Method for producing insulating metal element and insulating metal element

Abstract

A new method for producing an insulating metal element with stable electrical properties, 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 DEG C and Tm + 60 DEG C, Tm being the melting temperature of the thermoplastic polymer; e) applying the thermoplastic polymer to the surface of the metal element; f) cooling the coated metal element to a temperature above [(Tm + Tg) / 2-40 DEG C] and below [(Tm + Tg) / 2 + 40 DEG C], where Tm is the melting temperature of the thermoplastic polymer and Tg is the glass transition temperature of the thermoplastic polymer; g) stopping cooling for 2 s to less than 10 s; h) quenching the metal element with the covering layer to a temperature below 50 DEG C.

Description

Technical Field

[0001] The present invention relates to a method for producing an insulating metal component having stable electrical properties, to an insulating metal component having stable electrical properties obtained by said method, and to the use of said insulating metal component as a magnetic coil. Background Art

[0002] Insulating metal components are used in a variety of applications where it is necessary to insulate metal conductors. For example, it can be designed for cross-linked polyethylene (XLPE) insulated conductors for medium voltage lines and for XLPE insulated conductors or polyvinyl chloride (PVC) insulated conductors for low voltage lines.

[0003] Insulating metal components are also used in the stators of electric motors, for synchronous (permanent magnet) and asynchronous (induction) motors. New challenges for automotive electric motors include:

[0004] Higher motor speeds;

[0005] Compact design and maximized power / unit;

[0006] High reliability (short circuit, fire hazard);

[0007] Mass production;

[0008] High assembly speed;

[0009] Automated assembly;

[0010] Cost reduction.

[0011] As the battery voltage increases to more than 500V to increase the autonomy of electric vehicles and reduce the charging time, the insulating metal component used as a magnetic coil in an electric motor needs to be resistant to partial discharge at high voltages and thus exhibit a high partial discharge inception voltage (PDIV).

[0012] In addition, the insulating metal component used as a magnetic coil in an electric motor needs to exhibit stable electrical properties over time. The stable electrical properties mainly depend on the type of insulating layer, its tolerance to temperature changes, and its adhesion to the metal component.

[0013] Partial discharge inception voltage

[0014] When the insulating layer used as a barrier layer has defects such as internal voids, these defects will exhibit local ionization when exposed to high voltage. This ionization starts at a certain voltage and stops at a lower voltage. These are called the inception voltage and the extinction voltage. When a high voltage is applied to the barrier layer, the voltage will also accumulate across the voids. When the inception voltage is reached, the voids ionize and short-circuit themselves. When the voltage across the voids drops below the extinction voltage, the ionization stops. This action redistributes the charges within the barrier layer and is called partial discharge. If the barrier layer voltage continues to rise, another partial discharge cycle begins. If the barrier layer voltage is alternating current (AC) and large enough, the partial discharge cycle will repeat many times during the positive and negative peaks. If ionization starts and continues, it will damage the barrier layer, leading to a failure. If no discharge occurs, the barrier layer is not damaged. The inception voltage of each void tends to be constant. Therefore, the total charge redistributed within the barrier layer is a very good indicator of the number of voids and their likelihood of failure. Setting a very low limit for the allowable current caused by partial discharge in the test gives a very high confidence that no high voltage failure will occur.

[0015] Research has shown that there is a relationship (Dakin's formula) between the partial discharge inception voltage V, the thickness t of the insulating layer, and the relative permittivity ε of the insulator r as follows:

[0016] V = 163(t / ε r ) 0.46

[0017] Currently, the methods for producing insulating metal components resistant to PDIV include applying an enamel layer on copper wire. The enamel coating is used in combination with a paper liner in the stator. The paper-wound winding increases the insulation, but a lot of space is lost because the paper layer can be up to 500 μm thick.

[0018] US4471022A discloses a water-soluble polyimide, a wire with a coating layer, and a coating method. The insulating layer is composed of polyimide (PI), with at least 6 layers deposited, and each layer has a long curing time.

[0019] US9324476B2 discloses an alternative insulating winding wire including at least two layers. The first layer is an enameled polyamide-imide (PAI) layer, and the second layer is polyether ether ketone (PEEK) or polyaryl-ether-ketone (PAEK).

[0020] US9224523B2 discloses an insulating wire resistant to inverter surges, which also consists of an enamel layer and an extruded thermoplastic.

[0021] Increasing the thickness of the enamel (polyimide or polyamide-imide) layer increases the risk of defects and the production cost.

