Assembly and method for the electrophoretic dip coating of sheet material

Abstract

The invention relates to an assembly and a method for the electrophoretic dip coating of sheet material, wherein at least a portion of the total volume of the paint bath (LB) is formed in such a way or electrically insulated from other bath areas in such a way that the formed or the insulated portion of the paint bath (LB) forms an electrical ohmic specific resistance through which the substrate (S) passes, and which has a ratio of path length and cross-sectional area of the paint bath (LB) in the region of an entry section (ES) which is so high that the electrical field voltage forms at least one zero point along the entry section (ES), and / or a further formed or insulated portion forms an electrical ohmic specific resistance through which, over a part of its length, the substrate (S) passes in the feed direction thereof on its way from the position of the counter electrode to an emergence position, remote from the counter electrode, as an exit section (AS), and which, over another part of its length, is connected to the entry section (ES) so as to form an electrical return path between the entry section and the exit section (ES, AS).

Description

[0001] Arrangement and method for electrophoretic dip coating of web material

[0002] The invention relates to an arrangement and a method for the electrophoretic dip coating of web material according to the preamble of the first patent claim.

[0003] This involves the electrophoretic lacquer coating of web material, preferably wires, foils, or sheets, whereby the application of permanent electrical surface insulation or an anti-corrosion coating may be intended. A preferred area of ​​application is the production of lacquer-insulated winding wires, such as those used in the manufacture of electrical coils. Such coils are used as electronic components, in transformers, or in electrical power machines. Further application examples include sheet metal coils for transformer, stator, or rotor laminations in electrical machines and for the manufacture of housing parts or the coating of capacitor foils. The particular suitability for electrical surface insulation of all types is noteworthy because the invention also enables the selective correction of any defects.

[0004] The most advanced technology to date for the application of electrophoretic dip coating in the described field of application is known from the publication DE 10 2021 001 741 A1. It demonstrates how a coating on web materials can be advantageously removed using cathodic dip coating (CED) methods and describes a suitable setup. The method proposed therein differs from the previously known CED of piece goods primarily in its continuous process, in which the web material is conveyed through the coating bath at a constant speed, while a film layer is deposited on the substrate in the presence of an electric field and cured in a subsequent heat treatment process on the same production line. However, in practical application, the following problems arise that cannot be adequately solved with the currently known means and methods:

[0005] 1) Paint defects occur, e.g., in the form of bubbles, which are caused by the immersion process of the substrate surface continuously fed into the paint bath with the anode field permanently applied. This results in high partial current densities with spontaneous reaction of the paint colloids at the immersion point, even before uniform wetting of these areas can occur. Due to the spontaneity of the locally violent coagulation, the replenishment of fresh paint molecules by flow movement at this point is also problematic.

[0006] 2) When the web emerges from the coating bath, in addition to the coating film coagulated on the substrate surface by electrophoresis, coating components that are actually still in solution are also carried out of the bath. Because the web inevitably has to be guided over deflection rollers, this unwanted drag-out material subsequently compacts, dries, and caks. These unwanted particles cannot be completely removed in the subsequent rinsing bath and are subsequently cured. Residues also accumulate on the rollers, some of which later detach and adhere to the substrate. All of this leads to a partially greater and uneven coating thickness on the product, which noticeably reduces its quality.The use of mechanically acting scrapers, as suggested in the aforementioned document, is possible in principle, but leads to the continuous buildup of solid paint components and their drying at the contact point. These deposits then have to be constantly removed, which is not conducive to a continuous process with high paint utilization and the avoidance of waste products.

[0007] 3) At high feed rates of the substrate through the coating bath, as desired for good productivity, film formation can be impaired because a diffusion-controlled boundary layer of sufficient thickness can no longer form under such conditions. This relationship between the movement of the substrate and the depositability of electrocoating coatings is known, among other things, from [Beck F.: Elektrotauchlackierung an der rotating Scheibenelektrode. - Chemie- Ing.-Techn. 40 (1968) 12 pp. 575-581]. This, in turn, means that the possible feed rate of the web material is physically limited without further measures.

