EP1250195B1 - Method for producing insulations of electric conductors by means of powder coating - Google Patents

Abstract

A process for producing insulations for electrical conductors by powder coating, which results in aging which is improved compared to glass-mica or casting-resin insulation, and a powder which is suitable for such a process. The powder is applied a number of times in succession, in the form of individual layers which follow one another, until a layer thickness of <=10 mm is reached, and each of the individual layers undergoes intermediate heat curing before the next individual layer is applied. The intermediate curing of each individual layer uses a curing time which corresponds to 2-10 times the gel time of the powder used. Finally, the entire insulation undergoes final curing. The result of an electrical life test carried out on various specimens insulated with epoxy-resin powder which contains fine filler.

Description

Technical area

The invention relates to the insulation of electrical conductors of apparatus in the low to medium voltage range (i.e., up to about 50 kV) by powder coating. Likewise, the insulation in the high voltage range is possible, provided the conductors are not subjected to the full potential drop. The invention in particular relates to insulation of electrical conductors that are thermally and electrically loaded, such as insulation of electrical conductors or conductor bundles rotating electrical machines. Other examples of possible Applications are switchgear and transformers.

State of the art

The term "electrical aging" refers to the phenomenon that insulation under load has a finite lifetime which is in inverse proportion to the magnitude of the acting electric field. This relationship between lifetime and electric field strength is usually described graphically in the form of an aging curve. Very often, this curve can be described mathematically as power law, according to E = E 0 · t t 0 - 1 n . where E is the electric field in kV / mm, E _{0 is} the electric field at the lifetime t ₀ , t is the time in h, with t ₀ = 1 h, and n is the lifetime coefficient. In logarithmic representation of E and t, the above expression yields a straight line with slope -1 / n.

The life coefficient n may be characteristic of the type of insulation be designated. For example, for glass / mica insulation in electrical, rotating machines n = 7 to 9, for epoxy, cast resin insulation in switch construction n = 12 to 16 and for high-voltage cables, mostly insulated by extrusion n ≤ 35. Technically desirable is the lowest possible aging, This means a flat aging curve or the largest possible lifetime coefficient n, as it can be realized for example with cables.

The extrusion process used to make cable insulation is a continuous process, which is particularly suitable for the production of quasi-infinite, geometrically simpler structures. However, neither are the manufacturing process nor the materials used - mostly unfilled, pure Polyethylene - widely applicable. So can isolations of complex and small structures such as motor coils or interconnects in switchgear, can not be manufactured by this method. Likewise, the use of polyethylene for many possible applications not suitable because such PE isolations are used only up to about 90 ° C. can.

As a largely geometrieunabhängiges insulation process is the powder coating known. In contrast to extrusion, this isolation process is suitable even for very complex ladder structures. Theoretically, this could be a variety be isolated from medium voltage devices effectively and cost-effectively, for which the extrusion process is out of the question. At present there is a wide However, use contrary that with the known powder coating method and with the available coating materials no qualitatively sufficient Isolations are achievable.

The already known applications of powder coating are the insulation the single conductor of conductor bundles in generator construction, so-called Roebel rods, as well as the isolation of busbars. In both cases, the finished insulation but only weakly stressed. The tension between the individual conductors Roebel bars occurs, is at a few volts. Thus, the isolation itself with a layer thickness of the sub-conductor insulation of 50-200 μm electrically only weak loaded, i. with electric fields of E <1 kV / mm.

Both from US 4040993 and from US 4088809 d.ie the production of Epoxy resin known, by which electrostatic spraying or Spinal internally such a sub-conductor isolation can be generated. These insulations However, they are not suitable for high electrical loads from E> 3 kV / mm. In addition, with them only a small layer thickness of about 120 microns (<5 mils) realizable.

Because there is no counter electrode on the surface of the insulation, the Insulation in collecting machines also only weakly or not at all loaded. The electrical potential of the busbar is thus almost completely in the airspace mined above the layer. As a result, cavities in the epoxy layer interfere far less than in the present application. Try with a for Busbar coating powder used showed accordingly also an extreme content of holes.

The same applies to powders used to small electric motors or to provide parts of them with a thin layer of epoxy. This layer Primarily has the task of corrosion protection and is electric not or hardly charged.

