Insulated metal element

Abstract

Insulated metal element having stable electrical properties, excellent heat ageing resistance and controlled surface roughness is provided. The insulated metal element comprises a metal element and a thermoplastic polymer coating layer directly covering said metal element, the thermoplastic polymer coating layer comprises a thermoplastic polymer selected from the PAEK family, the thermoplastic polymer has a melt viscosity higher than 250 Pa.s, as measured according to ASTM D3835 at 400°C, at shear rate of 1000s-1; the thermoplastic polymer coating layer is in semi crystalline state, the relative crystallinity is between 20% and 50% as measured by DSC according to ASTM D3418, and the thermoplastic polymer coating layer has an outer surface roughness Ra between 0.01 and 10µm. The method to produce the same is also provided.

Description

Title: Insulated metal elementDescriptionTechnical Field

[0001] The invention relates to an insulated metal element having stable electrical properties, excellent heat ageing resistance and controlled surface roughness, and to a method to produce the same.Background Art

[0002] Insulated metal elements are used in several applications where a metal conductor needs to be insulated. For example it can design a cross-linked polyethylene (XLPE) insulated conductor for the medium- voltage lines and a XLPE insulated conductor or a polyvinyl chloride (PVC) insulated conductor for the low-voltage line.

[0003] Insulated metal elements are also used in the stator of electric motors, for both synchronous (permanent magnet) and asynchronous (induction) motors. New challenges for automotive electric motors include:- higher motor speed;- compact design and maximized power / unit;- high reliability (short circuits, fire hazard);- large production runs;- high assembly-speed;- automated assembly;- cost reduction.

[0004] With the increase of battery voltage above 500V to increase the electric vehicle autonomy and decrease the charging time, insulated metal elements used as magnet wires in electrical motors need to be resistant at high voltage against partial discharges and hence exhibit a high partial discharge inception voltage (PDIV).

[0005] Moreover, insulated metal elements used as magnet wires in electrical motors need to exhibit stable electrical properties with time. Stable electrical properties depend mainly on the type of insulation layer, its resistance to temperature variation and adhesion to the metal element.

[0006] When an insulation layer, acting as a barrier, has a defect such as an internal void, the defect will display localized ionization when exposed to high voltage. This ionization starts at one voltage and stops at a lower voltage. These are called the inception and extinction voltages. As high voltage is applied to the barrier, voltage will also build up across the void. When the inception voltage is reached, the void ionizes, shorting itself out. When the voltage across the void drops below the extinction voltage, ionization ceases. This action redistributes charge within the barrier and is known as partial discharge. If the barrier voltage continues to rise, another partial discharge cycle begins. If the barrier voltage is alternative current (AC) and is large enough, partial discharge cycles will repeat many times during the positive and negative peaks. If the ionization begins and continues, it can damage the barrier, leading to failure. If the discharge does not occur, the barrier receives no damage. The inception voltage of the individual voids tends to be constant. Therefore, the total charge redistributed within the barrier is a very good indicator of the number of the voids and their likelihood of becoming a failure. Setting a very low limit on the allowable current caused by partial discharges in testing gives a very high degree of confidence that high voltage failure will not occur.

[0007] In addition to local defects on the insulation layer, parameters that affect the partial discharge inception voltage are the insulation layer thickness, and the air gap between two insulation layers. Studies have shown that there is a relationship between the partial discharge inception voltage, V, and both the insulation layer thickness, t, and the relative permittivity erof the insulator (Dakin formula): V = 163(t / er)046

[0008] Traditional ways to produce insulated metal elements resistant against PDIV comprise the application of an enamel layer on a copper wire. Enamel coatings are used in combination with paper liners in the stator.Paper wrapped windings increase insulation, but much space is lost because paper layers can be up to 500pm thick.

[0009] US 4471022A discloses a water-soluble polyimide, a coated wire and the method of coating. The insulation layer consists of poly imide (PI), at least 6 layers are deposited with long curing time for each layer.

[0010] In the recent years alternative solutions have been developed comprising thermoplastic polymers alone or in combination with enamel coatings.

[0011] US 9324476B2 discloses an alternative insulated winding wire comprising at least two layers, the first one being an enamel polyamide-imide (PAI) layer, the second being polyether ether ketone (PEEK) or polyaryl-ether- ketone (PAEK).

[0012] US 9224523B2 discloses an inverter surge-resistant insulated wire, also consisting of an enamel layer and extruded thermoplastic.

[0013] Increasing the thickness of an enamel (polyimide or polyamide-imide) layer increases the risk of defects and increases the cost of production.

[0014] The cost of increasing the insulating layer could be decreased by using powder deposition or extrusion of thermoplastic polymers. However, due to the poor adhesion between thermoplastic polymers and the metal element, a primer or bonding layer is usually necessary according to prior art.

[0015] Solutions to improve adhesion of extruded thermoplastic directly on a metal element, without the need for a primer or a bonding layer are disclosed in US 2019 / 0131037A1 and W021041200A1 .

[0016] US 2019 / 0131037A1 describes an insulated electric conductor obtainable by a method in which the electric conductor is placed under a protective gas atmosphere and is bombarded with ions of the protective gas in a gas plasma in order to remove an oxide layer formed on a surface of the electric conductor and / or to increase the surface energy of the conductor. The insulating coating layer either comprises at least one insulating layer made of thermoplastic material, or the insulating layer and a plasticcontaining intermediate layer.

[0017] W021041200A1 discloses an insulated electrical conductor comprising an electrical conductor comprising an oxide layer on at least part of a surfaceof the electrical conductor, and an insulating coating on at least a portion of the oxide layer. Good adhesion between the electrical conductor and the insulating coating is obtained by heat-treating the coated electrical conductor.