[0008] The problems mentioned are specific to the continuous electrophoretic dip coating of web materials, whereas they are of little or no importance when processing piece goods. With piece goods, it is possible and common practice to reduce or switch off the voltage of the substrate when the workpieces are immersed and ascending. For example, a workpiece, e.g. a vehicle body, can be pulled through the bath on conductor rails, passing rail sections that are subjected to different voltages, so that a gradual switching of the voltage between the anode and cathode takes place along the passage path, as stated, for example, in DE 196 06 000 C1. Alternatively, the anode or cathode voltage can also be continuously varied synchronously with the process sequence of immersion, dwell, and ascending of workpieces or workpiece carriers, provided that only one batch, e.g.a workpiece carrier, is treated in an immersion bath or can be subjected to voltages independently of others. Neither method is applicable when processing web materials because they necessarily have a consistently uniform electrical potential, which cannot be interrupted and can be supplied with voltage individually. DE 10 2021 001 741 A1 instead proposes providing anode sections with different potentials. This is problematic in that sections with a lower potential partially become cathodes compared to those with a higher potential and thus cause cathodic reactions up to and including film formation on their surface, which, however, must not take place there but only on the substrate to be coated. Dripping from drip edges and subsequent multiple rinsing after the coating process, as is known from piece goods, is also not feasible in the same way with web materials.

[0009] Special designs of anodes or anode cells are known for distorting the electric field in the immersion bath to achieve preferential coating of certain regions of the substrate, or to disadvantage other regions that would be more heavily coated due to their geometry. For example, EP 1 201 791 A2 proposes an anode cell in which the effective area is reduced by means of a metallic shield. However, the metallic shield should preferably be covered with an insulating layer to prevent a cathodic effect. This raises the question of whether such a shield can even be effective in the manner described.Because the dip coating in which the processes take place has a conductivity orders of magnitude higher than such an insulation layer, at best a polarization can be expected there, but no effective influence on the field distribution in the bath due to the electrical potential of the shielding, as stated.

[0010] Furthermore, despite the many advantages of e-coating, it may be advisable to use anodic dip coating (ADC) instead of cathodic dip coating. This could be necessary, for example, due to special paint formulations that can only be produced for e-coating, as proposed in US Pat. No. 3,953,310 A for the multi-layer coating of insulated winding wires with an e-coating primer made of polyacrylic.

[0011] The object of the invention is to develop an arrangement and a method for the electrophoretic dip coating of web material, with which the disadvantages mentioned under 1) to 3) are eliminated and which is equally applicable to both KTL and ATL processes.

[0012] In particular, the immersion process is intended to achieve uniform surface wetting in the absence of an electrical current flow in this process area and a subsequent increasing current density curve along the path of the substrate through the paint bath.

[0013] Furthermore, a stripping process without mechanical contact with solid components should be enabled when the web emerges from the coating bath, preventing paint deposits on such components. To achieve this, the physical conditions for separating the paint solution from the film layer should be improved.

[0014] A solution must be found for high feed rates to prevent possible impairment of the film formation process.

[0015] In addition, process reliability and efficiency are to be improved by automatically adjusting the process parameters, for which special solutions for the acquisition and processing of suitable, continuously acquired measurement data are to be proposed.

[0016] This problem is solved by the features of the first and tenth patent claims. Advantageous embodiments emerge from the subclaims.

[0017] The following terms are agreed:

[0018] Web material is the term used to describe metallic or metallized raw material which is to be painted on its surface and which is of such a length that it cannot be painted over its entire extent at the same time, and which is preferably flexible enough to be provided to the painting process on unwinding reels or coils and removed again on take-up reels or coils.

[0019] When describing the coating processes themselves, the uncoated web material is referred to as the substrate, which is connected to an electrical reference potential, preferably zero or ground potential. One or more counter electrodes create an electric field relative to the reference potential and are either subjected to a more positive electrical potential as an anode or a more negative electrical potential as a cathode than the substrate. The terms "counter electrode," "anode," or "cathode" can also refer not only to a mere electrode, but also to a dialysis cell of a known design with a semipermeable membrane that surrounds the counter electrode but creates an electric field relative to the substrate as a whole in the same way.

[0020] The dip coating bath, also abbreviated to paint bath, or simply referred to as bath, contains the electrocoating paint and is passed through by the web material, where it is exposed to the actual film formation process, as is basically known from electrophoretic dip coating.

[0021] The entire paint bath volume is located in a bath vessel with a specific shape, the so-called bath geometry, which determines the spatial shape of the paint bath not only through external but possibly also through additional internal vessel shapes.

