only when the viscosity of the fluororesin paint is 10.0 Pa s or more at 25°C, but also when the viscosity is lower than 10.0 Pa s at 25°C, and thus completed the present invention.

[0006] One aspect of the present invention is a method for manufacturing a rectangular wire having a conductor having a substantially rectangular cross section and an insulating coating provided on a surface of the conductor, (1) covering the periphery with a fluororesin paint containing fluororesin powder and having a viscosity of 2.4 Pa s or more at 25°C, and then passing the conductor through a coating die to produce a conductor whose periphery is coated with the fluororesin paint; (2) irradiating the conductor, the periphery of which is coated with a fluororesin paint, with ionizing radiation in an oxygen-free environment at or above the crystalline melting point of the fluororesin; This is a method for producing a rectangular wire having the above structure.

[0007] Another aspect of the present invention is a fluororesin paint used to form an insulating coating for a rectangular wire, This fluororesin paint contains fluororesin powder and has a viscosity of 2.4 Pa·s or more at 25°C.

[0008] Yet another aspect of the present invention is a rectangular wire having a conductor having a substantially rectangular cross section and an insulating coating provided on a surface of the conductor, The insulating coating is a rectangular wire having a wall thickness deviation of 15% or less. Uneven thickness rate: The percentage of the thickness of the insulating coating covering the flat parts of the conductor minus the thickness of the insulating coating covering the corners of the conductor. Effect of the Invention

[0009] According to the present invention, it is possible to provide a rectangular wire having an insulating coating with good thickness uniformity. The rectangular wire also has good electrical properties, heat resistance, and chemical resistance. In the present invention, if the thickness of the insulating coating covering the corners of the conductor of a rectangular wire is not too thin compared to the thickness of the insulating coating covering the flat parts of the conductor, the rectangular wire is said to have good uniformity in the thickness of the insulating coating. [Brief description of the drawings]

[0010] [Figure 1] FIG. 2 is a cross-sectional view showing the configuration of a rectangular wire. [Diagram 2] FIG. 1 is a diagram for explaining a method for measuring and calculating the wall thickness deviation rate of an insulating coating of a rectangular wire. [Diagram 3] FIG. 2 is a diagram for explaining the configuration of a coating die used in manufacturing a rectangular wire. [Figure 4] FIG. 1 is a schematic diagram of an apparatus used for applying a fluororesin paint. [Diagram 5] 1 is a graph showing the correlation between the viscosity of a fluororesin paint and the wall thickness deviation rate. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] [Summary of the embodiment of the invention] First, the contents of the embodiments of the present invention will be listed and described. [1] A method for manufacturing a rectangular wire according to one embodiment of the present invention is a method for manufacturing a rectangular wire having a conductor having a substantially rectangular cross section and an insulating coating provided on a surface of the conductor, (1) covering the periphery with a fluororesin paint containing fluororesin powder and having a viscosity of 2.4 Pa s or more at 25°C, and then passing the conductor through a coating die to produce a conductor whose periphery is coated with the fluororesin paint; (2) irradiating the conductor, the periphery of which is coated with a fluororesin paint, with ionizing radiation in an oxygen-free environment at or above the crystalline melting point of the fluororesin; has.

[0012] According to the manufacturing method of the rectangular wire described in [1] above, it is possible to efficiently manufacture a rectangular wire having an insulating coating made of cross-linked fluororesin, the insulating coating having good uniformity in thickness. Furthermore, the resulting rectangular wire has an insulating coating that has good electrical properties, as well as good heat resistance and chemical resistance. Furthermore, because the insulating coating is made of fluororesin, which has a small coefficient of dynamic friction, the resulting rectangular wire is also suitable for reducing stress caused by friction between wires during coil winding or compression processing to increase the space factor, and therefore for reducing damage to the insulating coating caused by this stress.

[0013] [2] In the method for producing a rectangular wire according to [1] above, the dose of the ionizing radiation is preferably 10 kGy or more and 2000 kGy or less, and more preferably 30 kGy or more and 200 kGy or less. In these cases, the fluororesin can be sufficiently crosslinked to form an insulating coating having good mechanical properties such as excellent abrasion resistance, etc. In addition, the productivity is also excellent because excessive exposure to ionizing radiation is not required.

[0014] [3] In the method for producing a rectangular wire according to the above [1] or [2], the fluororesin is preferably at least one selected from the group consisting of PTFE, PFA and FEP. The fluororesin is more preferably PTFE. This is because these methods are suitable for forming a high-performance insulating film at low cost.