[0018] Controlling the crystallinity of a thermoplastic polymer can help optimize its properties for use as a wire insulator. The crystallinity of a thermoplastic polymer is crucial as it affects the polymer’s density, mechanical strength, and thermal and electrical conductivity. High levels of crystallinity typically result in materials that are denser, stiffer, and more resistant to heat and electrical current.

[0019] JPH02250206A discloses insulated electric wires having a PEEK insulating layer with crystallinity lower than 10% such that flexibility is obtained for winding, further subjected to a heat treatment to set the degree of crystallization of PEEK between 15 and 40% for improved hardness and chemical resistance.

[0020] US9691521 B2 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 the first thermoplastic layer, and the first thermoplastic layer has a relative crystallinity in the range of 20% to 50%.

[0021] US2018 / 005724A1 discloses a conductor wrapped in a PEEK tape layer having a crystallinity of at least 25%.

[0022] US5358786A discloses an insulated wire comprising a conductor, an inner insulation layer 0.1 -1mm comprising a halogen-free polymer, an intermediate insulation layer 0.001 mm to 0.5mm having a melting point <155°C, and an outer insulation layer 0.05mm to 1mm having a melting point>155°C.

[0023] CN117912771A provides an insulated wire comprising a PEEK resin insulation layer formed on the outside of the bare conductor wire wherein the crystallinity of the PEEK resin insulation layer covering the first side surface is a first preset value, and the PEEK resin insulation layer covering the second side surface is in an amorphous state.

[0024] In comparison with the traditional enamel layers consisting of thermoplastic polymer, insulated electric conductors comprising an extruded thermoplastic layer are more sensitive to heat ageing and their use in electric motors may require better cooling. Heat ageing is a phenomenon leading to degradation of mechanical properties when the thermoplastic polymer is exposed to elevated temperatures for a long period of time. In particular, the elongation at break may drop significantly, leading to embrittlement and ultimately to cracks or delamination of the insulating layer, possibly causing electric failure.

[0025] CN116994831 discloses an insulated wire comprising a PEEK resin insulation layer, used in winding in oil-cooled motors. Impregnating varnish is applied on the surface of the insulated wire, which is preliminary roughened by treatment with a plasma jet. The surface of the insulated wire is activated, the surface adhesion ability can be improved, and finally the adhesion between the PEEK insulated wire and impregnating varnish can be improved.

[0026] EP3754669A1 discloses an insulated wire containing a thermoplastic resin layer on a rectangular conductor, in which the insulated wire has at least one protruding part which is continuous in a longitudinal direction of said insulated wire, and a flat section. The disclosed insulated wire has a high conductor space factor to a slot cross-sectional area and secures a desirable gap area, whereby a coil that can be efficiently cooled by a cooling liquid or gas can be realized and is excellent in bending workability.

[0027] The thermal, mechanical, and electric properties of the insulated electric conductor thus not only depend on the adhesion between the conductor and the insulating coating, but also on the characteristics of the thermoplastic polymer of the insulating layer. Excellent heat ageing resistance is required to guarantee stable electric and mechanical properties in use.Disclosure of Invention

[0028] The technical problem to solve in view of the prior art is to provide an insulated electric conductor having stable electrical properties, said insulated electric conductor comprising an insulating layer which has good adhesion, excellent heat ageing resistance, and with controlled surface roughness.

[0029] As a result of intensive studies, the present inventors have first developed a method to produce an insulated metal element having an insulating layer which has good adhesion and stable electrical properties in use. The new method does not require protective atmosphere such as in US 2019 / 0131037 and does not require a heat treatment step after cooling such as in US2020 / 047379.

[0030] The method provides both good adhesion between the insulating layer and the metal element and a desired range of crystallinity. While further improving the heat ageing resistance it was found that surprisingly the surface roughness could also be adjusted without the need for surface modification either during an additional process step such as a plasma treatment or by a mechanical treatment such as shot peening or sand blasting, or by creating protrusion.

[0031] The present disclosure therefore provides a new method to produce an insulated metal element having both a desired range of crystallinity and a controlled surface roughness. The higher production speed obtainable with the new method and the limited number of processing steps allow significant cost reduction in comparison with other existing methods. With the method of the invention, less harmful chemicals such as solvents or additives are needed and less energy is consumed due to the reduced number of processing steps, providing a lower carbon footprint.

[0032] The method of the invention enables the production of new families of insulated wires, that fulfil the stringent requirements for high-end applications such as e.g. in stator of electric vehicles, and at the same have a low environmental impact.

[0033] Insulated metal elements having stable electrical properties in use, excellent heat ageing resistance and controlled surface roughness obtained by the new process are also disclosed.

[0034] It is a first object of the invention to provide a method to produce an insulated metal element having stable electrical properties, excellent heat ageing resistance and a controlled surface roughness. The method comprises the following steps: a) Providing a metal element; b) Providing a thermoplastic polymer selected from the PAEK family and having a melt viscosity higher than 250 Pa.s, as measured according to ASTM D3835 at 400°C, at shear rate of 1000s-1; c) Cleaning the surface of the metal element; d) Heating the metal element at a temperature between [Tm +20°C] and [Tm + 100°C], preferably at a temperature between [Tm +20°C] and [Tm + 60°C], more preferably at a temperature between [Tm +60°C] and [Tm + 100°C], Tm being the melting temperature of said thermoplastic polymer; e) Applying the thermoplastic polymer on the surface of the metal element; f) Holding the obtained coated metal element above a temperature of [Tm + 20°C] for 0.5s to 60s, Tm being the melting temperature of the thermoplastic polymer; g) Cooling the coated metal element to a temperature higher than [(Tm+Tg) / 2 - 40°C] and lower than [(Tm+Tg) / 2 + 60°C], with Tm the melting temperature of said thermoplastic polymer and Tg the glass transition temperature of said thermoplastic polymer; h) Stopping the cooling for 1 s to less than 20s; i) Quenching the coated metal element to a temperature below 50°C.