[0022] The entire passage from the immersion position in the paint bath to the emergence position from the paint bath, which may also include several changes of direction of the web material due to deflection rollers, is divided into three sections in the feed direction of the web material. In the order in which they pass through, these are referred to as the first inlet section, second the electrode chamber or, specifically for e-coating, the anode chamber, and third the outlet section. The electrode chamber is located between the counter electrode and the substrate, beginning in the feed direction with the front edge of the (first) counter electrode and ending with the rear edge of the (last) counter electrode. The inlet and outlet sections adjoin the electrode chamber before and after it in the feed direction.

[0023] To improve the readability of the description, all explanations are based on the example of a cathodic electrocoating (KTL). For an electrocoating (ATL), the potentials and charges are simply reversed: positive voltages become negative; the substrate is no longer connected as the cathode but as the anode; negative charge carriers become positive; and the function of the counter electrode changes from anode to cathode.

[0024] As already stated, one object of the invention is to reduce the electrical currents acting on the substrate both during immersion in the lacquer bath and upon exiting it. For this purpose, a suitable field distribution was created along the path of the substrate through the lacquer bath. The importance of current-free immersion has already been explained. However, a reduction in the electrical field before the substrate emerges from the lacquer bath is also very important. Thus, it was identified that the cause of the drag-out of non-crosslinked lacquer components is an unavoidable cohesion in the transition layer between the lacquer film and the lacquer colloids still in solution, which is intensified under the influence of a positive electrical field because the positively charged colloids in the lacquer solution acquire an additional force of attraction towards the cathode.After reaching saturation during film formation and the current density along the substrate surface decaying in the feed direction, a polarization occurs in the surrounding electric field, pulling the positively charged particles toward the cathode. This makes it difficult to wipe off the coating solution upon leaving the coating bath. Therefore, the solution according to the invention weakens the electric field at the exit point to such an extent that no significant polarization occurs.

[0025] Furthermore, according to the invention, the electrical conductivity of the paint bath is used to create precisely definable resistance paths, where a voltage drop is generated by the required current flow. This then leads to a specific potential profile within the paint bath along the direction of travel of the substrate. Furthermore, no additional electrodes are used to form the electric field and achieve a defined current density distribution along the substrate. Instead, ohmic volume resistances are specifically formed in the paint bath itself, and the current flow required anyway between the counter electrode and the substrate is technically utilized to create voltage differences at such volume resistances.

[0026] Due to its freely moving charge carriers, the paint bath has a specific ohmic conductivity, which can also be assumed to be approximately constant within constant bath parameters. Conductometric measurements on common paint formulations yield measured values ​​in the order of 1000 / jS / cm. Therefore, the paint bath essentially represents a volume conductor with a defined electrical resistance, which can be geometrically shaped as required and thus its electrical properties can be determined. For example, it is sufficient to increase the length of the paint bath or reduce its cross-sectional area to increase its resistance value.

[0027] Furthermore, it was observed that the resist film deposited on the substrate has a specific resistance of approximately 10 5times as large as that of the resist bath, so that even with film thicknesses of just a few micrometers, a contact resistance relevant for the formation of the electric field arises. This leads, for example, to almost the entire anode voltage dropping across the film layer in the area beneath the anode, while an almost constant field potential prevails in the surrounding resist bath. As measurement results show, it can also be assumed that the average specific resistance of the coagulated resist film depends little or not on its thickness. Thus, the cutting plane through the resist bath, which runs along the anode boundary, as well as any other cutting plane outside the anode that runs perpendicular to the direction of travel of the substrate, can be treated as an equipotential surface. While a field voltage that can be assumed to be constant acts beneath the anode, it decreases outside this area, as will be shown later.

[0028] Based on the model presented, the paint bath liquid is used according to the invention as an electrical volume resistivity with its nearly constant specific conductivity. Electrical resistances are formed using a suitably designed bath geometry, through the insertion of insulating walls, tubes, or chambers. To achieve high resistance values, the bath sections in question are to be guided in elongated vessels with a small cross-section, causing them to behave almost like linear conductors with a defined specific resistance. For low resistance values, however, particularly for forming electrical bridges or bypasses, shorter distances and larger cross-sections are preferred, as far as technically feasible.