[0015] [4] In any one of the methods for producing a rectangular wire according to [1] to [3] above, the fluororesin has a melt viscosity of 7×10 at 380° C. 5 It is preferable that the viscosity is less than Pa·s. In this case, the fluororesin is less likely to fibrillate when the insulating coating is formed, making it suitable for forming a coating of fluororesin paint around the conductor.

[0016] [5] A fluororesin paint according to one embodiment of the present invention is a fluororesin paint used for forming an insulating coating on a rectangular wire, It contains fluororesin powder and has a viscosity of 2.4 Pa·s or more at 25°C. The above-mentioned fluororesin paint is suitable as the fluororesin paint used in the above-mentioned method for producing rectangular wire.

[0017] [6] In the fluororesin coating material described in [5] above, the fluororesin is preferably at least one selected from the group consisting of PTFE, PFA and FEP, and more preferably PTFE. This is because these methods are suitable for forming a high-performance insulating film at low cost.

[0018] [7] In the fluororesin coating material according to [5] or [6] above, the fluororesin has a melt viscosity of 7×10 at 380°C. 5 It is preferable that the viscosity is less than Pa·s. In this case, the fluororesin is less likely to fibrillate when the coating is formed, making it more suitable as a material for forming an insulating coating for rectangular wires.

[0019] [8] The rectangular wire according to one embodiment of the present invention is A rectangular wire having a conductor with a substantially rectangular cross section and an insulating coating provided on the surface of the conductor, The insulating coating has a wall thickness deviation of 15% or less. Uneven thickness rate: The percentage of the thickness of the insulating coating covering the flat parts of the conductor minus the thickness of the insulating coating covering the corners of the conductor.

[0020] The rectangular wire described in [8] above has a small wall thickness deviation and good uniformity in the thickness of the insulating coating. In an insulated electric wire, if the thickness of the insulating coating is not uniform, the electrical insulation of the insulated electric wire is determined by the thin parts of the insulating coating, and the insulation may be insufficient. Therefore, it is important to increase the uniformity of the thickness of the insulating coating in order to ensure electrical insulation.

[0021] [Details of the embodiment of the invention] Specific examples of the embodiments of the present invention will be described below with reference to the drawings as appropriate. Note that the present invention is not limited to these examples. The scope of the present invention is defined by the claims, and is intended to include all modifications within the meaning and scope of the claims.

[0022] [Rectangular wire] 1 is a cross-sectional view showing an example of a rectangular wire according to an embodiment of the present invention, in which a cross section perpendicular to the longitudinal direction of the rectangular wire is shown. As shown in FIG. 1, a rectangular wire 10 according to an embodiment of the present invention is made up of a conductor 11 having a substantially rectangular cross section and an insulating coating 12 provided around the conductor 11. Conductor 11 has flat portion 11a and four corners 11b connecting flat portions 11a, and corners 11b are rounded. Therefore, corners 11b have a cross-sectional shape of an arc. In the present invention, a substantially quadrangle refers to a rectangle with four rounded corners.

[0023] The core wire used as the conductor 11 may be any known wire, and specific examples include metal wires such as copper wire, aluminum wire, aluminum alloy wire, nickel-plated copper wire, and silver-plated copper wire.

[0024] The insulating coating 12 is made of a fluororesin cross-linked by ionizing radiation. Since fluororesin has a small dielectric constant, it is suitable for thinning the thickness of the insulating coating 12. If the thickness of the insulating coating 12 can be thinned, the rectangular wire can be manufactured at low cost. The specific thickness of the insulating coating 12 may be appropriately selected depending on the required characteristics of the rectangular wire. Specific examples of the fluororesin will be described in detail later.

[0025] The insulating coating 12 has a thickness deviation rate of 15% or less. In the present invention, the wall thickness deviation rate is the percentage of the thickness T1 of the insulating coating 12a covering the flat portion 11a of the conductor 11 minus the thickness T2 of the insulating coating 12b covering the corners 11b of the conductor 11, and is calculated using the following formula (1).

[0026] Thickness unevenness rate (%)=[(T1-T2) / T1]×100...(1) In the rectangular wire 10, the closer the thickness deviation value is to zero, the smaller the difference between the thickness T1 of the insulating coating 12a covering the flat portion 11a of the conductor 11 and the thickness T2 of the insulating coating 12b covering the corners 11b of the conductor 11 is.

[0027] If the thickness deviation rate exceeds 15%, the thickness T2 of the insulating coating 12b covering the corners 11b will be thinner than the thickness T1 of the insulating coating 12a covering the flat portions 11a, and the insulating properties of the insulating coating may become insufficient. The thickness deviation is preferably 10% or less, more preferably 3% or less, and even more preferably 0% or less.