[0022] The cost increase of the insulating layer can be reduced by using powder deposition or extrusion of thermoplastic polymers. However, due to the poor adhesion between the thermoplastic polymer and the metal element, a primer or bonding layer is generally required according to the prior art.

[0023] Solutions for improving the adhesion of extruded thermoplastics directly on metal elements without the need for a primer or bonding layer are disclosed in US2019 / 0131037A1 and WO21041200A1.

[0024] US2019 / 0131037A1 describes an insulated electrical conductor obtainable by a method in which the electrical conductor is placed in a protective gas atmosphere and bombarded with ions of the protective gas in a gas plasma to remove the oxide layer formed on the surface of the conductor and / or increase the surface energy of the conductor. The insulating cover layer comprises at least one insulating layer made of a thermoplastic material or comprises an insulating layer and a plastic-containing intermediate layer.

[0025] WO21041200A1 discloses an insulated electrical conductor comprising an electrical conductor and an insulating cover layer, the electrical conductor comprising an oxide layer on at least a part of the surface of the electrical conductor, the insulating cover layer being on at least a part of the oxide layer. Good adhesion between the electrical conductor and the insulating cover layer is obtained by heat-treating the electrical conductor with the cover layer.

[0026] JPH02250206A discloses an insulated wire having a PEEK insulating layer with a crystallinity of less than 10% so as to obtain flexibility for winding, and the PEEK insulating layer is further subjected to heat treatment to set the crystallinity of PEEK between 15% and 40% to improve hardness and chemical resistance.

[0027] US9691521B2 discloses a conductor having a thermosetting resin layer and a plurality of thermoplastic layers, wherein the second thermoplastic layer has a relative crystallinity higher than that of the first thermoplastic layer, and the first thermoplastic layer has a relative crystallinity in the range of 20% to 50%.

[0028] US2018 / 005724A1 discloses a conductor wrapped in a PEEK tape layer having a crystallinity of at least 25%.

[0029] US5358786A discloses an insulated wire comprising a conductor, an inner insulating layer containing a halogen-free polymer of 0.1 - 1 mm, an intermediate insulating layer with a melting point < 155°C of 0.001 mm to 0.5 mm, and an outer insulating layer with a melting point > 155°C of 0.05 mm to 1 mm. Summary of the Invention

[0030] The present invention provides a new method for producing an insulating metal component having stable electrical properties during use. This new method does not require a protective atmosphere as in US2019 / 0131037 and does not require a heat treatment step after cooling as in US2020 / 047379. Compared with other existing methods, the higher production speed obtained by the new method results in a significant reduction in cost. The present invention solves the problems of the prior art by providing an alternative insulating component and a method for producing said insulating component.

[0031] A first object of the present invention is to provide a method for producing an insulating metal component having stable electrical properties. The method comprises the following steps:

[0032] a) providing a metal component;

[0033] b) providing a thermoplastic polymer;

[0034] c) cleaning the surface of the metal component;

[0035] d) heating the metal component at a temperature between Tm + 20 °C and Tm + 60 °C, where Tm is the melting temperature of the thermoplastic polymer;

[0036] e) applying the thermoplastic polymer to the surface of the metal component;

[0037] f) cooling the metal component with a coating 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;

[0038] g) stopping the cooling for 2 s to less than 10 s;

[0039] h) quenching the metal component with a coating to a temperature below 50 °C.

[0040] Each step with preferred embodiments is described below.

[0041] Provide a metal component

[0042] Preferably, the metal component is elongated and may have a circular or oval or irregular cross-section.

[0043] In an embodiment, the metal component has a square or rectangular cross-section.

[0044] The metal component consists of a pure metal or it may be a metal alloy.

[0045] For example, the metal component may be made of copper or a copper alloy.

[0046] Alternatively, the metal element can be made of aluminum or an aluminum alloy.

[0047] Alternatively, the metal element can be made of iron or steel.

[0048] The metal element can also include different metals. For example, a steel substrate can be coated with copper or a copper alloy. Another example is a steel substrate coated with aluminum or an aluminum alloy. Another example is a steel substrate coated with zinc or a zinc alloy.

[0049] The metal element can be a wire, a rod, or a tube.

[0050] Provide a thermoplastic polymer

[0051] Preferably, the polymer coating is thermoplastic, i.e., a substance that becomes plastic when heated and hardens when cooled, and is capable of repeating these processes.