Commercially available powders which meet the thermal requirements, but are electrically unsuitable. Such powders are mostly used for corrosion protection used in the field of chemical plant construction. The procedure for the preparation of such powders via hot blending, melting, cooling and Grinding corresponds to the general state of the art, as exemplified in US Pat US 4040993 is described.

Generally, with the known powder coating process for the production produced by electrical insulation layers with layer thicknesses d ≤ 0.1 mm (Powder paint). For the insulation thermally and electrically heavily loaded However, conductors are much larger layer thicknesses (e.g., d = 6 mm for 30 kV at a field strength of 5 kV / mm) and an improved lifetime coefficient is required.

Presentation of the invention

The invention seeks to avoid all these disadvantages. Its the task a method for producing insulation of electrical conductors by means of To provide a powder coating, which compared to glass mica or Giessharzisolierung has improved aging behavior.

According to the invention, this is achieved by using a method according to the preamble of claim 1, the powder up to a total thickness of the insulation of ≤ 10 mm successively, in the form of successive Single layers applied and each of the individual layers before applying the next single layer thermally cured. For intermediate hardening each single layer is maintained a curing time, which is 2-10 times the Gel time of the powder used corresponds. Completion is a final hardening the entire insulation.

For this purpose, a powder is used which contains at least one fusible and curable resin-hardener auxiliary system and at least one inorganic filler. In this case, the content of inorganic filler 5 to 50 weight percent, based on a closed density of the filler of up to 4 g / cm ³ . At least 3 percent by weight of the total mixture of the powder consist of fine filler with an average particle size d ₅₀ <3 microns. The remaining filler consists of coarse filler with a mean particle size d ₅₀ <30 μm. The course of the powder melting to a closed film is at least 25 mm and the gelation time of the melted powder is at least 40 s.

Due to the repeated application of thin individual layers of the powder and the subsequent thermal intermediate hardening of these individual layers arises on the one hand because of the associated reduction in blistering an insulation with a significantly improved quality and also clearly improved life coefficients, on the other hand, by applying additional single layers to the required for the particular application Layer thickness can be increased. Through the intermediate hardening reaches the respective outer layer a sufficiently high strength for applying the next single layer while still retaining enough untied Harder, to enter into chemical networking with the next single layer. Last but not least also contributes the composition of the powder, in particular the Inventive proportion of fine filler, to increase the service life of the insulation at.

Suitable coating method for applying the powder to be coated electrical conductors are the spray or vortex sintering or the thermal spraying of powder in the molten state. It can by a selection of resin-hardener adjuvant systems having a glass transition temperature of the thermosetting plastic of at least 130 ° C guaranteed be that isolation for all applications of medium voltage range can be used.

It is particularly advantageous for the intermediate thermal curing of the individual layers over a period of time which is 3-5 times the gel time of the one used Powder corresponds. In this way, with each single layer an optimal Relationship of strength and fortune, with the next single layer to enter into chemical crosslinking.

It is particularly useful if the individual layers with the lowest possible Layer thickness of ≤ 0.5 mm up to an optimum layer thickness of 0.2 mm are applied. This way, a complete, high quality Coating even complex surfaces as well as one for thermal and electrically highly loaded conductor suitable layer thickness can be realized.

Alternatively, either only single layers with a uniform Layer thickness or single layers of different layer thickness in any Order are applied to the electrical conductors to be insulated. moreover Can be used to apply single layers of powder of different composition be used. This makes it possible to produce an insulation which meets the expected requirements according to the conditions of use the insulated electrical conductor is fair.

1. Die Isolierung soll einsetzbar sein bis hin zu Wärmeklasse H, d.h. T_max = 180 °C im Dauerbetrieb. Da in der Elektrotechnik üblicherweise eine Wärmeklasse als Sicherheitsreserve verlangt wird, soll die Isolierung den Anforderungen der Wärmeklasse C, d.h. T_max = 205 °C genügen. Normalerweise gilt diese Anforderung als erfüllt, wenn der Temperatur-Index (TI) > Betriebstemperatur (T_op) ist. Über die Bestimmung des TI gibt die Norm IEC 218 Auskunft.

