Insulated metal element manufacturing method and insulated metal element

Abstract

A novel method for manufacturing an insulated metal element with stable electrical properties includes the steps of: a) providing a metal element; b) providing a thermoplastic polymer; c) cleaning the surface of the metal element; d) heating the metal element at a temperature of Tm+20°C to Tm+60°C, where Tm is the melting temperature of the thermoplastic polymer; e) spreading the thermoplastic polymer on the surface of the metal element; f) cooling the coated metal element to a temperature higher than [(Tm+Tg) / 2-40°C] and lower than [(Tm+Tg) / 2+40°C], where Tm is the melting temperature of the thermoplastic polymer and Tg is the glass transition temperature of the thermoplastic polymer; g) stopping the cooling for less than 2 s to 10 s; and h) quenching the coated metal element to a temperature lower than 50°C.

Description

[Technical Field]

[0001] Technical Field The present invention relates to a method for producing an insulated metal element having stable electrical properties, an insulated metal element having stable electrical properties obtained by said method, and the use of said insulated metal element as a magnet wire. [Background technology]

[0002] Background technology Insulated metal elements are used in several applications where metal conductors need to be insulated. For example, for medium voltage lines, cross-linked polyethylene (XLPE) insulated conductors can be designed, and for low voltage lines, XLPE insulated conductors or polyvinyl chloride (PVC) insulated conductors can be designed.

[0003] Insulated metal elements are also used in the stators of electric motors, both synchronous (permanent magnet) and asynchronous (induction) motors. New challenges for automotive electric motors include: -Higher motor speed, -Compact design and maximum power / unit, -High reliability (short circuit, fire hazard), -Mass production, -High assembly speed, -Assembly automation, -Cost reduction, included.

[0004] With the increase in battery voltages beyond 500V to improve the autonomy and reduce charging times of electric vehicles, insulated metal elements used as magnet wire in electric motors must have high voltage resistance to partial discharge and therefore must exhibit a high partial discharge inception voltage (PDIV).

[0005] Additionally, insulated metal elements used as magnet wire in electric motors must exhibit stable electrical properties over time, which depend primarily on the type of insulating layer, its resistance to temperature fluctuations, and its adhesion to the metal element.

[0006] Partial discharge inception voltage If an insulating layer acting as a barrier contains defects, such as internal voids, and is exposed to high voltage, these defects will exhibit localized ionization. This ionization begins at a certain voltage and stops at a lower voltage. These are called the inception and extinction voltages. When a high voltage is applied to the barrier, the voltage across the void also increases. When the inception voltage is reached, the void ionizes and shorts itself out. When the voltage across the void falls below the extinction voltage, ionization stops. This process redistributes charge within the barrier, known as a partial discharge. If the barrier voltage continues to increase, another partial discharge cycle begins. If the barrier voltage is alternating current (AC) and sufficiently large, partial discharge cycles can repeat many times between positive and negative peaks. If ionization begins and continues, it can damage the barrier and lead to failure. If no discharge occurs, the barrier is not damaged. The inception voltage of an individual void tends to be constant. Therefore, the total charge redistributed within the barrier is a very good indicator of the number of voids and their likelihood of failure. By setting a very low limit on the allowable current caused by partial discharges in the test, a very high degree of confidence can be obtained that no high voltage fault will occur.

[0007] Partial discharge inception voltage V, thickness t of the insulating layer, and relative permittivity ε of the insulator r Research has shown that there is a relationship between both (Dakin's formula): V=163(t / ε r ) 0.46

[0008] Current methods for producing insulated metal elements that are resistant to PDIV involve applying an enamel layer over copper wire. Enamel coatings are used in combination with paper liners in stators. Paper-covered windings improve insulation, but they also result in significant space loss because the paper layer can be up to 500 μm thick.

[0009] US Patent No. 4,471,022A discloses a water-soluble polyimide, a coated wire, and a coating method. The insulating layer is made of polyimide (PI), and at least six layers are deposited with a long curing time for each layer.