[0035] Each step of the method with preferred embodiments is described hereunder.

[0036] A) Providing a metal element

[0037] The metal element is preferably elongated and can have a round or oval or profiled cross section.

[0038] In a preferred embodiment the metal element has a square or rectangular cross-section.

[0039] The metal element consists of pure metal or it can be a metallic alloy.

[0040] For instance, the metal element can be made of copper or copper-alloy.

[0041] Or the metal element can be made of aluminum or aluminum alloy.

[0042] Or the metal element can be made of iron or steel.

[0043] The metal element can also comprise different metals. For instance, 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. Yet another example is a steel substrate coated with zinc or a zinc alloy.

[0044] The metal element can be a wire, a rod, or a tube.

[0045] The metal element is preferably provided on a carrier, such as a coil, reel, spool, or spider.

[0046] B) Providing a thermoplastic polymer

[0047] The polymer used for the insulating coating is a thermoplastic, i.e. a substance that becomes plastic on heating and hardens on cooling, and is able to repeat these processes.

[0048] There exist different thermoplastic polymers commercially available, having different properties due to their different molecular weight. Low molecular weight thermoplastic polymers usually exhibit low melt viscosity and better adhesion. High molecular thermoplastic weight polymers exhibit high melt viscosity and are more difficult to process because of entanglement of longer polymer chains.

[0049] However, low molecular weight thermoplastic polymers were found to have lower elongation and less thermal resistance after their application on a metal element compared to high molecular weight thermoplastic polymers.

[0050] The molecular weight depends on the type of polymers. Preferably the provided thermoplastic polymer belongs to the family of poly(aryl ether ketone) (PAEK), for instance poly(ether ether ketone) (PEEK), or poly(ether ketone) (PEK), or poly(ether ketone ketone) (PEKK).

[0051] Preferably the polymer consists of PEEK.

[0052] PEEK with a molecular weight in the range of 20,000 to 27,000 g / mol are typically considered low molecular weight thermoplastics. These polymers are easier to process due to their lower melt viscosity.

[0053] High molecular weight PEEK generally has a molecular weight in the range of 27,000 to 37,000 g / mol or higher. These polymers are tougher and more impact-resistant, but their higher melt viscosity can make them more challenging to process, especially in small or complex molds.

[0054] The melt viscosity of several thermoplastic polymers was measured according to ASTM D3835 at 400°C, at shear rate of 1000s-1.

[0055] The inventors have found that thermoplastic polymers with a low melt viscosity, e.g. below 250 Pa.s had not satisfactory heat ageing resistance.

[0056] Therefore, the thermoplastic polymer provided has a melt viscosity at 400°C, at shear rate of 1000s-1equal or higher than 250Pa.s, preferably higher than 275Pa.s, more preferably higher than 300Pa.s.

[0057] The inventors have found that with the new process, despite the high melt viscosity, it was possible to obtain very good adhesion between the thermoplastic polymer and the metal element and control the crystallinity and surface roughness of the insulating thermoplastic coating.

[0058] C) Cleaning the surface of said metal element

[0059] The metal element has preferably a degreased surface.

[0060] Surface preparation is done by electrolytic cleaning, assisted chemical treatment (e.g. ultrasonic cleaning), plasma, laser ablation, or any combination thereof.

[0061] D) Heating said metal element at a temperature between Tm + 20°C and Tm + 100°C, preferably at at temperature between Tm + 20°C and Tm + 60°C, more preferably at a temperature between [Tm +60°C] and [Tm + 100°C], Tm being the melting temperature of said thermoplastic polymer

[0062] Heating can be done by means of induction, resistive heating, gas oven, plasma, or any combination thereof.

[0063] The step of heating the metal element can be done preferably by means of medium frequency induction or by means of low pressure plasma or by combining both heating means. Medium frequency induction and / or low pressure plasma are preferred heating means because high heating rate can be obtained.

[0064] As an example, when the melting temperature Tm of said thermoplastic polymer is 340°C said metal element is heated at a temperature between 360°C and 440°C.

[0065] E) Applying said thermoplastic polymer on the surface of said metal element

[0066] The polymer coating is applied directly on the surface of the hot metal element.

[0067] The polymer coating layer can be applied by any technique known in the art, for example by extrusion or powder coating. Preferably, the coating layer is an extruded coating layer, as can be identified by observing the polymer chain orientation in the coating layer.

[0068] The polymer coating layer has preferably a thickness in the range 20pm to 500pm, for example between 30pm and 400pm or between 40pm and 300pm. More preferably the polymer coating layer has a thickness in the range 50pm to 200pm.

[0069] F) Holding the obtained coated metal element above a temperature of fTm + 2O°C1 for 0.5s to 60s, Tm being the melting temperature of the thermoplastic polymer:

[0070] This step is essential to control the surface roughness of the coated metal element. It was found that no holding or holding for less than 0.5s beforecooling resulted in an uncontrolled rough surface, while long holding time before cooling resulted in a smooth surface.

[0071] If the temperature of the coated metal element dropped too quickly below temperatures of [Tm+20°C], or if the time above [Tm+20°C] was less than 0.5 seconds, the resulting surface had an undesired roughness of Ra>10pm.

[0072] The roughness Ra is measured on the finished or semi-finished product, i.e. on the coated metal element at room temperature.

[0073] To avoid the temperature drops too quickly, a temperature holding zone may be used. Said temperature holding zone may comprise insulation elements and heating elements or hot air.

[0074] Heating elements may comprise a muffle furnace, and / or any heating device such as an inductor, an infrared heater or a plasma generator or any combination thereof.

[0075] In case additional heat is used, the temperature of the coated metal element should preferably stay below Tm+100°C.

[0076] The inventors have found that holding for more than 60s did not bring additional benefit to the product.