[0052] In one embodiment, the polymer is selected from the family of poly(aryl ether ketone) (PAEK), such as poly(ether ether ketone) (PEEK), or poly(ether ketone) (PEK), or poly(ether ketone ketone) (PEKK).

[0053] Preferably, the polymer consists of PEEK.

[0054] The polymer coating can be applied by any technique known in the art, such as by extrusion or powder coating. Preferably, the coating is an extrusion coating, which can be identified by observing the orientation of the polymer chains in the coating.

[0055] Preferably, the polymer coating has a thickness in the range of 20 μm to 500 μm, such as a thickness between 30 μm and 400 μm or between 40 μm and 300 μm.

[0056] Clean the surface of the metal component

[0057] Preferably, the metal element has a degreased surface.

[0058] Surface preparation is carried out by electrolytic cleaning, assisted chemical treatment (such as ultrasonic cleaning), plasma, laser ablation, or any combination thereof.

[0059] Heat the metal component at a temperature between Tm + 20°C and Tm + 60°C, where Tm is the melting temperature of the thermoplastic polymer

[0060] Heating can be carried out by induction, resistance heating, gas oven, plasma, or any combination thereof.

[0061] As an example, when the melting temperature Tm of the thermoplastic polymer is 340 °C, the metal element is heated at a temperature between 360 °C and 400 °C.

[0062] Apply the thermoplastic polymer to the surface of the metal component

[0063] A polymer coating layer is applied to the hot metal element by extrusion or powder coating.

[0064] Cool the metal component with a coating 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

[0065] Controlled cooling can be achieved by spraying a gas, such as N2 or compressed air, onto the surface of the metal element with the coating layer. Preferably, controlled cooling is carried out by immersion in water or by spraying water onto the surface of the metal element with the coating layer. Other controlled cooling techniques using a mixture of gas and water can also be used.

[0066] The duration and intensity of the cooling should be adjusted such that the surface of the metal element with the coating layer reaches a temperature higher than [(Tm + Tg) / 2 - 40°C] and lower than [(Tm + Tg) / 2 + 40°C]. Reheating above (Tm + Tg) / 2 + 40°C should not occur after the first cooling step.

[0067] Stop cooling for 2 s to less than 10 s

[0068] To maintain the surface of the metal element with the coating layer between [(Tm + Tg) / 2 - 40°C] and [(Tm + Tg) / 2 + 40°C] for 2 s to less than 10 s at the end of the first cooling step, a temperature holding zone can be used. The temperature holding zone can include insulating elements and heating elements or hot air.

[0069] The temperature holding time between 2 s and 10 s determines the crystallization rate, ensuring good adhesion and stable electrical properties of the insulated metal element. In particular, a holding time less than 2 s results in low crystallinity and poor adhesion of the coating layer.

[0070] Quench the metal component with a coating to a temperature below 50°C

[0071] Regarding the first 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 metal element with the coating layer. Preferably, quenching is carried out by immersion in water or by spraying water onto the surface of the metal element with the coating layer. Other controlled cooling techniques using a mixture of gas and water can also be used.

[0072] In a preferred embodiment, the above steps c) to h) are carried out on a production line, where the metal element runs on the production line at a linear speed higher than 40 m / min (such as 50 m / min, such as 100 m / min). 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.

[0073] A second object of the present invention is to provide an insulating metal element having stable electrical properties, said insulating metal element comprising a metal element and a polymer coating, said polymer coating being semi-crystalline with a crystallinity of 20% - 40%.

[0074] The metal element is covered 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.

[0075] Conductors of the prior art always include an adhesion layer between the metal element and the polymer coating because there is usually no adhesion between the polymer and the metal.

[0076] The method for producing the insulating element of the present invention as described above allows suppression of the intermediate layer or the bonding layer. Thus, the insulating element of the present invention only comprises a core metal element, a conductor and a polymer coating.

[0077] By controlling the process parameters (especially by the polymer deposition rate such as extrusion) and controlling the cooling, adhesion between the polymer coating and the metal element is obtained. The controlled cooling causes the polymer coating to have a desired range of crystallinity, which is a feature of the present invention. The crystallinity is between 10% and 40%, more preferably between 15% and 35%, and even more preferably between 20% and 35%.

[0078] The metal element with a polymer coating of the present invention has a partial discharge inception voltage of more than 800 Vrms, preferably more than 900 Vrms, and more preferably more than 1000 Vrms at 20 °C.