[0010] US Patent No. 9,324,476 B2 discloses an alternative insulated winding comprising at least two layers, the first being an enameled polyamideimide (PAI) layer and the second being a polyetheretherketone (PEEK) or polyaryletherketone (PAEK).

[0011] US Pat. No. 9,224,523 B2 also discloses an inverter surge resistant insulated wire made of an enamel layer and an extruded thermoplastic material.

[0012] Increasing the thickness of the enamel (polyimide or polyamideimide) layer increases the risk of defects and increases manufacturing costs.

[0013] The cost of adding an insulating layer can be reduced by using powder deposition or extrusion of thermoplastic polymers, however, due to poor adhesion between the thermoplastic polymer and the metal elements, a primer or bonding layer is usually required according to the prior art.

[0014] Solutions to improve the direct adhesion of extruded thermoplastics to metal elements without the need for a primer or bonding layer are disclosed in US Patent Application Publication No. 2019 / 0131037 A1 and WO 21041200 A1.

[0015] US Patent Application Publication No. 2019 / 0131037A1 describes an insulated conductor obtained by placing the conductor in a protective gas atmosphere and exposing the conductor to ions of the protective gas in a gas plasma in order to remove an oxide layer formed on the surface of the conductor and / or to increase the surface energy of the conductor. The insulating coating layer either includes at least one insulating layer made of a thermoplastic material or includes an insulating layer and a plastic-containing intermediate layer.

[0016] WO 21041200A1 discloses an insulated conductor comprising a conductor comprising an oxide layer on at least a portion of the surface of the conductor and an insulating coating on at least a portion of the oxide layer, whereby good adhesion between the conductor and the insulating coating is obtained by heat treating the coated conductor.

[0017] JP 02250206A discloses an insulated wire having a PEEK insulation layer with a crystallinity of less than 10% to provide flexibility for winding, which is further subjected to a heat treatment to set the crystallinity of the PEEK to 15-40% to improve hardness and chemical resistance.

[0018] U.S. Patent No. 9,691,521 B2 discloses a conductor having a thermosetting resin layer and multiple thermoplastic layers, where the second thermoplastic layer has a higher relative crystallinity than the first thermoplastic layer, and the first thermoplastic layer has a relative crystallinity in the range of 20% to 50%.

[0019] US Patent Application Publication No. 2018 / 005724A1 discloses a conductor covered with a PEEK tape layer having a crystallinity of at least 25%.

[0020] U.S. Patent No. 5,358,786A discloses an insulated wire including a conductor, a 0.1 to 1 mm inner insulation layer containing a halogen-free polymer, a 0.001 to 0.5 mm middle insulation layer having a melting point below 155°C, and a 0.05 to 1 mm outer insulation layer having a melting point above 155°C. Summary of the Invention [Problem to be solved by the invention]

[0021] Disclosure of the Invention The present disclosure provides a novel method for producing insulated metal elements that have stable electrical properties during use. The novel method does not require a protective atmosphere, as in U.S. Patent Application Publication No. 2019 / 0131037, or a post-cooling heat treatment step, as in U.S. Patent Application Publication No. 2020 / 047379. The higher production rates achieved with the novel method allow for significant cost savings compared to other existing methods. The present invention addresses the problems of the prior art by providing an alternative insulating element and a method for producing said insulating element. [Means for solving the problem]

[0022] A first object of the present invention is to provide a method for manufacturing an insulated metal element having stable electrical characteristics, the method comprising the steps of: a) providing a metal element; b) providing a thermoplastic polymer; c) cleaning the surface of the metal element; d) heating the metal element at a temperature between Tm+20°C and Tm+60°C, where Tm is the melting temperature of the thermoplastic polymer; e) applying the thermoplastic polymer onto the surface of the metal element; f) cooling the coated metal element to a temperature greater than [(Tm+Tg) / 2-40°C] and less than [(Tm+Tg) / 2+40°C], where Tm is the melting temperature of the thermoplastic polymer and Tg is the glass transition temperature of the thermoplastic polymer; g) stopping the cooling for less than 2 seconds to 10 seconds; h) quenching the coated metal element to a temperature below 50°C; Each step with a preferred embodiment will be described below.