[0028] When the thickness deviation ratio is a negative value, it means that the thickness T2 of the insulating coating 12b covering the corners 11b is thicker than the thickness T1 of the insulating coating 12a covering the flat portions 11a. In this case, even if T2 is sufficiently thicker than T1 (even if the thickness deviation ratio value is small), no significant inconvenience occurs, but the preferred lower limit of the thickness deviation ratio is -25%, and the more preferred lower limit is -15%. A rectangular wire having such a configuration is used, for example, in motor windings. A rectangular wire having such a configuration can be manufactured, for example, by the manufacturing method of a rectangular wire according to an embodiment of the present invention.

[0029] In the above rectangular wire, the thickness T1 and the thickness T2 are measured by the following methods. FIG. 2 is a diagram for explaining a method for measuring the thicknesses T1 and T2 of the insulating coating used in calculating the wall thickness deviation rate. An optical microscope image of the cross section of the rectangular wire 10 is obtained, and based on the obtained optical microscope image of the rectangular wire, the film thicknesses at positions 1, 3, 5, 7 and 2, 4, 6, 8 shown by circled numbers in FIG. 2 are measured. At this time, optical microscope images of the cross section of the rectangular wire 10 are taken for three cross sections at intervals of 50 cm, and the film thickness is measured at eight points on each cross section. Therefore, film thicknesses at 8 points x 3 = 24 points can be obtained. Then, the average value of the film thicknesses obtained at the central parts of each flat portion, numbers 1, 3, 5, and 7, among the circled numbers in Figure 2, is calculated, and this value is set as the thickness T1 of the insulating coating 12 covering the flat portion of the conductor 11. In addition, the average value of the film thicknesses obtained at positions 2, 4, 6, and 8 among the circled numbers in FIG. 2 is calculated, and this value is set as the thickness T2 of the insulating coating 12 covering the corners of the conductor 11.

[0030] [Method of manufacturing rectangular wire] The method for manufacturing a rectangular wire according to an embodiment of the present invention is as follows: (1) covering the periphery with a fluororesin paint containing fluororesin powder and having a viscosity of 2.4 Pa s or more at 25°C, and then passing the conductor through a coating die to produce a conductor whose periphery is coated with the fluororesin paint; (2) irradiating the conductor, the periphery of which is coated with a fluororesin paint, with ionizing radiation in an oxygen-free environment at or above the crystalline melting point of the fluororesin; has.

[0031] In the above step (1), first, a fluororesin paint containing a fluororesin powder and having a viscosity of 2.4 Pa·s or more at 25° C. is prepared. The fluororesin paint is prepared by dispersing fluororesin powder and a thickener in a solvent mainly composed of water. The solvent is not particularly limited as long as it is mainly composed of water, and may be composed of only water or may contain other components in addition to water. Here, "mainly composed of water" means that the proportion of water in the solvent is 50% by volume or more. Other components that may be contained in the solvent include, for example, water-soluble organic dispersion media such as alcohol.

[0032] The particle size of the fluororesin powder may be, for example, 0.1 μm or more and 0.5 μm or less in terms of average primary particle size. The average primary particle size is measured by the following method. A calibration curve is prepared between the transmittance per unit length of a projected light having a wavelength of 550 nm for an aqueous dispersion of a fluororesin with a polymer concentration adjusted to 0.22% by mass and the average primary particle diameter determined by measuring the unidirectional diameter in a transmission electron microscope photograph.The transmittance is then measured for the aqueous dispersion to be measured, and the average primary particle diameter is calculated based on the calibration curve.

[0033] Examples of the fluororesin include PTFE (polytetrafluoroethylene resin), PFA (tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer), FEP (tetrafluoroethylene-hexafluoropropylene copolymer), polyvinylidene fluoride (PVDF), tetrafluoroethylene-ethylene copolymer (ETFE), polychlorotrifluoroethylene (PCTFE), chlorotrifluoroethylene-ethylene copolymer (ECTFE), polyvinyl fluoride (PVF), fluoroolefin-vinyl ether copolymer, vinylidene fluoride-tetrafluoroethylene copolymer, vinylidene fluoride-hexafluoropropylene copolymer, and the like. These may be used alone or in combination of two or more kinds. The fluororesin is preferably at least one of PTFE, PFA and FEP, and more preferably PTFE. PTFE is one of the cheapest fluororesins and is particularly suitable for forming insulating films that are low cost and have good electrical properties and heat and chemical resistance.