[0079] The polymer coating is applied to the hot metal element by extrusion or powder coating.

[0080] The insulating metal element of the present invention obtained by the method is resistant to partial discharge at high voltages, has stable electrical properties in use, and is easier and cheaper to produce than insulating metal elements of the prior art.

[0081] A preferred use of the insulating metal element with stable electrical properties according to the present invention is as a hairpin wire for a rotating or stationary component of an electric motor. In this particular case, the insulating metal element comprises Cu or a Cu alloy as the metal element and PEEK or a thermoplastic polymer from the PAEK family. BRIEF DESCRIPTION OF THE DRAWINGS

[0082] Figure 1 is a schematic cooling curve.

[0083] Figure 2 is a chart of the PEEK crystallinity varying with the temperature holding time. DETAILED DESCRIPTION

[0084] Manufacturing different insulated metal components according to the disclosed method:

[0085] a) Provide rectangular copper with a cross-sectional dimension of 3.7 mm × 2 mm and a corner radius > 0.3 mm as a metal component on a carrier.

[0086] b) Provide PEEK as a thermoplastic polymer. Commercial PEEK is obtained, for example, from Solvay or Victrex. The melting temperature Tm and the glass transition temperature Tg are measured by DSC and are found to be 340 °C and 150 °C, respectively.

[0087] c) Unwind the rectangular copper wire from the carrier at a linear speed of 40 m / min, and clean and heat the copper wire on the production line by plasma.

[0088] d) Measure the surface temperature of the rectangular copper wire at the outlet of the heating device by an infrared camera and set it to 380 °C, which is Tm + 40 °C.

[0089] e) Apply PEEK on the surface of the copper wire by extrusion under ambient conditions, i.e., without a protective atmosphere.

[0090] f) Then cool the copper wire with a PEEK coating in water to 250 °C, which is higher than [(Tm + Tg) / 2 - 40 °C] and lower than [(Tm + Tg) / 2 + 40 °C], where the melting temperature Tm of PEEK = 340 °C and the glass transition temperature Tg of PEEK = 150 °C.

[0091] g) Adjust the cooling length in water so that the metal component with a coating reaches the desired temperature of 250 °C. Stop cooling and, to avoid further cooling, blow hot air at 250 °C to keep the temperature constant for a time selected between 2 s and 10 s. At a linear speed of 40 m / min, the length of the blowing device varies between 1.35 m and 6.7 m. At a higher linear speed, e.g., 120 m / min, the length of the blowing device will be between 3.3 m and 16.7 m. However, for a short holding time, no hot air blowing is required.

[0092] h) At the outlet of the stopped cooling length, quench the metal component with a coating in cold water to a temperature below 50 °C.

[0093] Finally, wind the cooled insulated metal component on the carrier at the winding unit.

[0094] Figure 1 is a schematic cooling curve illustrating steps f), g), and h) of the method. At Figure 1Above, the letter A represents the temperature at which the thermoplastic polymer is applied to the heated metal element. In this illustrative example, A = 380 °C. The letter B represents 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 represents the end of step g), i.e., the start of the quenching step after a temperature holding time of 2 to 10 seconds. The letter D represents the end of the quenching step when the insulated metal element reaches a temperature below 50 °C.

[0095] Several samples were produced with different stop cooling times. After all other identical steps, additional samples with only 1 second and 40 seconds of stop cooling time were also produced.

[0096] According to ASTM D3418 - 15, the percentage crystallinity of the thermoplastic polymer is determined from the heat of fusion and cold crystallization measured by DSC and the reference heat of fusion of 100% crystalline thermoplastic polymer. Approximately 10 mg of the thermoplastic polymer was removed from the insulated metal element by, for example, scratching or rubbing. Heating and cooling rates of 10 °C / min were used to generate the heat flow curves. The heat of fusion ΔHm and cold crystallization ΔHc were determined by integrating the area (J / g) under the peaks. The percentage crystallinity was determined using the following equation:

[0097] % crystallinity = 100 * [ΔHm - ΔHc] / ΔHm°

[0098] where ΔHm° is the heat of fusion of the fully crystalline polymer, which is 130 J / g for PEEK.

[0099] Figure 2 is a graph of the crystallinity versus the stop cooling time. Too short a temperature holding time between the first cooling step and the quenching step results in a low crystallinity or completely amorphous polymer coating, leading to poor adhesion between the polymer and the metal element and unstable electrical properties. A very long stop cooling time results in the highest crystallinity values. However, too long a stop cooling time may result in variations in the coating thickness and unstable electrical properties.