[0023] Provision of metal elements The metal element is preferably elongated and may have a circular, oval or shaped cross section.

[0024] In one embodiment, the metal element has a square or rectangular cross section.

[0025] The metal element may consist of a pure metal or may be a metal alloy.

[0026] For example, the metal element may be made of copper or a copper alloy.

[0027] Alternatively, the metal element may be made of aluminum or an aluminum alloy.

[0028] Alternatively, the metal element may be made of iron or steel.

[0029] The metal elements may also comprise different metals. For example, a steel substrate may be coated with copper or a copper alloy. Another example is a steel substrate coated with aluminum or an aluminum alloy. Yet another example is a steel substrate coated with zinc or a zinc alloy.

[0030] The metal element may be a wire, a rod, or a tube.

[0031] Thermoplastic polymer offering The polymeric coating is preferably a thermoplastic material, that is, a material that becomes plastic when heated and hardens when cooled, and these processes can be repeated.

[0032] In one embodiment, the polymer is selected from the poly(aryl ether ketone) (PAEK) family (e.g., poly(ether ether ketone) (PEEK), or poly(ether ketone) (PEK), or poly(ether ketone ketone) (PEKK)).

[0033] Preferably, the polymer comprises PEEK.

[0034] The polymer coating layer may be applied by any technique known in the art, such as by extrusion or powder coating. Preferably, the coating layer is an extrusion coating layer, which can be identified by observing the polymer chain orientation of the coating layer.

[0035] The polymer coating layer preferably has a thickness in the range of 20 μm to 500 μm, for example, 30 μm to 400 μm, or 40 μm to 300 μm.

[0036] Cleaning the surface of the metal element The metal element preferably has a degreased surface.

[0037] The surface treatment may be performed by electrolytic cleaning, assisted chemical treatment (eg, ultrasonic cleaning), plasma, laser ablation, or any combination thereof.

[0038] Heating the metal element at a temperature of Tm+20°C to Tm+60°C (Tm is the melting temperature of the thermoplastic polymer) Heating can be by induction, resistive heating, gas oven, plasma, or any combination thereof.

[0039] As an example, if the melting temperature Tm of the thermoplastic polymer is 340°C, the metal element is heated to a temperature of 360°C to 400°C.

[0040] Applying the thermoplastic polymer onto the surface of the metal element. The polymer coating is applied onto the hot metal element by extrusion or powder coating.

[0041] Cooling the coated metal element to a temperature above [(Tm + Tg) / 2 - 40°C] and below [(Tm + Tg) / 2 + 40°C], where Tm is the melting temperature of the thermoplastic polymer and Tg is the glass transition temperature of the thermoplastic polymer. Controlled cooling can be achieved by spraying a gas, e.g., N2 or compressed air, onto the surface of the coated metal element. Preferably, controlled cooling is achieved by immersion in water or by spraying water onto the surface of the coated metal element. Other controlled cooling techniques that mix gas and water can also be used.

[0042] The duration and intensity of the cooling should be adjusted so that the surface of the coated metal element reaches a temperature higher than [(Tm + Tg) / 2 - 40°C] and lower than [(Tm + Tg) / 2 + 40°C]. After the first cooling step, no reheating above (Tm + Tg) / 2 + 40°C should occur.

[0043] Cooling stops for less than 2s to 10s At the end of the first cooling step, a temperature holding zone may be used to maintain the surface of the coated metal element at [(Tm + Tg) / 2 - 40°C] to [(Tm + Tg) / 2 + 40°C] for 2 to less than 10 seconds, and may include insulating and heating elements or hot air.

[0044] The temperature holding time of 2 to 10 seconds determines the crystallization rate and ensures good adhesion and stable electrical properties of the insulated metal element. In particular, a holding time of less than 2 seconds will result in low crystallinity and poor adhesion of the coating.