[0077] G) Cooling the coated metal element to a temperature higher than r(Tm+Tg) / 2 - 4O°C1 and lower than RTm+Tg) / 2 + 6O°C1, with Tm the melting temperature of said thermoplastic polymer and Tg the glass transition temperature of said thermoplastic polymer

[0078] Controlled cooling can be obtained by means of spraying a gas, e.g. N2 or compressed air on the surface of the coated metal element. Preferably, controlled cooling is done by immersion in water or by spraying water on the surface of the coated metal element. Other controlled cooling techniques mixing gas and water may also be used.

[0079] The duration and intensity of the cooling should be adjusted such that the surface of the coated metal element reaches a temperature higher than [(Tm+Tg) / 2 - 40°C] and lower than [(Tm+Tg) / 2 + 60°C],

[0080] Preferably, no reheating above (Tm+Tg) / 2 + 60°C should occur after the first cooling step.

[0081] H) Stopping the cooling for 1s to less than 20s

[0082] Stopping the cooling means keeping or holding the surface of the coated metal element within the temperature range [(Tm+Tg) / 2 - 40°C] and [(Tm+Tg) / 2 + 60°C]. To keep the surface of the coated metal element between [(Tm+Tg) / 2 - 40°C] and [(Tm+Tg) / 2 + 60°C] for 1s to less than 20s at the end of the first cooling step, a temperature holding zone may be used. Said temperature holding zone may comprise insulation elements and heating elements or hot air.

[0083] The temperature holding time between 1s and 20s determines the crystallinity rate, ensures good adhesion and stable electrical properties of the insulated metal element. In particular, holding times lower than 1s cause low crystallinity, and bad adhesion of the coating.

[0084] I) Quenching the coated metal element to a temperature below 50°C

[0085] As for the first cooling step, the final cooling step, or quenching, can be obtained by means of spraying a gas, e.g. N2 or compressed air on the surface of the coated metal element. Preferably, quenching is done by immersion in water or by spraying water on the surface of the coated metal element. Other controlled cooling techniques mixing gas and water may also be used. The temperature of the coated metal element after quenching should be below 50°C, preferably below 30°C. The temperature of the coated metal element should be preferably higher than 0.5°C.

[0086] In a preferred embodiment, above described steps c to i are executed in a production line wherewith said metal element is running at a linear velocity higher than 40m / min, e.g. 50m / min, e.g. 100m / min. The holding time in step f as well as the temperature range and temperature holding time at the end of the first cooling (steps g and h) are independent from the linear velocity of said metal element.

[0087] In another embodiment, after the quenching step i a second layer of thermoplastic polymer is applied on the surface of the coated insulated metal element. In that case the time above [Tm+20°C] in step f should be more than 0.5s but short enough, such that the resulting surface has a roughness Ra between 1 pm and 10pm. The maximum time depends on the geometry of the metal element, and its temperature before the cooling step. The region of time and temperature leading to a desired roughness Ra between 1 pm and 10pm corresponds to zone C as illustrated in FIG.1 and further described by means of examples below.

[0088] The second layer of thermoplastic polymer is applied on the coated insulated metal element e.g. by means of extrusion or powder coating.

[0089] Preferably, the coated insulated metal element has a temperature below 200°C, more preferably below 150°C, more preferably below 100°C before the second layer of thermoplastic polymer is applied on the coated insulated metal element so that the surface roughness of the first thermoplastic polymer layer is not modified by the application of the second layer of thermoplastic polymer.

[0090] The method provides indeed sufficient surface roughness to significantly improve adhesion between the first thermoplastic polymer layer and the second thermoplastic polymer layer.

[0091] The total polymer coating comprising the first thermoplastic polymer layer and the second thermoplastic polymer layer has preferably a thickness in the range 20pm to 500pm, for example between 30pm and 400pm or between 40pm and 300pm. More preferably the total polymer coating layer has a thickness in the range 50pm to 200pm.The second thermoplastic polymer coating layer may comprise any type of thermoplastic polymer and is not limited to the family of PAEK. Thermoplastic polymers suitable for the second thermoplastic polymer coating layer can be selected in the list of PAEK, including PEEK, PEKK, PEK. Thermoplastic polymers suitable for the second thermoplastic polymer coating layer can also be selected in different families of thermoplastic polymers, such as e.g. polyphenylene sulfide (PPS),polyesther sulfide (PES), thermoplastic polyimide (PI), polytetrafluoroethylene (PTFE), polyphenyl sulfone (PPSLI), liquid crystal polymer (LPC) and polyamide (PA).The surface roughness of the second thermoplastic polymer coating layer is preferably between 0.01 pm and 1 m, e.g. between 0.01 pm and 0.25pm, or between 0.25pm and 0.4pm, or between 0.4pm and 1 pm.

[0092] A third or fourth layer with the same or different thermoplastic polymers can be applied. The coated metal element may additionally be further coated with any additional coating layer such as e.g. varnish.

[0093] It is a second object of the invention to provide an insulated metal element having stable electrical properties, and excellent heat ageing resistance.

[0094] The insulated metal element comprises a metal element and a thermoplastic polymer coating layer directly covering the metal element. The thermoplastic polymer coating layer comprises a thermoplastic polymer selected from the family of poly(aryl ether ketone) (PAEK), for instance poly(ether ether ketone) (PEEK), or poly(ether ketone) (PEK), or poly(ether ketone ketone) (PEKK).

[0095] Preferably the thermoplastic polymer coating layer consists of a thermoplastic polymer, i.e. it does not contain fillers such as solid particulate material or blended polymers.

[0096] Preferably the thermoplastic polymer consists of PEEK.

[0097] The thermoplastic polymer has a melt viscosity higher than 250 Pa.s, as measured according to ASTM D3835 at 400°C, at shear rate of 1000s’1.