2. Die Isolierung soll im Dauerbetrieb elektrisch stark belastbar sein, d.h. mit E > 3 kV/mm, insbesondere E ≥ 5 kV/mm. Als Feldstärke E wird hier die effektive Wechselspannung U_eff, dividiert durch die Dicke d der Isolierung auf der Flachseite des Leiters bezeichnet, also E = U_eff/d. Mit E = 5 kV/mm und einer angestrebten Maximalspannung von 50 kV ergibt sich, dass die Isolierung in Dicken bis zu 10 mm herstellbar sein soll.

3. Geringe elektrische Verluste (Richtwert tan δ < 0.3) bis hin zur Maximaltemperatur, da sich die Isolierung bei E=5 kV/mm und grösseren dielektrischen Verlusten selbst aufheizt und ein Versagen durch Wärmedurchschlag auftreten kann.

4. Weitestgehend frei von Hohlräumen (meist Gaseinschlüsse), welche bei Betrieb zu elektrischen Teilentladungen (TE) und frühzeitigem dielektrischen Versagen führen können.

5. Resistent gegen TE oder Oberflächenentladungen kleiner Energie. Dadurch wird das Isoliersystem fehlertolerant gegenüber begrenzten Qualitätsschwankungen.

6. Frei von scharfkantigen leitfähigen Einschlüssen (z.B. Metallspänen), welche zu lokal stark überhöhten Feldern und ebenfalls zu frühzeitigem Versagen führen.

The most important requirements for the finished insulation are the following:

1. The insulation should be usable up to thermal class H, ie T _max = 180 ° C in continuous operation. Since in electrical engineering usually a thermal class is required as a safety reserve, the insulation should meet the requirements of the thermal class C, ie T _max = 205 ° C. Normally, this requirement is considered met when the temperature index (TI) is> operating temperature (T _op ). The standard IEC 218 provides information on the determination of the TI.

2. The insulation should be electrically strong in continuous operation, ie with E> 3 kV / mm, in particular E ≥ 5 kV / mm. The field strength E here is the effective AC voltage U _eff , divided by the thickness d of the insulation on the flat side of the conductor, that is to say E = U _eff / d. With E = 5 kV / mm and an aspired maximum voltage of 50 kV it follows that the insulation in thicknesses up to 10 mm should be produced.

3. Low electrical losses (guideline tan δ <0.3) up to the maximum temperature, since the insulation heats up itself at E = 5 kV / mm and greater dielectric losses and a breakdown by thermal breakdown may occur.

4. Largely free of voids (usually gas inclusions), which can lead to electrical partial discharges (TE) and premature dielectric failure during operation.

5. Resistant to TE or surface discharges of low energy. This makes the isolation system fault-tolerant to limited variations in quality.

6. Free from sharp-edged conductive inclusions (eg metal shavings), which leads to locally greatly inflated fields and also to premature failure.

Special properties of the powder can be taken from the dependent claims become.

Short description of the drawing

The only figure shows the result of an electrical life test various Test specimens, isolated with feinfüllerhaltigem applied according to the invention Epoxy resin powder, with horizontal life in hours, vertically the Field strength in kV / mm are shown.

Way to carry out the invention

The polymer-based powder according to the invention contains at least one non-crosslinked system consisting of resin, hardener and auxiliaries, as well as electrically insulating inorganic fillers. The auxiliaries influence, for example, the curing time or the process, it being possible to use auxiliaries known from the prior art. Electrically insulating inorganic fillers to about 50 weight percent in amounts of about 5 on fillers with a closed density contain up to 4 g / cm ³ of. The filler is either entirely as a fine filler with a mean particle size d ₅₀ <3 microns, especially d ₅₀ <1 micron, more preferably d ₅₀ between 0.01 and 0.3 microns, or as a mixture of fine filler and coarse filler with d ₅₀ <30 microns, in particular between 3 and 20 microns, before. The proportion of fine filler in the total mixture of the powder should be at least 3%, in particular at least 5%, and the polymer to be formed from resin and hardener should be a thermoset having a glass transition temperature of at least 130 ° C. in the crosslinked state.

Preferred fine fillers have a mean diameter d ₅₀ of about 0.2 microns, although finer fillers can be used, which has a positive effect on the corona resistance but negative effect on the flow properties (thixotropy) of the molten insulating material.

Preferably, the total filler content is about 40%. If the filler has an average closed density greater than 4 g / cm ³ , the limit and preferred values given hereinabove and below may be higher.