[0045] Rapid cooling of coated metal elements to temperatures below 50°C As with the initial cooling step, the final cooling step or quenching can be achieved by spraying a gas, such as N2 or compressed air, onto the surface of the coated metal element. Preferably, the quenching is achieved by immersion in water or by spraying water onto the surface of the coated metal element. Other controlled cooling techniques that mix gas and water can also be used.

[0046] In a preferred embodiment, steps c) to h) are carried out on a production line in which the metal element travels at a linear speed of more than 40 m / min (e.g., 50 m / min, e.g., 100 m / min), and the temperature range and temperature holding time at the end of the first cooling step are independent of the linear speed of the metal element.

[0047] A second object of the present invention is to provide an insulated metal element having stable electrical properties, the insulated metal element comprising a metal element and a polymer coating, the polymer coating being in a semi-crystalline state and having a crystallinity of 20% to 40%.

[0048] The metal element is coated with a thermoplastic polymer coating, there is no intermediate layer between the polymer and the metal element, and the polymer coating is in a semi-crystalline state.

[0049] Prior art conductors always include an adhesive layer between the metal element and the polymer coating, since adhesion between the polymer and the metal is usually not present.

[0050] The above-described method for manufacturing the insulating element of the present invention allows for the removal of intermediate or bonding layers, so that the insulating element of the present invention comprises only the core metal element, the conductor, and the polymer coating.

[0051] Adhesion between the polymer coating and the metal element is achieved by controlling the process parameters, particularly the polymer deposition rate, e.g., by extrusion, and by controlling the cooling. Controlling the cooling allows for the polymer coating to have the desired crystallinity range, which is a feature of the present invention. The crystallinity range is 10% to 40%, more preferably 15% to 35%, and even more preferably 20% to 35%.

[0052] Metal elements having a polymer coating of the present invention have a partial discharge inception voltage at 20° C. of greater than 800 Vrms, preferably greater than 900 Vrms, and more preferably greater than 1000 Vrms.

[0053] The polymer coating is applied to the hot metal element by extrusion or powder coating.

[0054] The insulated metal elements of the present invention obtained by the described method are resistant to partial discharges at high voltages, have stable electrical properties in use, and are simpler and cheaper to manufacture than prior art insulated metal elements.

[0055] A preferred use of the insulated metal element with stable electrical properties according to the invention is as a hairpin wire in the rotating or stationary parts of an electric motor, in that particular case said insulated metal element comprises Cu or a Cu alloy as the metal element and a thermoplastic polymer of the PEEK or PAEK type. [Brief explanation of the drawings]

[0056] Brief description of the figures included in the drawing [Figure 1] 1 is a schematic cooling curve. [Figure 2] 1 is a plot of PEEK crystallinity as a function of temperature hold time. DETAILED DESCRIPTION OF THE INVENTION

[0057] Modes for Carrying Out the Invention Several different insulated metal elements were fabricated according to the disclosed method. a) A copper rectangle having cross-sectional dimensions of 3.7 mm x 2 mm and a corner radius of more than 0.3 mm was provided as a metal element on a carrier. b) PEEK was provided as the thermoplastic polymer. Commercially available PEEK was obtained, for example, from Solvay or Victrex. By DSC, both the melting temperature Tm and the glass transition temperature Tg were measured and found to be 340°C and 150°C, respectively. c) A rectangular copper wire was unwound from a carrier at a line speed of 40 m / min, cleaned, and heated by plasma on the production line. d) The surface temperature of the rectangular copper wire at the outlet of the heating device was measured by an infrared camera and set to 380°C, which is Tm+40°C. e) PEEK was applied by extrusion onto the surface of the copper wire under ambient conditions, i.e. without a protective atmosphere. f) The PEEK coated copper wire is then cooled in water to 250°C, which is a temperature higher than [(Tm + Tg) / 2 - 40°C] and lower than [(Tm + Tg) / 2 + 40°C], where Tm = 340°C (the melting temperature of PEEK) and Tg = 150°C (the glass transition temperature of PEEK). g) The cooling length in water was adjusted so that the coated metal element reached the desired temperature of 250°C. To stop the cooling and prevent further cooling, hot air at 250°C was blown in for a selected time of 2 to 10 seconds to maintain the temperature constant. At a line speed of 40 m / min, the air blower length varied from 1.35 m to 6.7 m. At higher line speeds, e.g., 120 m / min, the air blower length varied from 3.3 m to 16.7 m. However, if the holding time was short, hot air blower was not required. h) At the exit of the cooling stop length, the coated metal element was quenched in cold water to a temperature below 50°C.