[0098] The inventors have found that thermoplastic polymers with a low melt viscosity, e.g. below 250 Pa.s had not satisfactory heat ageing resistance.

[0099] Therefore, the thermoplastic polymer coating layer has a melt viscosity at 400°C, at shear rate of 1000s’1equal or higher than 250Pa.s, preferably higher than 275Pa.s, more preferably higher than 300Pa.s.

[0100] The thermoplastic polymer coating layer is in semi crystalline state, the relative crystallinity is between 20% and 50% as measured by DSC according to ASTM D3418.

[0101] The average roughness of the outer surface of the thermoplastic polymer coating is measured according to ISO 21920-2:2021 e.g. by using a laser scanning confocal microscope. The arithmetic mean height is the average of the absolute values along the sampling length. For roughness profiles, Ra stands for the arithmetic mean roughness. For sake of simplification, Ra is further refering to surface roughness in the description.

[0102] The thermoplastic polymer coating layer has an outer surface roughness Ra between 0.01 pm and 10pm, preferably between 0.01 pm and 8pm, more preferably between 0.01 pm and 6pm.

[0103] The insulated metal element of the invention, obtained with the described method, is resistant at high voltage against partial discharges, has stable electrical properties in use, excellent heat ageing resistance and is easier and cheaper to produce than insulated metal elements of the prior art.

[0104] Thanks to the new method described above, several embodiments of the invention can be obtained. By adjusting the time and temperature within the ranges of the inventive method, in particular in steps d) and f), different products can be obtained offering different advantages.In a first embodiment the insulated metal element of the invention comprises one layer of thermoplastic polymer coating directly covering said metal element. The thermoplastic polymer coating layer has a very low Ra value, between 0.01 pm and 0.4pm, preferably between 0.01 pm and 0.25pm. The insulated metal element of the first embodiment provides high PDIV, low space factor, and has a very smooth surface. The inventors have found that insulated metal element of the invention wherein the thermoplastic polymer coating layer has a low Ra value, between 0.01 pm and 0.4pm, preferably between 0.01 pm and 0.25pm were significantly more resistant to heat ageing compared to insulated metal elements with higher Ra value.

[0105] In a second embodiment the insulated metal element of the invention comprises one layer of thermoplastic polymer coating directly covering said metal element. The thermoplastic polymer coating layer has a low Ra value between 0.4pm and 1 pm. The insulated metal element of thesecond embodiment still provides high PDIV, low space factor, and has a slightly rough surface, visible by naked eye, providing some friction, thereby allowing better positioning of insulated metal elements next to each other in e.g. a stator of a motor, and / or allowing come cooling thanks to the larger surface area in contact with the air or any coolant such as a gas or a liquid.

[0106] In a third embodiment the insulated metal element of the invention comprises one first layer of thermoplastic polymer coating directly covering said metal element. The first thermoplastic polymer coating layer has a Ra value between 1 pm and 10pm, preferably between 1 pm and 8pm, more preferably between 1 pm and 6pm. A second thermoplastic polymer coating layer is directly covering the first thermoplastic polymer coating layer. The high Ra of the first thermoplastic polymer coating provides improved adhesion for powder deposition or extrusion of the second thermoplastic polymer coating layer.

[0107] It is remarkable that when the surface roughness increases it becomes visible with naked eyes from Ra values above 0.4pm. With Ra values above 1 pm the surface aspect has a typical texture, that can be compared to a sharkskin or a snakeskin. This typical texture provides very good adhesion between the first thermoplastic polymer coating layer and a second thermoplastic polymer coating layer deposited on top of the first layer. The second thermoplastic polymer coating layer can comprise any thermoplastic polymer or a blend of thermoplastic polymer with or without filler so that the properties of the insulated metal element can be finetuned according to the requirements. Preferably, the first layer of thermoplastic polymer coating has a thickness equal to or inferior to one third of the sum of the first and the second thermoplastic polymer coating layer thicknesses.

[0108] Impregnating varnish can also be applied on the surface of the insulated metal element. In contrast with CN116994831 , wherein the surface of an insulated wire is preliminary roughened by treatment with a plasma jet, thetypical sharkskin or snakeskin texture of the insulated metal element of the invention provides a better surface adhesion at no additional cost.Brief Description of Figures in the Drawings

[0109] FIG. 1 . is a drawing illustration the preferred time / temperature ranges in step f to control the surface roughness of the thermoplastic polymer coating layer.

[0110] FIG. 2. Represents an insulated metal element of the invention according to a first embodiment, with low Ra.

[0111] FIG. 3. Represents an insulated metal element of the invention according to a second embodiment, with medium Ra.

[0112] FIG. 4. Represents an insulated metal element of the invention according to a third embodiment, with a first thermoplastic polymer coating layer with a high Ra and a second polymer coating layer directly covering the first thermoplastic polymer coating layer.Mode(s) for Carrying Out the Invention