[0029] Simple resistance paths, which embody a two-terminal network with two external "connections," can be formed, for example, in the form of a channel in an insulating piece of pipe. In such cases, the resistance can be easily calculated using equation (1a), see below, and integrated into a resistance network of other such elements in series or parallel circuits, just like an electrical component. Hydraulic connections between parts of the paint bath therefore always lead to electrical "contacts" at these points. In this way, bypasses, return lines, or other channels can be connected to the paint bath at suitable points; these then fulfill an electrical function, although not necessarily a hydraulic one. The fact that a hydraulic connection is also created in this process is generally not a problem.

[0030] Preferably, the invention provides for volume resistors formed in this way not only to be used as electrical two-terminals, but also to be directly traversed by the substrate, with traversal in the direction of their longitudinal extension along a developing voltage gradient being of particular interest. Because the substrate absorbs a transverse electrical current that depends on the partially distributed film formation, this results in a current density function distributed along the feed direction with a simultaneous increase in the deposited film thickness. This embodiment is particularly interesting along the inlet section and will be explained in more detail later.

[0031] But such volume resistances can also be coupled as two-poles away from the movement paths of the substrate, e.g., to form connections between different areas of the paint bath and thus equalize potentials.

[0032] According to the invention, two basic forms of electrical paint bath resistors are preferably used:

[0033] 1) Elongated vessels, tubes, or chambers that are part of the coating bath form a higher-resistance inlet path for the substrate before it passes through the actual anode chamber and / or an outlet path before it leaves the coating bath. In both cases, the goal is to reduce the field voltage outside the anode chamber with increasing distance, either in the opposite direction to or in the direction of advance.

[0034] 2) Arrangement of two-pole, preferably low-ohmic, electrical resistors in the form of vessels, tubes, or chambers filled with paint bath fluid. Due to their geometry, they meet the required resistance values. These resistors are additionally integrated into the hydraulic system of the paint bath as an electrical bridge or bypass, without having to fulfill a mandatory hydraulic function. In the following text, such regions will generally be referred to as return lines. It may also be useful to use a different electrolyte or dialysis fluid instead of the paint bath fluid in parts of such a system, and to separate these from the paint bath by means of a semipermeable membrane in such a way that ion exchange occurs but no direct mass transport across these boundaries.

[0035] In essential contrast to any type of electrode or other electrically active element that could be placed in the paint bath to influence the field, the resistors formed with the paint bath liquid itself do not cause charge reversal or ionization effects at interfaces because no interfaces are formed. To generate voltage drops across these resistors, the current between the anode and the substrate, which is appropriately redirected and modified across them, is sufficient.

[0036] While the inlet and outlet sections are used to gradually change the field voltage for the substrate as it is guided through the paint bath, a return line is primarily suitable for reducing voltages on the substrate before it leaves the paint bath. This is because the outflowing substrate is not very suitable as a potential sink due to the paint film already formed there having only a low conductance. In contrast to the inlet section, an increased substrate current density cannot be used for voltage division because the substrate in the outlet section no longer allows any significant cathode current. Thus, without further measures, almost the full anode potential would still be present at the emergence position. To ensure that a current can still flow and the voltage can be reduced without additional auxiliary electrodes, the only option is the additional use of the inlet section, namely where the conductance is still high.In order to be able to electrically connect both areas, such a short circuit between the outlet and inlet sections is suitable and necessary.

[0037] With a stationary substrate and a fixed anode voltage, the anode current would converge towards zero over time because the film layer grows, thus also continuously increasing the electrical resistance and developing saturation, at which point further film deposition comes to a halt. In this case, generating voltage drops in the formed resistor networks using the method described would also be difficult. However, this is not the intended mode of operation, because the task requires a continuous pass. If the substrate is continuously advanced, fresh and bare substrate surfaces are constantly fed into the coating bath. With a constant feed and film formation process, an anode current of approximately constant strength can also flow continuously, leading to the desired voltage drops in the volume resistances.

[0038] As demonstrated, it is possible to create a suitable current density distribution along the substrate in the inlet section by using a geometrically elongated, and thus high-resistance, design of the paint bath, e.g., in the form of a slender tube, and by locally limiting the anode area in the paint bath. This ensures that no spontaneous current flow occurs, especially during immersion. If a resistance value in the paint bath is to be further increased under spatially restricted conditions, the insertion of a labyrinth of insulating discs and ribs is also recommended, reducing the cross-sectional area in favor of the length.