[0058] A winding device finally wound the cooled insulated metal element onto a carrier.

[0059] FIG. 1 is a schematic cooling curve illustrating steps f), g), and h) of the method. In FIG. 1, the letter A indicates the temperature at which the thermoplastic polymer is applied to the heated metal element. In this illustrative example, A=380°C. The letter B indicates the end of step f), i.e., the target temperature after the first cooling step. In this illustrative example, B=250°C. The letter C indicates the end of step g), i.e., the start of the quenching step after a temperature hold time of 2-10 seconds. The letter D indicates the end of the quenching step when the insulated metal element reaches a temperature below 50°C.

[0060] Several samples were prepared with various cooling pause times. Additional samples with a cooling pause of only 1 s and a cooling pause of 40 s were also prepared following all other identical steps.

[0061] The percent crystallinity of a thermoplastic polymer was determined from the heat of fusion and heat of cold crystallization measured by DSC and the reference heat of fusion of a 100% crystalline thermoplastic polymer according to ASTM D3418-15. Approximately 10 mg of thermoplastic polymer was removed from the insulating metal element, for example by scraping or grating. Heat flow curves were generated using heating and cooling rates of 10°C / min. The heat of fusion, ΔHm, and the heat of cold crystallization, ΔHc, were determined by integrating the area under the peaks (J / g). The percent crystallinity was determined using the following equation: Crystallinity (%)=100*[ΔHm-ΔHc] / ΔHm° where ΔHm° is the heat of fusion of a fully crystalline polymer, which for PEEK is 130 J / g.

[0062] Figure 2 is a plot of crystallinity as a function of cooling stop time. A temperature hold time between the initial cooling step and the quenching step that is too short can result in low crystallinity or a completely amorphous polymer coating, leading to poor adhesion between the polymer and the metal element and unstable electrical properties. A very long cooling stop time produces the highest crystallinity values. However, a cooling stop time that is too long can result in coating thickness inconsistencies and unstable electrical properties.

[0063] The optimum was found by controlling the cooling stop time between 2s and 10s.

[0064] Three insulated metal elements were fabricated according to the disclosed method and compared with two prior art insulated metal elements.

[0065] Two reference samples from the prior art were selected, namely REF.1 and REF.2, in both samples the metal element consisted of copper with a purity of 99.9% and containing less than 400 ppm O2.

[0066] In both samples, the metal elements had a rectangular shape with a width of 3.7 mm and a height of 2 mm, and corner radii of greater than 0.3 mm.

[0067] REF.1 was coated with a 103 μm thick enamel layer made of PAI obtained by several deposition and curing cycles.

[0068] REF.2 was coated with a first enamel layer made of PAI with a thickness of 38 μm and a second polymer layer made of PEEK with a thickness of 112 μm. The total thickness of the insulating layers was 150 μm.

[0069] Samples INV.1 to INV.3 were obtained using the same initial metal element, namely a rectangular copper piece with cross-sectional dimensions of 3.7 mm x 2 mm and corner radii greater than 0.3 mm.

[0070] The three samples produced according to the disclosed method had PEEK coating thicknesses ranging from 40 μm to 300 μm.

[0071] To test the adhesion, a 10 mm strip of the coating was pulled from the metal element substrate through a calibrated opening and the stripping force was measured. All samples measured a stripping force of over 40 N / mm.

[0072] Additionally, adhesion was tested according to the IEC 60317 standard by elongation testing using an incision through the polymer coating.