[0113] Different insulated metal elements were produced according to the disclosed method: a) In some of the below examples rectangular shaped copper with section dimensions 3.7mm x 2mm and >0.3mm corner radius were provided as metal element on a carrier. In some other examples round steel wires with a diameter of 1 ,2mm were provided to prove the feasibility of the method of the invention, particularly on the control of the surface roughness. b) PEEK was provided as thermoplastic polymer. The commercial PEEK was obtained from e.g. Solvay or Victrex. Several grades were tested. From Victrex™ , grades 90G, 150 / 151 G, XPI150, 381 G, 450G and 650G were investigated. From Solvay™, Ketaspire™ KT-880NT, KT-857NT, KT-851 NT, KT-820NT and KT- 81 ONT were investigated.For each thermoplastic polymer grade the melt viscosity was measured in a rheometer at 400°C, at shear rate of 1000s-1according to the ASTM D3835 standard method.The melting temperature Tm and glass transition temperature Tg of investigated thermoplastic polymers were measured by DSC and were found to be approximately 340°C and 150°C, respectively. c) The rectangular shaped copper wire or the round steel wire were unwound from the carrier at a linear velocity of 40m / min or more, cleaned and heated in line by means of plasma or by induction. d) The surface temperature of the rectangular shaped copper wire or of the round steel wire at the exit of the heating device was measured by infra red camera and was set to different temperatures between Tm+20°C and Tm+100°C, e.g. 380°C, which is Tm+40°C. e) PEEK was applied under ambient conditions, i.e. without protective atmosphere, by means of extrusion on the surface of the hot metal element, i.e. the copper or steel wire. f) The PEEK-coated metal element was then held to a temperature above 360°C, which is Tm+20°C for a time between 0.5s and 60s. The time was set by varying the length of the holding zone before cooling and / or the linear velocity of the metal element. The temperature was was measured by infra red camera and was held above 360°C by targeting a temperature before cooling of e.g. 380°C or 400°C by means of slow cooling or by adding heat by means of induction heating; g) The PEEK-coated metal element was then cooled in water to 250°C, which is a temperature higher than [(Tm+Tg) / 2 - 40°C] and lower than [(Tm+Tg) / 2 + 60°C], with Tm = 340°C, the melting temperature of PEEK and Tg = 150°C, the glass transition temperature of PEEK. h) The cooling length in water was adjusted such that the coated metal element reached the desired temperature of 250°C. The cooling was stopped and to avoid further cooling, hot air at 250°C was blown to keep the temperature constant for a time selected between 1s to 20s. At the line velocity of 40m / min, the air blowing installationvaried between 0.7m and 13.3m. At higher line velocity, e.g. 120m / min, the air blowing installation would be between 2m and 40m long. For short holding times, however, hot air blowing is not needed. i) At the exit of the stop cooling length, the coated metal element was quenched in cold water to a temperature below 50°C.

[0114] The cooled insulated metal element was finally wound on a carrier at a take up unit.

[0115] The inventors have found that thermoplastic polymers having a melt viscosity of less than 250 Pa.s were not suitable for the current process for two reasons:- In all cases the heat ageing resistance was bad, i.e. the thermoplastic polymer coating became brittle;- It was more difficult to control the surface roughness of the thermoplastic polymer coating.

[0116] Heat ageing resistance was tested by reheating samples of coated metal elements for 336h at 180°C and by measuring the elongation at break of the thermoplastic polymer coating. In samples comprising thermoplastic polymers having a melt viscosity at 400°C 1 1000s-1below 250Pa.s such as in comparative examples CE1 and CE2, multiple cracks appeared in the thermoplastic polymer coating after 5% elongation.

[0117] In samples produced with the inventive method and comprising thermoplastic polymers having a melt viscosity at 400°C 1 1000s-1above 250Pa.s no cracks were visible until the elongation at break was reached. The elongation at break reached values above 35% in those samples with high melt viscosity.

[0118] The average roughness of the samples was measured by using a laser scanning confocal microscope VK-X3100 from Keyence. Line roughness measurements were done in the direction of the longest dimension (the length of the metal element), on a distance of 12mm. The rough data obtained by laser and optical analyses were used to calculate several roughness parameters. The arithmetic mean height is the average of theabsolute values along the sampling length. For roughness profiles, Ra stands for the arithmetic mean roughness.

[0119] The inventors have found that surprisingly the Ra value could be adjusted within desired ranges by controlling the time and temperature of the coated metal element at the exit of the application process, the extrusion die in the present examples, and before the cooling step.

[0120] FIG.1 shows preferred time / temperature ranges in step f to control the surface roughness of the thermoplastic polymer coating layer. On FIG.1 zone A indicates a temperature range above Tm+100°C. This range of temperatures should be avoided as degradation and loss of properties of the thermoplastic polymer coating layer may occur.

[0121] Zone F indicates a temperature range below Tm+20°C. Cooling should occur before the coated metal element reaches Tm+20°C to allow better control of roughness and crystallinity.

[0122] In the temperature range between Tm+100°C and Tm+20°C, zone B indicates holding or slow cooling times below 0.5s, being too short to ensure optimal control of roughness and crystallinity.

[0123] Zones C, D and E are preferred processing ranges of times and temperatures to control the roughness between 0.01 pm and 10pm. Zone C provides roughness values in the range of 1 pm to 10pm. Zone D provides roughness values in the range 0.4pm to 1 pm. Zone E provides roughness values in the range 0.01 pm to 0.4pm.

[0124] The time above Tm+20°C should not exceed 60s, preferably it should be less than 40s, more preferably less than 30s.

[0125] The time and temperature required to reach a desired average roughness value are not independant. Shorter time is needed at higher temperature and longer time is needed at lower temperature as illustrated in FIG.1 . The time and temperature regions to reach a desired average roughness also depend on the type of metal element and its dimensions. For instance a steel element having a round cross section will have different cooling rates compared to a copper element or an aluminum element with a rectangular cross section. Examples are provided in tables 1 and 2 for both a rectangular copper wire with a rectangular cross section and a steel wirewith a round cross section. In some cases a heat source was used to prevent the coated metal element to cool too quickly and increase the time in a preferred temperature range thereby controlling the surface roughness. For the examples reported in table 1 the heat source was provided by a medium frequency inductor.

[0126] The person skilled in the art will be able to establish a reference graph as illustrated in FIG.1 for each type of metal element of any dimensions without difficulty, thereby linking the time and temperature ranges to obtain average roughness within desired ranges.

[0127] At the end of the holding step f coated metal elements were cooled to the target temperature of 250°C, and held for a time between 2s and 10s to control the crystallinity of the thermoplastic polymer between 20 and 50%. Samples were finally quenched in water to a temperature below 50°C.