[0039] If the substrate passes through an inlet section which is shaped to a volume resistance before it reaches the immediate anode chamber, a controlled increase of the voltage field in the feed direction is achieved.

[0040] According to the method, the electrophoretic dip coating of web material according to the invention is carried out in a system as described above, comprising a paint bath through which the web material in the form of the substrate to be coated passes at a continuous feed rate, wherein an electric field is formed between the substrate and at least one counter electrode, which leads to the deposition of a paint film on the surface of the substrate during the pass, wherein at least a part of the paint bath forms an electrical ohmic volume resistance through which the substrate passes as an inlet section in its feed direction on the way from an immersion position to a position of the counter electrode remote therefrom,

[0041] - wherein an electric field voltage builds up across this volume resistance in interaction with a temporally and spatially progressive film formation around the substrate, which forms at least one zero point along the inlet section and / or

[0042] - wherein the part of the paint bath which forms an electrical ohmic volume resistance is traversed over part of its length by the substrate in its feed direction on the way from the position of the counter electrode to an emergence position remote therefrom as an outlet section, wherein the paint bath is connected over a further part of its length to the inlet section in such a way that an electrical return connection between the inlet and outlet sections takes place via this further part of the paint bath (LB).

[0043] The substrate coated with the system and the process has a very uniform layer that adheres well to the substrate. Flat wires, round wires, and other continuous metallic materials, such as strip material, can be coated. Particularly in the lacquer coating of flat wires and micro-flat wires in various dimensions, e.g., 0.1 mm x 3.5 mm to 1 mm x 5 mm, as well as round wires with diameters of, for example, 0.28 mm to 3 mm, very thin and uniform layers have been achieved. The invention is explained in more detail below with the accompanying drawings. They show:

[0044] Figure 1: Illustration a) the section through the model with a round wire as substrate, illustration b) the model according to illustration a) with an electrical equivalent circuit,

[0045] Figure 2 shows a diagram according to the deposition equation for the continuously passing substrate according to its annular partial surface A Cu along a length of A x in the development,

[0046] Figure 3 shows the course of stress and film thickness using an example with real parameters,

[0047] Figure 4 shows a reaction vessel as an embodiment,

[0048] Figure 5 the hydraulic system of a circulation circuit with the associated fittings,

[0049] Figure 6 two inlet nozzles,

[0050] Figure 7 shows the exemplary structure of an anode tube as a dialysis cell,

[0051] Figure 8 shows a reaction vessel in an alternative design,

[0052] Figure 9 shows an example of the wiring of a measuring electrode and the anode current control.

[0053] In the arrangement according to the invention, a paint bath LB is continuously passed through by the substrate S to be coated, an electric field being formed between the substrate S and at least one counter electrode, which field leads to the deposition of a paint film on the surface of the substrate S during the pass. The paint bath LB is shaped or electrically insulated from other bath areas by electrically insulating vessels, partitions, tubes or chambers, at least for part of its total volume, such that the shaped or insulated part of the paint bath LB forms an electrical ohmic volume resistance, which is passed through by the substrate S in its feed direction on the way from an immersion position EP to a position of the counter electrode remote therefrom as an inlet section ES.This part of the paint bath LB has such a large quotient of path length and cross-sectional area that the electric field voltage, which builds up across this volume resistance in interaction with the temporally and spatially progressive film formation around the substrate S, forms at least one zero point along the inlet section ES.

[0054] Furthermore, it is provided according to the invention that the paint bath LB is shaped or electrically insulated from other bath areas by electrically insulating vessels, partitions, tubes or chambers over a further part of its total volume in such a way that this shaped or insulated part of the paint bath LB forms an electrical ohmic volume resistance, which is traversed over a part of its length by the substrate S in its feed direction on the way from the position of the counter electrode to a remote emerging position as an outlet section AS and is connected over a further part of its length to the inlet section ES in such a way that this forms an electrical return between the inlet and outlet sections ES, AS.Figure 1, illustration a) shows the section through the model of the inlet section (ES) with a round wire as substrate S, which passes through a cylindrically shaped paint bath LB with the cross-sectional area A from left to right at a constant feed rate v. The immersion position EP of the substrate S is located at x=0, while the anode An, which forms the counter electrode here, in the form of a hollow cylinder is reached and passed from the same substrate position only later at a distance x=1. / is the front edge of the anode An at the end of the inlet section ES.