The fine filler and coarse filler may be different materials have different hardness. It is also within the scope of the present invention that the fine filler or the coarse filler or the fine filler and the coarse filler Mixtures of fillers of equal or different hardness are.

In order to prevent abrasion during the production of the insulating material or its processing for insulation, which is essential in particular in today's conventional use of steel or carbide equipment in the compounding and milling of the insulating material, the coarse filler must have a Mohs hardness, preferably at least one hardness unit below that of steel and hard metal (Mohs hardness of about 6). When using hard fillers, such as quartz powder (hardness 7), the processing leads to metallic abrasion, preferably in the form of chips in the sub-mm range. These are incorporated into the insulation and, due to their needle-like geometry, lead to points with a locally very greatly increased electric field strength, from which experience has shown that electrical breakdown can be triggered. Microscopic investigations revealed a surface density of such metallic particles of 1-3 / 100 mm ² when using SiO ₂ as coarse filler.

The abrasion is avoided by using "soft" fillers (Mohs hardness ≤ 4) such as chalk meal and / or by using finer fillers with d ₅₀ << 1 .mu.m. Such fine fillers moreover have the advantage that they can prevent or at least greatly retard the electrical breakdown even in the presence of defects such as cavities or metallic inclusions (see in this regard US Pat. No. 4,760,296, DE 40 37 972 A1). In these two documents, the life-prolonging effect is achieved by complete or partial replacement of the coarse filler by fillers with particle sizes in the nanometer range (0.005 to 0.1 microns maximum grain size). Nanofillers, however, have the unpleasant property of greatly increasing the melt toughness of the powder mixture (thixotropic effect). This interferes with both the production of the powder and its processing. For the present application it has been found that TiO ₂ powder with average grain sizes of about 0.2 microns as a complete or partial replacement for coarse filler does not lead to an adverse increase in melt viscosity and still has the life-prolonging effects in the nature of nano-fillers. In this way, insulation with low electrical aging could be realized.

To avoid metal abrasion, it would also be possible, all contact surfaces for Insulating material to be provided with a protective coating, e.g. with a ceramic coating, or certain means of production e.g. made of ceramic. Such a Replacement or partial replacement of metal parts is currently very expensive. Although the abrasion at e.g. Ceramic surfaces the electric field and thus the insulating effect Nevertheless, the rule that the coarse filler has a hardness is still valid which should have at least about a Mohs' hardness below that of the means of production or container, i. in a ceramic coating a hardness of usually about 8 with a maximum Mohs' hardness of about 7th

The electrically insulating inorganic fillers are preferably selected from carbonates, silicates and metal oxides, which may also be present in the form of minced minerals. Examples of such fillers are, for example, TiO ₂ , CaCO ₃ , ZnO, wollastonite, clay and talc, with TiO ₂ , ZnO and clay especially as fine filler and CaCO ₃ , wollastonite and talc having particle sizes of about 10 μm (average particle size d ₅₀ ) being especially specific are suitable as coarse filler.

Fillers of the desired grain size can be obtained in various ways be, e.g. by special precipitation methods, combustion processes, etc. but also by mechanical comminution, all of these methods being optional can be coupled with a fractionation or sieving process.

The risk of abrasion through the use of hard fine filler is far less critical because fine-grained abrasives are generally much less effective as a coarse abrasive.

The presence of at least 5 weight percent filler and at least 3 weight percent, preferably at least 5 weight percent filler is essential because the filler is electrically insulating, increases mechanical strength, improves thermal conductivity, lowers the coefficient of thermal expansion, increases UV resistance and contributes to viscosity adjustment , The fines filler is also essential for increasing corona resistance, while the coarse filler allows for an increase in filler content with less increase in viscosity than would be the case with fines. Filler contents above 50 percent by weight based on fillers with a closed density of up to 4 g / cm ³ and a maximum grain size of 20 microns and too high Feinfüllergehalte are critical, since problems occur due to excessive viscosity both in the production of the insulating material and in its processing ,

Preferred thermosets for the matrix of insulating materials of the present invention in the cured state have a glass transition temperature of 130 ° C - 200 ° C, preferably 150 ° C - 180 ° C.

Since the inventive insulating material for a good insulation, as they are for the preferred applications is required, bubble-free or at least as much as possible should be bubble-free, should the resin-hardener excipient system of thermoset be such that it hardens without release of volatile substances.