[0034] When the fluororesin is PTFE, the PTFE may be homo-PTFE, modified PTFE, or a mixture of both. Homo-PTFE refers to a homopolymer of tetrafluoroethylene. Modified PTFE refers to a copolymer of tetrafluoroethylene and a monomer other than tetrafluoroethylene (also called a "modified monomer"). The upper limit of the content of the structural unit derived from the modified monomer in the modified PTFE is preferably 1% by mass, more preferably 0.5% by mass. As the modified monomer, a known monomer may be used, and one type may be used alone, or two or more types may be used.

[0035] The above-mentioned fluororesin can be produced by a conventionally known production method. Specifically, PTFE can be produced, for example, by emulsion polymerization of tetrafluoroethylene (TFE) in the presence of an aqueous dispersion medium, a surfactant, and a radical polymerization initiator. Examples of the surfactant include fluorine-containing surfactants such as perfluoroalkyl carboxylic acids and their salts, and fluorine-containing sulfonic acids and their salts. Examples of the radical polymerization initiator include persulfates such as ammonium persulfate and potassium persulfate, and water-soluble organic peroxides such as disuccinic acid peroxide and tert-butyl hydroperoxide. During the synthesis of the PTFE, a chain transfer agent may be used to control the molecular weight (melt viscosity) of the resulting PTFE. Furthermore, an emulsion stabilizer such as paraffin wax, a pH adjuster such as ammonium carbonate, and the like may also be used. As the fluororesin, commercially available products can also be used.

[0036] The above fluororesin has a melt viscosity of 7×10 at 380°C. 5 The upper limit of the melt viscosity at 380°C is preferably 6×10 5 Pa s is more preferable, 5×10 5 Pa·s is more preferred. When the melt viscosity of the fluororesin before irradiation with ionizing radiation is within the above-mentioned preferable upper limit range, the mechanical properties of the formed insulating coating may be further improved. On the other hand, a fluororesin (especially PTFE) having a melt viscosity exceeding the above-mentioned preferable upper limit may have poor melt fluidity and may have a tendency to fibrillate, which may result in poor film-forming properties and make it difficult to form a smooth insulating coating. On the other hand, the lower limit of the melt viscosity of the above fluororesin at 380°C is 1 × 10 2 Pa s is preferred, 1×10 3 Pa s is more preferable, 1×10 4 Pa s is more preferable, and 1×10 5 Pa·s is particularly preferred. If the melt viscosity is less than the preferred lower limit, the amount of volatile components at high temperatures increases, and volatilization is likely to occur during irradiation with ionizing radiation or during baking (heat treatment) before irradiation, which may result in a deterioration in the mechanical properties of the insulating coating formed. In addition, the insulating coating formed may be discolored.

[0037] Next, a method for measuring the melt viscosity of a fluororesin at 380° C. will be described. (Method X) The melt viscosity of a fluororesin at 380° C. is a value measured by the following method X. First, a molded body is prepared as a sample by the following procedure. 3 g of powdered fluororesin is filled into a cylindrical mold with an inner diameter of 30 mm and compressed to a final pressure of 30 MPa. By maintaining the compression at 30 MPa for 2 minutes, a disk-shaped sample (diameter 30 mm, thickness approximately 2 mm) is obtained. The obtained sample is removed from the mold and cut into a disk shape with a diameter of 27 mm, and then clamped between the parallel plate test stand with a diameter of 25 mm of Anton Paar's "Rheometer MCR500". The sample is held at 380°C for 5 minutes. The plate distance is then adjusted to 1.5 mm, the sample protruding from the plate is removed, and the sample is held at 380°C for 30 minutes until the stress in the sample is sufficiently relaxed. The melt viscosity is measured at 380°C in a vibration mode with a deformation amount of 0.2% and a shear rate of 0.1 (1 / s). In the case of parallel plates, the deformation amount during measurement is the ratio of the vibration amplitude of the outermost circumference of the parallel plates to the sample thickness.

[0038] (Method Y) The melt viscosity of the fluororesin at 380° C. can also be measured by the following method Y. The value measured by method X and the value measured by method Y are substantially equal. Method Y is a measurement method conforming to ASTM D 1238. Specifically, using a Shimadzu flow tester and a 2φ-8L die, a 2g sample that has been preheated to the measurement temperature (380°C) for 5 minutes can be measured under a load of 0.7 MPa while maintaining the sample at the above temperature.

[0039] (Method Z) On the other hand, the melt viscosity at 380°C of a high molecular weight fluororesin that does not melt and flow can be determined by the following Method Z, which determines the melt viscosity from the elongation (creep) in the molten state by thermomechanical analysis (TMA). Specifically, a creep test can be performed using a solid viscoelasticity spectrometer (SII NanoTechnology's "EXSTAR 6000DMS") according to the following procedure to determine the (specific) melt viscosity.