[0055] The bath vessel is not shown here. The horizontal position of the paint bath LB could also be rotated 90° clockwise, so that the immersion position is then at the surface of the paint bath LB, which can be contained in a cylindrical, electrically non-conductive vessel. Further possibilities and details of the technical implementation can be found in the following examples.

[0056] Simply because the film formation process must be progressive over time and can only begin after immersion at the earliest, an increasing film thickness dp(x) results along the inlet section of the substrate S, which is shown exaggerated to scale in the model sketch for better visibility. In practice, the film thickness dp(x) is in the range of approximately

[0057] 5...30 m often only reach a few percent of the substrate diameter, while the substrate cross-section will, in turn, be within a few percent of the paint bath cross-section. It is also currently unknown where exactly film formation begins and what progression the film thickness takes along the path x, which is why the figure shows some increase in thickness along the entire path.

[0058] Both the cross-sectional shape of the substrate S and that of the paint bath LB can deviate arbitrarily from the circular shape shown. All of the findings presented below are also applicable without restriction to any other shape—even if, for example, it has a very large length-to-width ratio and more closely resembles a long, narrow rectangle, such as sheet metal or flat wire. Of course, the cross-sectional shape of the paint bath LB should always be adapted to that of the substrate S around it, whenever possible.

[0059] In the illustrated configuration as a long, slender structure, the lacquer bath LB acts electrically like a technical wire-wound resistor. For example, if the diameter of the lacquer bath LB is 14 mm, practical experience shows resistances in the order of 60 kOhm per meter. As with a wire-wound resistor, partial taps in the bath volume are also possible along its entire length.

[0060] The further mathematical-physical derivations and findings that were specifically developed for the present invention are not trivial and are not apparent from the previously known literature, which is why they are presented and explained in detail below.

[0061] As a result of all preliminary considerations, it is obvious to describe the model with an electrical equivalent circuit according to Figure 1b, which resembles the long telecommunications line known from communications engineering. The inlet section from the immersion position x=0 to the edge of the anode An at x=1 can be modeled as a chain circuit of individual voltage dividers G, and R, each of which occupies a length element LE with the constant partial length A x. The series resistors R represent the series resistance of the paint bath LB, are the same everywhere, assuming a constant cross-sectional area A, and are calculated according to

[0062] R=^- -Ax and r = -^—=^-= const , (1a, 1 b)

[0063] A Ax A with the specific bath resistance p B . Equation (1b) calculates a length-specific longitudinal resistance r for further model calculations.

[0064] Instead of the transverse resistances that form in the paint film, transverse conductances G are used in accordance with the conduction theory, because these, like the series resistances, are proportional to the length units considered. They are calculated according to

[0065] K -u r " G. K -u "

[0066] G,=— - A x and g, = ~ ~ = ~i — ^const , (2a, 2b) d Fi A xd Fj with the specific conductance of the deposited coagulated film layer, the circumferential length Ucu of the substrate, and the thickness dpi of the film layer. Because the thickness of the film layer must vary at different locations along the travel direction, the transverse conductance is also dependent on i and therefore not constant. Equation (2b) calculates a length-specific transverse conductance gi.

[0067] The electrical resistance of a metallic substrate S is negligible compared to the much higher series resistances of the paint bath, which is why a continuous line is drawn as the reference potential in the equivalent circuit. In the anode compartment, the series resistances are eliminated because the same anode potential is applied everywhere and no voltage gradient occurs. Due to the slender shape of the paint bath, it is assumed that a constant potential Ui develops in every plane x> from the left edge of the anode An toward the immersion position EP.

[0068] For the voltages and potentials given in the equivalent circuit, for each line element, according to Kirchhoff’s theorems

[0069] J i+i = I i + ^ I i = I i + u i -G l -^ x and (3)

[0070] U l+l =U l +AU l =U l +I t-r A x (4) From this, the system of equations similar to the telegraph equation is derived which with Ax -> 0 in the differential form passes over.

[0071] Furthermore, for any node t

[0072] P)

[0073] Due to the spatial dependence of the conductances g(x), the general solution of the telegraph equation is not applicable here. Rather, there is an interaction between the increase in film thickness and its resulting conductance, on the one hand, and the course of the voltages and partial currents along the length x, on the other. Imagine, for example, that the film thickness increases more rapidly due to a higher partial current. The partial current would be smaller in the next length range, resulting in a smaller increase in thickness in the next step, and so on.