In order to avoid bubbles during curing, it is also preferred that the resin-hardener adjuvant system has a gel time, which at best in it or at the to be coated Surface adsorbed water or other volatile substances allows to escape from the insulating layer before this too much solidified, so that at best emerged pores at this exit respectively Can close bubbles.

The mixture of resin, hardener and organic additives should have a melting point have a maximum of 200 ° C, in which it is essential that the Melting point is below the activation temperature of the curing reaction, or that the curing reaction proceeds very slowly at the melting temperature, and can be stopped substantially when cooled. This is necessary to to prevent a far-reaching hardening already in the production of the insulating material. The curing properties can be adjusted by adding suitable substances It must be ensured that such substances are nonvolatile or completely outgassed within the gel time. Preferably, the mixture has made of resin, hardener and organic auxiliaries a melting point of at least 50 ° C, in particular from 70 ° C - 120 ° C. In exceptional cases, the Melting point of resin and / or hardener at up to about 200 ° C. Such a high one Melting point, however, is because of the activation of the curing reaction, which is usually in a similar, if not deeper, area is problematic. Curing is usually carried out in a temperature range from 70 ° C to 250 ° C, preferably in a range of 130 ° C to 200 ° C.

To meet the high demands on the glass transition point of the thermoset To be able to, it is preferred that the thermoset is highly cross-linked, respectively has a high crosslink density. A preferred thermoset is a Epoxy resin. Epoxy resin is i.a. preferred because both the carboxylic acid anhydride as well as the amine curing without release of volatile substances from the Resin resp. the hardener takes place. Furthermore, epoxy resin is usually crosslinking and the crosslink density can be increased by using di- or polyanhydrides as curing agents or polyamines and / or as a resin multifunctional, branched-chain Epoxy resins are used. To lower the volatility of the components and to increase the glass transition point are aromatic group-containing Resins and / or hardener preferred.

As already indicated above, the inventive insulating material additives contain auxiliaries, such as activators, accelerators, pigments etc., such substances are preferably low volatility.

For some applications of the new insulation, in particular in the field of rotary electric machines, a use of the insulation in thermal class H (T _max = 180 ° C) is necessary. For this, the glass transition temperature (T _g ) should be in this temperature range, preferably between 130 ° C and 200 ° C. Glass transition temperatures significantly higher than 200 ° C on the one hand difficult to realize and on the other hand lead to a material that is quite brittle in the room temperature. In order to meet the requirement of mechanical stability in class H, in addition to a T _g in the region of 180 ° C., the filler content is important, which should be> 10% by volume with such high requirements, which is about 23% by weight at a closed density of 4 g / cm ³ equivalent.

An insulation for the medium voltage and lower high voltage range thermally and electrically highly loaded electrical conductor is preferably characterized manufactured, that the electrical conductors to be coated at least partially be covered with an inventive insulating material, whereupon the Insulating material to a temperature above the melting and activation temperature for the curing of the resin-hardener excipient system of thermosetting and brought held there until gelation. The application of the powder can on different Species occur, e.g. by spraying with and without electrostatic charging or in a fluidized bed.

The above-mentioned freedom from bubbles is due both to the choice of process control as well as determined by different material properties. It's important, that the insulating material has a sufficiently low viscosity in the liquid state, to go well, and that the gel time is long enough for all the bubble-forming Admixtures (e.g., adsorbed water) may evaporate. This requirement after long gel times, the trend of powder coating is opposite, which to achieve high throughput times in thin-film painting the gel times deliberately set low by adding accelerators (typically 15 seconds (s)). By reducing the proportion of accelerators, however, can be the gel times of commercial powders without difficulty at times of ≥ 60s, preferably 80-160s, which are sufficient for the present application are long. The viscosity of spray powders is usually not considered separate Size measured and specified; but instead the so-called process, which results from viscosity and gel time, specified. bubble-free Layers are achieved when the drain> 25 mm, preferably 30-50 mm, is.

To blistering at best on the surface of the to be coated electrical conductor or volatiles present in the insulating material (e.g. adsorbed and absorbed water) in addition to minimize and preferably To completely prevent it, a stratified order of isolation has emerged extremely advantageous, the thickness of a single layer being 0.05-0.3 mm, preferably 0.2 mm.