[0040] First, a sample is prepared in the following manner. 80 g of powder is filled into a cylindrical mold with an inner diameter of 50 mm, and the final pressure is approximately 352 kg / cm 2Gradually apply pressure for about 30 seconds until the final pressure is reached. After holding this final pressure for 2 minutes, the molded body is removed from the mold. The resulting cylindrical molded body (diameter 50 mm) is sintered for 90 minutes in an air electric furnace heated to 371°C, and then cooled to 250°C at a rate of 1°C / min. After holding at 250°C for 30 minutes, the sintered body is removed from the furnace. Next, the resulting cylindrical sintered body is cut along the side to obtain a strip-shaped sheet with a thickness of 0.50 mm. A small piece with a width of 5 mm and a length of 15 mm is cut from the resulting strip-shaped sheet, and the width and thickness are accurately measured to calculate the cross-sectional area. Next, a sample mounting bracket is attached to both ends of this small piece (sample) so that the mounting distance is 1.0 cm. Furthermore, this metal-sample assembly is placed in a cylindrical furnace, and the temperature is raised from room temperature to 380°C at a rate of 20°C / min. After maintaining the temperature for about 5 minutes, a load of about 15 g is applied. From the curve of the change in elongation over time, the elongation between 60 and 120 minutes after loading is read, and the ratio to the time (60 minutes) is calculated. The (specific) melt viscosity (η) can be calculated from the following relational formula (2).

[0041]

number

[0042] In the above formula, W is the tensile load (g), Lr is the length of the sample at 380°C (cm), and g is the gravitational constant (980 cm / s 2 ), dLr / dT is the rate of elongation over time from 60 to 120 minutes (cm / sec), and Ar is the cross-sectional area of ​​the sample at 380°C (cm 2 Here, from a separately obtained thermal expansion measurement, Lr / Ar can be calculated using the following formula (3). Lr / Ar = 0.80 × L (length at room temperature) / A (cross-sectional area at room temperature) (3)

[0043] At 380°C as mentioned above, 7×10 5 Fluororesin with a melt viscosity of Pa·s or less is a low molecular weight fluororesin. When the fluororesin having such a low molecular weight is PTFE, the molecular weight of the PTFE is preferably 600,000 or less in number average molecular weight. When the fluororesin is PTFE, the preferred lower limit of the number average molecular weight is 10,000. The number average molecular weight of PTFE is a value measured according to the method of S. Wu (Polymer Engineering & Science, 1988, Vol. 28, 538; ibid., 1989, Vol. 29, 273). When such low molecular weight PTFE is used as a fluororesin, it is suitable for forming an insulating coating while avoiding fibrillation of the PTFE.

[0044] The above fluororesin paint has a viscosity of 2.4 Pa s or more at 25° C. If the viscosity of the above fluororesin paint at 25° C. is less than 2.4 Pa s, when an insulating coating is formed around a conductor, the thickness of the insulating coating covering the corners of the conductor will be too thin, making it impossible to form an insulating coating with a uniform thickness. The viscosity of the above fluororesin paint at 25°C is measured using a BH type viscometer.

[0045] The fluororesin paint preferably has a viscosity at 25° C. of 5.5 Pa·s or more, and more preferably 16.0 Pa·s or more, because this is more suitable for improving the uniformity of the insulating coating.

[0046] The fluororesin paint contains a thickener to satisfy the above viscosity. Examples of the thickener include nonionic surfactants, acrylic emulsions, urethane-modified polyethers, cellulose-based thickeners, etc. These thickeners may be used alone or in combination of two or more kinds.

[0047] The upper limit of the viscosity of the fluororesin coating material at 25° C. is not particularly limited, but is usually about 50.0 Pa·s.

[0048] In addition to the fluororesin and thickener, the fluororesin paint may further contain various additives, such as surfactants, pigments, dyes, inorganic fillers, organic fillers, lubricants, adhesion improvers, etc. On the other hand, when the proportion of fluororesin contained in the insulating coating is high, the insulating coating exhibits good electrical properties and mechanical properties, so that these additives do not need to be added.

[0049] In the present process (1), the conductor is covered with the fluororesin paint, and then the conductor is passed through a coating die to produce a conductor whose periphery is coated with the fluororesin paint. That is, by carrying out the present process (1), an uncrosslinked fluororesin coating film is formed around the conductor 11. Here, for example, a fluororesin paint is applied to the surface of the conductor (core wire), and then the conductor covered with the fluororesin paint is passed through a coating die to remove excess fluororesin paint and smooth the coated surface, after which a heat treatment (baking treatment) is performed as necessary. Such a series of treatments consisting of applying the fluororesin paint, passing the conductor through the coating die, and baking the fluororesin paint may be carried out only once, or may be repeated two or more times.