[0074] In order to be able to capture these relationships analytically, the function of the thickness increase will now be derived.

[0075] The decisive factor for film formation on the substrate S is the release of charge carriers from its surface, the intensity of which is known to depend on the current density – this is ultimately what defines it. Thus, at least in a first approximation, it can be assumed that Faraday's deposition law (8a, 8b) applies to the deposition of colloids from the coating bath onto a substrate S of any size. with the deposited mass m or the deposited volume V with the substance density p and the current flow l(t) during the time t in the time interval T, as is known from the electrolysis of inorganic substances. E denotes the electrochemical equivalent according to with the molar mass M, the valence z, and the Faraday constant F = 96485 As / mol. The volume of a deposited film on the substrate S is therefore proportional to the total amount of charge that flowed over this section of the substrate S, which generally applies to any substrate surface statically located in the paint solution. The electrochemical equivalent of electrodeposition paints is not easy for the user to calculate from a molecular formula, but can be determined experimentally as a guideline for the paint used in each case by solving equation (8a) for E and determining a deposited mass for a defined amount of charge. For a specific paint formulation, this value can be considered sufficiently constant with fixed bath parameters.

[0076] To derive the deposition equation for the continuously passing substrate S, its annular partial surface A is determined according to Figure 2. Cualong a length of Ax in the /

[0077] Consider a development that moves step by step with an average speed v=-^= const. The step size is therefore always A x and the residence time in each step A t. The film volume over the partial area is calculated as V= u Cu -d Fi -Ax >

[0078] Each step position now belongs to a conduction element according to Figure 1b, so that an arbitrarily assumed current profile is passed through step by step as shown in Figure 2. After reaching a step i, the total charge quantity acting on the considered substrate section is calculated from the sum of the partial charges. Using equations (8b) and (7), the film thickness is calculated according to

[0079] Using equation (2), the length-specific transverse conductance is determined after passing through the positions 1 ... I as

[0080] The approach to this equation was developed for a single, isolated section that was advanced to position i. However, film formation also occurred simultaneously for each preceding and subsequent section of the substrate S, so that equation (11) applies simultaneously to any other positions adjacent to each other.

[0081] All derivations up to equation (11) are valid for arbitrarily small step sizes A x , so that after the transition zlx — > 0 for any position x on the substrate S the continuous function can be written as [ ]. For shortened notation, the process constant ki summarizes all effective fundamental constants and stationary parameters. Together with equations (6), this completely and uniquely describes the entire system with its interactions between the increase in film thickness and the course of the voltages and currents along the length x.

[0082] After insertion, the system of equations (6) is from which the differential equation From the general solution and the boundary condition f7(I=0) = 0, which can be seen from the equivalent circuit diagram, one obtains the partial solution • (15)

[0083] From this, the further differential equation (13a) is derived for which the general solution for the current function It should be recalled that this function describes the current in the longitudinal direction along the inlet section ES (see Figure 1) and should therefore not be confused with the current density at the substrate S.

[0084] Inserted into (15), (12) and (2), the voltage, conductance and film thickness curves are calculated according to

[0085] With the boundary condition that the node voltage at the transition to the anode An must be equal to the anode voltage, the integration constant is calculated according to (18) as

[0086] (21)

[0087] The most important finding from the obtained equations is that there is a zero for current, field voltage, and layer thickness at x=C and not, as one might expect for the conduction model, an asymptotic approach to zero. Further out for x <C, in Vorschubrichtung gesehen also vor dieser Nullstelle, ist ein erneuter Anstieg der Funktionen physikalisch ausgeschlossen, nachdem die Feldspannung bereits auf Null abgesunken war. Die Filmabscheidung auf dem Substrat S beginnt somit erst dann, wenn der betrachtete Substratpunkt die Koordinate x=C passiert hat, egal wie lange das Lackbad LB vorher bereits durchlaufen wurde. Hieraus leitet sich im Umkehrschluss die wichtige Bedin- gung for the minimum distance between the immersion position and the edge of the anode An, in order to immerse the substrate S into the paint bath LB without stress. Only under this condition are the derived equations valid. Otherwise, at x=0, the field becomes discontinuous and abrupt coagulation occurs on the substrate S, which should not be allowed.