To build up layers with d> 0.2 mm, the application of the individual layers repeated until the desired layer thickness. After every shift application The system consisting of resin, hardener, auxiliaries and fillers is accordingly its gel time approx. 60 - 300 s tempered, whereby it for melting, to Water release and partial hardening comes. In addition, by the Use of different powder compositions locally different passages within the single layers or locally different layer thicknesses of the entire insulation are generated. In this way, the insulation can be optimally adapt to the surface to be coated.

embodiments Example 1:

An epoxy resin powder which contains 40% by weight of TiO ₂ having an average particle size d ₅₀ = 0.2 μm was used to apply insulation of d = 0.5 mm to Cu plates of 200 mm × 200 mm. The powder was not optimized for slow gel times and therefore contained bubbles with diameters up to 0.3 mm in diameter. Electrodes of 80 mm diameter were applied to the plates. Subsequently, the samples were aged at 16 kV / mm under oil. Due to the bubbles, the samples were partially discharged (TE) during the test. After 2600 hours (h), the tests were stopped without a breakdown being observed.

In the counterexample, quartz flour with d ₅₀ = 10 μm was used as the filler. None of the samples reaches a lifetime of more than 1 h in the aging test.

Example 2:

Cu profiles with lxbxh = 600 x 15 x 50 mm and edge radius 2.5 mm were coated with epoxy powder (with TiO ₂ filler 35%) and a drain of 50 mm. The layer thickness was 0.5-1 mm. Except for a few and very small bubbles (<50 microns), the insulation is completely void-free, as revealed by microscopic examinations on sections. The TE field strengths, defined by the detection of a TE level of> 5 pC, were 18-25 kV / mm. The tan δ of the material remained below 10% in the range from room temperature to 200 ° C, so that only small electrical losses occurred.

Example 3:

Like example 2, however, 35% CaCO ₃ with d ₅₀ about 7 μm and only 5% fine filler (TiO ₂ ) were used as fillers. The results of the TE measurement were the same as in Example 2

Example 4:

The test pieces manufactured in 2 and 3 were subjected to an electrical life test subjected. The result of the test is shown in the single figure. It exists no significant difference with regard to the two types of fillers. A big part The data points shown correspond to samples that have not yet been broken through are; the final achievable life curve is even flatter than that shown in the figure. In cases where there was a breakdown, This was usually at the edge of the profile, where the specified field strength by a factor of 1.7 compared to the homogenous field strength (related stress U / d with d = layer thickness) is excessive (in the characteristic shown is this Field elevation factor not yet included). The lifetime characteristic is extremely flat, which means that the material has a low electrical Aging undergoes and the permanent field strength, the expected lifetime of 20 years, not significantly lower than those in the short-term test measured breakdown field strength. The lifetime coefficient n was about 33.

Example 5:

With epoxy resin powders containing 40% TiO ₂ fine filler, an insulation of 10 mm total thickness was prepared in 56 layers.

Claims (9)

1. Process for production of insulation for electrical conductors by means of powder coating, based on thermoplastics, characterized in that

   a) the powder is applied a number of times successively, up to a total insulation thickness of ≤ 10 mm, in the form of successive individual layers,

   b) each of the individual layers is thermally intermediately cured before the application of the next individual layer,

   c) during the intermediate curing of each individual layer, a curing time is complied with which corresponds to 2-10 times the gelling time of the powder being used,

   d) the final curing of the overall insulation is carried out.
2. Process according to Claim 1, characterized in that the thermal intermediate curing is carried out over a time period which corresponds to 3-5 times the gelling time of the powder that is used.
3. Process according to Claim 1 or 2, characterized in that the individual layers are applied with a layer thickness of ≤ 0.5 mm.
4. Process according to Claim 3, characterized in that the individual layers are applied with a layer thickness of ≤ 0.3 mm, in particular with a layer thickness of 0.2 mm.
5. Process according to one of Claims 1 to 4, characterized in that only individual layers with a uniform layer thickness are applied.
6. Process according to one of Claims 1 to 4, characterized in that individual layers with different layer thicknesses are applied.
7. Process according to one of the preceding claims, characterized in that powders of different composition are used for application of the individual layers.
8. Process according to one of Claims 1 to 7, characterized in that the powder is applied by means of spray or swirl sintering.
9. Process according to one of Claims 1 to 7, characterized in that the powder is applied in the liquid state by means of thermal sprays.

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