[0073] The PDIV of different samples was measured according to the IEC 60664-1 and 61800-5-1 standards.

[0074] To measure the PDIV, a bundled sample pair was prepared. A test voltage (50 Hz AC, RMS) was applied to one conductor of the bundled pair, while the other was connected to earth. The test voltage was gradually increased until a partial discharge above a 10 pC level was recorded on the measurement capacitor.

[0075] The table below reports the measured PDIV values ​​for two reference samples and three inventive samples.

[0076] [Table 1]

[0077] The measured PDIV value was calculated using the DAKIN formula V = 163(t / ε r ) 0.46 is in good agreement with

[0078] The coating thickness t given in the table needs to be doubled in the formula because sample pairs are tested. r was estimated to be 3.9 for enamel (PAI) and 3.1 for PEEK, depending on the type of coating.

[0079] All of the inventive samples achieved high PDIV values ​​exceeding 800V.

[0080] The polymer-coated metal elements of the present invention, associated with a more cost-effective manufacturing process, are particularly suitable for use in hairpin wires in the rotating or stationary parts of electric motors.

Claims

1. 1. A method for manufacturing an insulated metal element having stable electrical characteristics, comprising: a) providing a metal element; b) providing a thermoplastic polymer; c) cleaning the surface of the metal element; d) heating the metal element at a temperature between Tm+20°C and Tm+60°C, where Tm is the melting temperature of the thermoplastic polymer; e) applying the thermoplastic polymer onto the surface of the metal element; f) cooling the coated metal element to a temperature above [(Tm+Tg) / 2-40°C] and below [(Tm+Tg) / 2+40°C], where Tm is the melting temperature of the thermoplastic polymer and Tg is the glass transition temperature of the thermoplastic polymer; g) stopping the cooling for between 2 seconds and less than 10 seconds; h) quenching the coated metal element to a temperature below 50°C; A method comprising:

2. 2. The method of claim 1, wherein the first cooling step f) is not followed by reheating of the metal element.

3. 10. The method of claim 1, wherein the cooling step f) and the quenching step h) are carried out in water.

4. 10. The method of claim 1, wherein hot air is used to stop the cooling at a temperature greater than [(Tm + Tg) / 2 - 40°C] and less than [(Tm + Tg) / 2 + 40°C], where Tm is the melting temperature of the thermoplastic polymer and Tg is the glass transition temperature of the thermoplastic polymer.

5. 2. The method of claim 1, wherein steps c) to h) are carried out on a production line in which the metal element travels at a linear speed of greater than 40 m / min.

6. 10. An insulated metal element having stable electrical properties, obtained by the method of claim 1, comprising a metal element and a thermoplastic polymer coating, wherein the thermoplastic polymer coating is in a semi-crystalline state and has a crystallinity of 10% to 40%.

7. 7. The insulated metal element having stable electrical characteristics according to claim 6, wherein the partial discharge inception voltage (PDIV) at 20°C is higher than 800 Vrms.

8. 7. The insulated metal element with stable electrical properties according to claim 6, wherein the metal element has a rectangular or square cross section.

9. 7. The insulated metal element with stable electrical properties according to claim 6, wherein the metal element is a copper wire, rod, or tube.

10. 7. The insulated metal element with stable electrical properties according to claim 6, wherein the metal element is a wire, rod, or tube made of aluminum or an aluminum alloy.

11. 7. The insulated metal element with stable electrical properties according to claim 6, wherein the metal element is a steel wire, rod, or tube.

12. 7. The insulated metal element with stable electrical properties according to claim 6, wherein the thermoplastic polymer coating comprises or consists of a PAEK-based thermoplastic polymer.

13. 7. The insulated metal element with stable electrical properties according to claim 6, wherein the thickness of the thermoplastic polymer coating is in the range of 60 μm to 500 μm.

14. Use of the insulated metal element with stable electrical properties according to claim 6 as a hairpin wire in a rotating or stationary part of an electric motor.

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