[0100] The best results were found by controlling the stop cooling time between 2 s and 10 s.

[0101] Three insulated metal elements were produced according to the disclosed method and compared with two insulated metal elements of the prior art.

[0102] Two reference samples from the prior art were selected, namely, REF.1 and REF.2. In both samples, the metal element was composed of copper with a purity of 99.9% and containing less than 400 ppm O2.

[0103] In two samples, the metal element has a rectangular shape with a width of 3.7 mm and a height of 2 mm, and the corner radius > 0.3 mm.

[0104] REF.1 is covered with an enamel layer 103 μm thick composed of PAI, which is obtained through multiple deposition and curing cycles.

[0105] REF.2 is covered with a first enamel layer 38 μm thick composed of PAI and a second polymer layer 112 μm thick composed of PEEK. The total thickness of the insulation layer is 150 μm.

[0106] Samples INV.1 to INV.3 are obtained with the same starting metal element, i.e., rectangular copper with a cross-sectional size of 3.7 mm × 2 mm and a corner radius > 0.3 mm.

[0107] Three samples produced according to the disclosed method have a PEEK coating thickness ranging between 40 μm and 300 μm.

[0108] To test the adhesion, a strip of the 10 mm coating portion from the metal element substrate is pulled through a calibrated opening and the peel force is measured. A peel force higher than 40 N / mm is measured in all samples.

[0109] In addition, according to the IEC60317 standard, the adhesion is tested by an elongation test with a cut through the polymer coating.

[0110] The PDIV of different samples is measured according to standards IEC 60664-1 and 61800-5-1.

[0111] To measure the PDIV, pairs of samples are made. A test voltage (AC 50 Hz, RMS) is applied to one conductor of the pair, while the other conductor is grounded. The test voltage is gradually increased until a partial discharge with a measured capacitance higher than the 10 pC level is recorded.

[0112] The following table reports the measured PDIV values of two reference samples and three samples of the present invention.

[0113]

[0114] The measured PDIV values are in very good agreement with the DAKIN formula V = 163(t / ε r ) 0.46 Very consistent.

[0115] When testing pairs of samples, the coating thickness t shown in the table needs to be doubled in the formula. The relative dielectric constant ε r Depends on the type of coating, and the enamel (PAI) is estimated to be 3.9 and PEEK is 3.1.

[0116] Obtain a high PDIV value higher than 800V in all invention samples.

[0117] The metal element with a polymer coating layer of the present invention is related to a more cost-effective production process and is particularly suitable for hairpin wires used in rotating or stationary components of electric motors.

Claims

1. A method for producing an insulating metal component with stable electrical properties, the method comprising the following steps: a) Providing a metal component; b) Providing a thermoplastic polymer; c) Cleaning the surface of the metal component; d) Heating the metal component 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 component; f) Cooling the metal component with the coating 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 2 s to less than 10 s; h) Quenching the metal component with the coating to a temperature lower than 50°C.

2. The method according to claim 1, wherein, The metal component is not reheated after the first cooling step f).

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

4. The method according to claim 1, wherein, Hot air is used to stop the cooling at 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.

5. The method according to claim 1, wherein, Steps c) to h) are carried out on a production line, where the metal component runs on the production line at a linear speed higher than 40 m / min.

6. An insulating metal component with stable electrical properties obtained by the method according to claim 1, the insulating metal component comprising a metal component and a thermoplastic polymer coating, the thermoplastic polymer coating being semi-crystalline with a crystallinity rate between 10% and 40%.

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

8. The insulating metal element with stable electrical properties according to claim 6, wherein, The metal component has a rectangular or square cross-section.

9. The insulating metal element with stable electrical properties according to claim 6, wherein, The metal component is a wire, rod or tube made of copper.

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

11. The insulating metal element with stable electrical properties according to claim 6, wherein, The metal component is a wire, rod or tube made of steel.

12. The insulating metal element with stable electrical properties according to claim 6, wherein, The thermoplastic polymer coating comprises a thermoplastic polymer from the PAEK family or consists of a thermoplastic polymer from the PAEK family.

13. The insulating metal element with stable electrical properties according to claim 6, wherein, The thickness of the thermoplastic polymer coating ranges from 60 μm to 500 μm.

14. Use of the insulating metal component with stable electrical properties according to claim 6 as a hairpin wire for a rotating or stationary component of an electric motor.

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