[0128] The percent crystallinity of the thermoplastic polymer was determined from the heats of melting and cold crystallization as measured via DSC and the reference heat of melting of the 100% crystalline thermoplastic polymer according to ASTM D3418-15. Approximately 10mg of thermoplastic polymer was removed from the insulated metal element by e.g. scraping or grating. Heating and cooling rates of 10°C / min were used to produce the heat flow curves. The heats of melting, DHm and cold crystallisation, DHc were determined by integrating the areas (J / g) under the peaks. The percent crystallinity was determined using the following equation: %Crystallinity = 100*[DHm- DHc] / DHm° where DHm° is the heat of melting of a fully crystalline polymer, which is 130 J / g for PEEK.

[0129] A too short temperature holding time between the first cooling step and the quenching step leads to low crystallinity or a completely amorphous polymer coating, causing bad adhesion between the polymer and the metal element, and unstable electrical properties. Very long stop cooling times lead to the highest crystallinity value. However, too long stop cooling time may cause variations of the coating thickness and unstable electrical properties.

[0130] An optimum was found by controlling the stop cooling time between 2 and 10s.

[0131] Tables 1 and 2 below provide illustrative examples of the invention.

[0132] Table 3 provides comparative examples which are out of the scope of the invention.

[0133] Table 1 - Examples of samples according to the invention

[0134] Table 2 - Examples of samples according to the invention

[0135] Table 3 - Comparative examples out of the scope of the invention

[0136] In table 1 examples Ex1 and Ex2 refer to rectangular copper wires with section dimensions 3.7mm x 2mm and >0.3mm corner radius. Other examples Ex3 and Ex4 in table 1 , and Ex5 to Ex8 in table 2 refer to steel samples with a round section, of diameter 1 ,2mm.

[0137] All samples in tables 1 and 2 were coated according to the method of the invention with a thermoplastic polymer having a melt density of more than 250Pa.s at 400°C, measured at shear rate of 1000s-1. Commercial grades 381 G and XPI150 from Victrex, and Ketaspire 851 NT from Solvay were found suitable for the method of the invention. To control the roughness, samples were held in step f) for different times between 0.5s and 60s above Tm+20. Tm was measured by DSC to be approximately 340°C.

[0138] Samples corresponding to Ex2, Ex3 and Ex4 were held in a zone of time and temperature referred to as zone E in FIG.1 . The surface roughness Ra, measured by means of a laser scanning confocal microscope was below 0.4pm. The cross section of samples according to Ex2, Ex3 and Ex4 is schematically represented in FIG.2, which corresponds to the first embodiment wherein a thermoplastic polymer coating layer 202 is directly covering a metal element 201.

[0139] Samples corresponding to EX1 , Ex5 and EX6 were held in a zone of time and temperature referred to as zone D in FIG.1 . The surface roughness Ra, measured by means of a laser scanning confocal microscope was between 0.4pm and 1 pm. The cross section of samples according to Ex1 , Ex5 and Ex6 is schematically represented in FIG.3, which corresponds to the second embodiment wherein a thermoplastic polymer coating layer 302 is directly covering a metal element 301 . The visual aspect of the surface of samples Ex1 , Ex5 and Ex6 can be described as very fine snakeskin or sharkskin.

[0140] Samples corresponding to EX7 and EX8 were held in a zone of time and temperature referred to as zone C in FIG.1 . The surface roughness Ra, measured by means of a laser scanning confocal microscope was between 1 pm and 10pm. The cross section of samples according to Ex7, and Ex8 is schematically represented in FIG.4, which corresponds to the third embodiment wherein a thermoplastic polymer coating layer 402 is directly covering a metal element 401 . In FIG.4 a second thermoplastic polymer coating layer 403 is represented on top of the first thermoplastic polymer coating layer 402. The visual aspect of the surface of samples Ex7 and EX8 before application of the second or further thermoplastic polymer coating layers 1 , Ex7 and Ex8 can be described as snakeskin or sharkskin.

[0141] Comparatives examples CEx1 , CEx2 and CEx3 refer to rectangular copper wire were with section dimensions 3.7mm x 2mm and >0.3mm corner radius, while CEx4 refers to a steel wire with a round section, of diameter 1 ,2mm.

[0142] A sample obtained according to the conditions of comparative example CEx4 in table 3 was held in a zone of time and temperature referred to as zone B in FIG.1 . The surface roughness Ra, measured by means of a laser scanning confocal microscope was above 10pm and was found to be irregular, i.e. with more variation or a higher standard deviation compared to that of samples Ex3 to Ex8, which were produced according to the method of the invention.

[0143] The samples corresponding to comparative examples CEx1 and CEx2 were coated according to the method of the invention with a thermoplastic polymer having a melt density of less than 250Pa.s at 400°C, measured at shear rate of 1000s-1. Commercial grades 151 G from Victrex™, and Ketaspire™ 880NT from Solvay™ were found unsuitable for the method of the invention, because after ageing at 180°C for 336h the thermoplastic polymer coating showed multiple cracks below 5% elongation.

[0144] The samples corresponding to comparative examples CEx3 was coated according to the method of the invention with a thermoplastic polymer having a melt density of more than 250Pa.s at 400°C, measured at shear rate of 1000s-1. Commercial grade Ketaspire™ 851 NT from Solvay™ was used for CEx3. However, in step g) no holding time of 2s to 10s between [(Tm+Tg) / 2 - 40°C] and [(Tm+Tg) / 2 + 60°C] was used and the sample was cooled directly below 50°C, leading to a very low crystallinity or amorphous state of the thermoplastic polymer coating layer. In this specific sample the adhesion between the thermoplastic polymer coating layer and the metal element was bad.

[0145] In all samples Ex1 to Ex8 according to the invention, very good adhesion between the thermoplastic polymer coating layer and the metal element was measured.