[0050] FIG. 3 is a diagram for explaining the configuration of the coating die, and is a schematic diagram of the coating die 20 as viewed from the outlet side. In this step (1), a coating die 20 as shown in Fig. 3 is used. The coating die 20 has a through hole in its center, and an opening 23 on the exit side of the through hole has a shape similar to the cross-sectional shape of the conductor (core wire) 11. By passing the conductor 11 through the through hole of such a coating die 20, it is possible to remove the fluororesin coating material applied in excess to the surface of the conductor 11 and to smooth the applied surface. The through-holes are configured so that the cross-sectional area of ​​the cross-section perpendicular to the direction in which the conductor passes decreases continuously or intermittently from the inlet side to the outlet side (opening 23 side) of the coating die 20. The shape of opening 23 may be similar to the cross-sectional shape of conductor 11 as described above, or may be a shape that has the same ratio of length to width as the rectangular cross-section of conductor 11 but has a corner R dimension (rounding radius) that is different from the corner R dimension (rounding radius) of conductor 11.

[0051] The above-mentioned series of processes may be carried out, for example, using the device shown in FIG. FIG. 4 is a schematic diagram of an apparatus used for applying fluororesin paint. An apparatus 50 used for coating with a fluororesin paint includes a paint tank 51, a coating die 20, a baking furnace 54, and a cooling section 55. The apparatus 50 is configured such that the conductor 11 moves (runs) from below to above by pulleys 56 and 57, passing through the paint tank 51, the coating die 20, the baking furnace 54, and the cooling section 55. Therefore, in the above series of processes, the conductor 11 unwound from a unwinding bobbin (not shown) is guided to the lower side of the device 50 via a pulley 56, and then guided to the upper side of the device 50 via a pulley 57 so as to pass through a paint tank 51 containing the fluororesin paint 30, the opening of the coating die 20, a baking furnace 54, and a cooling section 55. The conductor 11 is then repeatedly passed through the device 50 until the film formed on the surface of the conductor 11 reaches a predetermined thickness, thereby producing a conductor 11' whose periphery is coated with a fluororesin. Thereafter, the conductor 11' whose periphery is coated with a fluororesin paint is sent to the next crosslinking process.

[0052] In the device 50, the above-mentioned series of processes can be repeated as many times as necessary. When the above-mentioned series of processes are repeated, the size and shape of the opening of the coating die used in the second or subsequent times may be the same as or different from that of the coating die used in the previous time. If they are different, it is sufficient that at least one of the dimensions of the length, width, and corner radius is larger.

[0053] The apparatus 50 is a vertical apparatus in which a paint tank 51, a coating die 20, a baking furnace 54, and a cooling section 55 are arranged in this order from below, but the above series of processes may also be performed using a horizontal apparatus in which the paint tank, the coating die, the baking furnace, and the cooling furnace are arranged in parallel.

[0054] The temperature and time for the baking treatment of the fluororesin coating in step (1) may be appropriately selected depending on the type of fluororesin.

[0055] In the above step (2), the conductor 11', the periphery of which is painted with a fluororesin paint, is irradiated with ionizing radiation in an oxygen-free environment and at a temperature equal to or higher than the crystalline melting point of the fluororesin. The above-mentioned irradiation with ionizing radiation is carried out in a substantially oxygen-free environment in order to prevent oxidation of the conductor 11 during irradiation. In the present invention, the absence of oxygen specifically refers to a vacuum (5.0×10 -4 Torr or less) or in an inert gas atmosphere such as nitrogen (oxygen concentration 100 ppm or less).

[0056] The ionizing radiation is applied after the fluororesin is heated to a temperature above its crystalline melting point, which activates the molecular motion of the molecular chains and allows an efficient crosslinking reaction to occur. The crystalline melting point of the fluororesin varies depending on the type of resin, molecular weight, etc., but when the fluororesin is PTFE, the crystalline melting point is, for example, 320°C or higher and lower than 350°C. Therefore, when the fluororesin is PTFE, the heating temperature in step (2) can be, for example, equal to or higher than the crystalline melting point and in the range of 320°C to 360°C.

[0057] The crystalline melting point of the above fluororesin is measured by the following method using differential scanning calorimetry. A differential scanning calorimeter (SII NanoTechnology's "X-DSC7000") that has been temperature calibrated in advance using indium and lead as standard samples is used. 3 mg of the fluororesin is placed in an aluminum pan (crimp container) and heated in a temperature range of 230 to 380°C at a heating rate of 10°C / min under a nitrogen flow of 40 ml / min. The minimum point of the melting peak in the above range is measured and this minimum point is regarded as the crystalline melting point of the fluororesin.

[0058] Examples of the ionizing radiation to be irradiated include electron beams, gamma rays, X-rays, neutron beams, and high-energy ions. As a method for irradiating with these ionizing radiations, a conventionally known method can be used.

[0059] The lower limit of the dose of the ionizing radiation to be irradiated is preferably 10 kGy, more preferably 30 kGy, and even more preferably 50 kGy, while the upper limit of the dose of the ionizing radiation to be irradiated is preferably 2000 kGy, more preferably 1000 kGy, more preferably 400 kGy, even more preferably 200 kGy, and particularly preferably 125 kGy. If the dose of ionizing radiation is less than the lower limit, the crosslinking reaction may not proceed sufficiently, making it impossible to form an insulating coating having excellent mechanical properties such as abrasion resistance, etc. On the other hand, if the dose of ionizing radiation exceeds the upper limit, productivity may decrease.

[0060] By going through these steps (1) and (2), the rectangular wire according to the embodiment of the present invention can be manufactured. EXAMPLES

[0061] The best mode for carrying out the present invention will be explained by way of examples (test examples). The examples are not intended to limit the scope of the present invention.

[0062] [Preparation of fluororesin paints A to E] The nonionic surfactant Octopol 45 (Sanyo Chemical Industries Co., Ltd.) was added as a thickener to the dispersion liquid EK-3700C21R (Daikin Industries Co., Ltd.) containing PTFE as the fluororesin component, and the mixture was mixed uniformly using a blade mixer to prepare fluororesin paints A to E with different viscosities.

[0063] [Preparation of fluororesin paints F to H] By adding the thickening agent Metolose (Shin-Etsu Chemical Co., Ltd.) to Lubron L5 (Daikin Industries, Ltd.), a PTFE powder, and dispersing it in water, fluororesin paints F to H with different viscosities were prepared.

[0064] The amounts of each component and the viscosity of the fluororesin paint are as shown in Table 1 below. The crystalline melting point of the PTFE contained in the above dispersion EK-3700C21R is 327°C. The crystalline melting point of the PTFE that constitutes Lublon L5 is 330°C.

[0065] The viscosity (Pa·s) of the fluororesin paints A to H was measured at 25°C using a BH type viscometer (Toki Sangyo Co., Ltd. VISCOMETER, rotor No. 2 and 6, rotation speed 20 rpm). The melt viscosity of the fluororesin at 380°C was measured by the above-mentioned measurement method. Specifically, the PTFE contained in EK-3700C21R was measured by the above-mentioned method Z, and the PTFE constituting Lublon L5 was measured by the above-mentioned method X.

[0066] [Table 1]

[0067] (Test Examples 1 to 8) Using the device shown in Fig. 4, a conductor with a length x width of 1 mm x width of 4 mm and a roughly rectangular cross section with rounded corners was immersed in any one of fluororesin paints A to H, and then passed through a coating die with a through hole at a speed of 3.5 m / min. The coating die used here had a roughly rectangular cross section opening on the exit side of the through hole whose length and width were longer than those of the cross section of the conductor, and whose corner radius (rounding radius) was the same as that of the conductor. Thereafter, the plate was passed through a baking oven and baked at 360° C. for 60 minutes. Here, a series of treatments consisting of immersion in a fluororesin coating material, passing through a coating die, and baking treatment was carried out once (Test Examples 1 to 5) or repeatedly four times (Test Examples 6 to 8).

[0068] Next, the fired fluororesin coating was irradiated with an electron beam as an ionizing radiation at 200 kGy at 350°C in a nitrogen atmosphere (oxygen concentration 5 ppm or less) to form an insulating coating made of cross-linked fluororesin (cross-linked PTFE) around the conductor, thereby producing a rectangular wire. Here, the electron beam was accelerated to 1.03 MeV by an electron beam accelerator "Sagatron" manufactured by NHV Corporation and then irradiated.

[0069] For the rectangular wires of Test Examples 1 to 8, the wall thickness deviation (%) was calculated by the method already explained. That is, optical microscope images of the cross section of the rectangular wire were taken at three locations at midpoints in the longitudinal direction of the rectangular wire, spaced 50 cm apart. Based on the optical microscope images of each cross section, the thickness T1 of the insulating coating covering the flat portion of the conductor and the thickness T2 of the insulating coating covering the corners of the conductor were obtained using the method described above. The thickness deviation rate (%) was calculated from the obtained thicknesses T1 and T2 using the above formula (1). The results are shown in Table 2.