[0146] To test adhesion, a strip of 10mm coating section from the metal element substrate was pulled through a calibrated opening and the stripping force was measured. Stripping forces above 40N / mm were measured in all samples Ex1 to Ex8.

[0147] Additionally, adhesion was tested according to IEC60317 standards, by means of elongation tests with incision through the polymer coating.

[0148] The PDIV of the different samples was measured according to the standards IEC 60664-1 and 61800-5-1.

[0149] To measure PDIV, lashed pairs of samples were made. The test voltage (AC 50Hz, RMS) was applied at one conductor of the lashed pair, while the other was connected to earth. The test voltage was gradually increased until partial discharge was registered with the measuring capacitor above a level of 10pC.

[0150] High PDIV values, above 800V were obtained in all invention samples.

[0151] The metal element having a polymer coating of the present invention, associated with a more cost-effective production process, is particularly suitable for use in hairpin wire for rotating or static parts of an electric motor.

Claims

Claims1 . Method to produce an insulated metal element, comprising the steps: a) Providing a metal element; b) Providing a thermoplastic polymer selected from the PAEK family, said thermoplastic polymer having a melt viscosity higher than 250 Pa.s, as measured according to ASTM D3835 at 400°C, shear rate of 1000s-1; c) Cleaning the surface of said metal element; d) Heating said metal element at a temperature between [Tm +20°C] and [Tm + 100°C], Tm being the melting temperature of said thermoplastic polymer; e) Applying said thermoplastic polymer on the surface of said metal element to obtain a coated metal element; f) Holding the obtained coated metal element at above [Tm + 20°C] for 0.5s to 60s, Tm being the melting temperature of said thermoplastic polymer; g) Cooling said coated metal element to between [(Tm+Tg) / 2 - 40°C] and [(Tm+Tg) / 2 + 60°C], with Tm the melting temperature of said thermoplastic polymer and Tg the glass transition temperature of said thermoplastic polymer; h) Stopping the cooling for between 1s and 20s; i) Quenching the coated metal element to a temperature below 50°C and above 0.5°C.

2. Method as in claim 1 said coated metal element is kept in a temperature holding zone during the holding step f).

3. Method as in claims 1 or 2 wherein additional heat is provided by a heating device to said coated metal element during the holding step f).

4. Method as in claim 3 wherein said heating device is an inductor, or an infrared heater, or a plasma generator.

5. Method as in claim 1 wherein steps c) to h) are executed in a production line wherewith said metal element is running at a linear velocity higher than 40m / min.

6. Method as in any of claims 1-5 further comprising the stepj) Applying a second thermoplastic polymer directly on the surface of said coated metal element with the provision that the surface of said coated metal element has a roughness Ra>1 pm after step i).

7. Insulated metal element comprising a metal element and a thermoplastic polymer coating layer obtained according to the method of claims 1-5, said thermoplastic polymer coating layer comprising or consisting of a thermoplastic polymer, said thermoplastic polymer having a melt viscosity higher than 250 Pa.s, as measured according to ASTM D3835 at 400°C, at shear rate of 1000s-1, said thermoplastic polymer coating layer is in semi crystalline state, the relative crystallinity is between 20% and 50% as measured by DSC according to ASTM D3418, said thermoplastic polymer coating layer has an outer surface roughness Ra between 0.01 and 0.25pm according to ISO 21920-2:2021.

8. Insulated metal element comprising a metal element and a thermoplastic polymer coating layer obtained according to the method of claims 1-5, said thermoplastic polymer coating layer comprising or consisting of a thermoplastic polymer, said thermoplastic polymer having a melt viscosity higher than 250 Pa.s, as measured according to ASTM D3835 at 400°C, at shear rate of 1000s-1, said thermoplastic polymer coating layer is in semi crystalline state, the relative crystallinity is between 20% and 50% as measured by DSC according to ASTM D3418, said thermoplastic polymer coating layer has an outer surface roughness Ra between0.25 and 0.4pm according to ISO 21920-2:2021.

9. Insulated metal element comprising a metal element and a thermoplastic polymer coating layer obtained according to the method of claims 1-5, said thermoplastic polymer coating layer comprising or consisting of a thermoplastic polymer, said thermoplastic polymer having a melt viscosity higher than 250 Pa.s, as measured according to ASTM D3835 at 400°C, at shear rate of 1000s-1,said thermoplastic polymer coating layer is in semi crystalline state, the relative crystallinity is between 20% and 50% as measured by DSC according to ASTM D3418, said thermoplastic polymer coating layer has an outer surface roughness Ra between 0.4 and 1 pm according to ISO 21920-2:2021.

10. Insulated metal element comprising a metal element and a thermoplastic polymer coating layer obtained according to the method of claims 1-5, said thermoplastic polymer coating layer comprising or consisting of a thermoplastic polymer, said thermoplastic polymer having a melt viscosity higher than 250 Pa.s, as measured according to ASTM D3835 at 400°C, at shear rate of 1000s-1, said thermoplastic polymer coating layer is in semi crystalline state, the relative crystallinity is between 20% and 50% as measured by DSC according to ASTM D3418, said thermoplastic polymer coating layer has an outer surface roughness Ra betweenl m and 10pm according to ISO 21920-2:2021.11 . Insulated metal element as in claims 7-10 wherein said thermoplastic polymer is selected from the PAEK family.

12. Insulated metal element as in claim 10 further comprising one or more additional thermoplastic polymer coating layers on top of said thermoplastic polymer coating layer.

13. Insulated metal element as in claims 12 wherein said one or more additional thermoplastic polymer coating layers comprise one or more thermoplastic polymer(s) different from the thermoplastic polymer of the first thermoplastic polymer coating layer or a blend of thermoplastic polymers different from the thermoplastic polymer of the first thermoplastic polymer coating layer.

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