[0070] In addition, the correlation between the difference (T1-T2) between the thickness T1 of the insulating coating covering the flat parts of the conductor and the thickness T2 of the insulating coating covering the corners of the conductor, obtained when calculating the thickness deviation rate (%), and the viscosity of the fluororesin paint is shown in a graph in Figure 5.

[0071] Furthermore, the fibrillation properties of the fluororesins contained in the fluororesin coating materials A to H were evaluated by the following method. The results are shown in Table 2. [Evaluation of fibrillation properties] The evaluation was performed in accordance with ASTM D 4895. Specifically, the evaluation is as follows. 50 g of fluororesin powder and 10.25 g of hydrocarbon oil (Exxon's "Isopar G" (registered trademark)) as an extrusion aid were mixed in a glass bottle for 3 minutes and aged at room temperature (25±2°C) for 1 hour. Next, the mixture was filled into an extrusion die (with a constriction angle of 30° and an orifice (orifice diameter: 2.54 mm, orifice length: 2 mm) at the lower end) with a cylinder (inner diameter 25.4 mm), and a load of 1.2 MPa was applied to the piston inserted into the cylinder and held for 1 minute. Immediately after that, the mixture was extruded from the orifice at room temperature at a ram speed of 20 mm / min. If continuous strands were obtained and the obtained unsintered molded product had strength, it was evaluated as having fibrillation properties. If continuous strands were not obtained, or if continuous strands were obtained but had no substantial strength or elongation (for example, if the elongation was 0% and the product broke when pulled), it was evaluated as not having fibrillation properties. The results are shown in Table 2. In Table 2, the presence of fibrillating properties was indicated by "x", and the absence of fibrillating properties was indicated by "o". In addition, in this evaluation, the fibrillation properties of the fluororesins contained in the fluororesin coating materials A to E were evaluated using, as a measurement sample, powder obtained by volatilizing water from the dispersion liquid EK-3700C21R.

[0072] [Table 2]

[0073] As shown in Table 2 and FIG. 5, according to the method for manufacturing a rectangular wire according to an embodiment of the present invention, a fluororesin paint having a viscosity of 2.4 Pa·s or more at 25°C is used to form an insulating coating on the rectangular wire, thereby making it possible to manufacture a rectangular wire with good uniformity in film thickness. [Explanation of symbols]

[0074] 10. Rectangular wire 11 Conductor (core wire) 11a Flat area 11b Corner 12 Insulation film 20 Coating Dies 23 Opening 30 Fluorine resin paint 50 equipment 51 Paint tank 54 Baking oven 55 Cooling section 56, 57 Pulleys

Claims

1. A method for manufacturing a rectangular wire having a conductor with a substantially rectangular cross section and an insulating coating provided on a surface of the conductor, (1) a step of covering the periphery with a fluororesin paint containing a fluororesin powder and a solvent, the viscosity of which at 25°C is 2.4 Pa·s or more, the solvent containing 50% by volume or more of water, and then passing the resulting conductor through a coating die to produce a conductor whose periphery is coated with the fluororesin paint; (2) irradiating the conductor, the periphery of which is coated with a fluororesin paint, with ionizing radiation in an oxygen-free environment and at or above the crystalline melting point of the fluororesin; A method for manufacturing a rectangular wire having the above structure.

2. 2. The method for producing a rectangular wire according to claim 1, wherein the amount of irradiation with the ionizing radiation is 10 kGy or more and 2000 kGy or less.

3. 3. The method for producing a rectangular wire according to claim 2, wherein the amount of irradiation with the ionizing radiation is 30 kGy or more and 200 kGy or less.

4. The method for manufacturing a rectangular wire according to any one of claims 1 to 3, wherein the fluororesin is at least one selected from the group consisting of PTFE, PFA and FEP.

5. The method for producing a rectangular wire according to claim 4, wherein the fluororesin is PTFE.

6. The fluororesin has a melt viscosity of 7×10 at 380° C. ５ The method for producing a rectangular wire according to any one of claims 1 to 5, wherein the hardness is Pa·s or less.

7. A fluororesin paint used to form an insulating film for rectangular wires, The composition contains a fluororesin powder and a solvent, and has a viscosity of 2.4 Pa s or more at 25°C; The solvent is a fluororesin paint containing 50 volume % or more of water.

8. The fluororesin coating material according to claim 7, wherein the fluororesin is at least one selected from the group consisting of PTFE, PFA and FEP.

9. The fluororesin coating material according to claim 8, wherein the fluororesin is PTFE.

10. The fluororesin has a melt viscosity of 7×10 at 380° C. ５ The fluororesin paint according to any one of claims 7 to 9, having a viscosity of not more than Pa·s.

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