[0088] The film thickness at the transition from the inlet section ES into the anode chamber is calculated from (20), (21) and (12b) with x= / zu

[0089] If one wanted to change the functional dependencies of x in equations (17) to (21), the cross-section along the length x could be designed differently. If the paint bath LB is designed along the inlet section, for example, in the shape of a truncated cone, its cross-sectional area increases quadratically, whereby equation (1b) changes to and therefore the equations derived from it must also be adjusted.

[0090] In the area below the anode An there are no voltage drops in the direction of travel, which is why no series resistances R are entered in representation b) of Figure 1 and from (6) it follows

[0091] From this, the differential equation develops in the same way as shown above with U=UA with the general solution

[0092] Inserted into (10), the film thickness under the anode An is

[0093] While the film thickness along the inlet section in front of the anode chamber increases quadratically according to (20), it only increases with x below the anode An.

[0094] In developing (26) to (28), it was initially assumed that 7=0 is located at the left edge of the anode An and that the substrate S is just being immersed there and film formation is starting - hence the use of x instead of x. In fact, Xi must be shifted along x such that this current is as large as that from (17) and (21) with =Z. After equalizing the currents, an offset at the left anode edge, i.e. for =Z, is calculated as

[0095] Inserted into (27), the current at the anode edge is and is therefore as large as the equation (17) gives for this position.

[0096] In summary, for arrangements according to a) and b) of Figure 1, in which the substrate S is pulled as a web through the coating bath LB at a constant speed in the manner shown, the following conclusions can be drawn on the basis of the developed equations:

[0097] 1) The resist bath LB and the film layer deposited on the substrate S interact to form an electrical conduction system distributed over the entire length of the pass. This system can be divided into finite length elements as a discretized model according to diagram b) of Figure 1. The voltage dividers cascaded in this way can be considered as a conduction network.

[0098] 2) After switching on the substrate feed, an equilibrium according to equations (17) to (21) is established after an initial settling time, as long as the conditions remain constant. The geometric dimensions of the resist bath LB and its resistivity, the specific conductance and specific mass of the film layer, the circumferential length of the substrate S and its feed rate, and the anode voltage were derived as conditions influencing the equilibrium.

[0099] 3) The electric field is formed along the inlet section ES before reaching the anode chamber over a length that depends on the conditions mentioned in 2). It begins in the direction of travel after the immersion position EP with a zero point, from which the field voltage increases steadily, and ends at the anode edge, where the full anode voltage is already in effect.

[0100] 4) If the calculated film formation length with C<0 is greater than the inlet distance ES, i.e., the distance / between the immersion position EP and the anode edge, a discontinuity forms, which causes a cathode current to flow during immersion, and the system of equations is no longer valid. According to the task, this scenario should be eliminated by selecting the appropriate process parameters and the inlet distance length.

[0101] 5) If, on the other hand, the calculated length for film formation with C>0 is shorter than the inlet distance ES, then there is an area in the direction of travel behind the immersion position EP in which no film deposition is taking place. In practice, this case C>0 should always apply in order to reliably fulfill the intended purpose. The zero point of the current function would shift forward towards the immersion position EP according to equation (21) if the substrate S is fed more slowly into the paint bath, whereas it would shift increasingly towards the anode An as the feed speed of the substrate S increases. As the feed speed increases, the achievable film thickness that is deposited under otherwise unchanged conditions also decreases. This can be used to apply thinner layers. For this purpose, a balance must be made between the feed speed and the anode voltage in conjunction with all other process parameters.

[0102] 6) Along the inlet section ES, the film thickness increases quadratically at a constant cross-sectional area according to equation (20). To linearize this increase, if necessary, the shape of the coating bath LB can be designed to create a conical cross-sectional constriction along the inlet section.

[0103] Figure 3 shows the variation of stress and film thickness using an example with real parameters. It shows a three-way division of the passage into a region x <C vor Beginn der Filmbildung, den Bereich C<x<l mit beginnender Filmbildung bei stetig steigender Feldspannung und den Anodenraum x> l with constant anode voltage. The first part could be extended arbitrarily to the left without changing the position or shape of the two curves with respect to x=l or changing the function values ​​in x <C von Null verschieden wären. Während die Spannung einen sehr markanten und im Übergang zum Anodenraum sogar unstetigen Verlauf zeigt, wirkt der Anstieg der Filmdicke im betrachteten Bereich beinahe gleichmäßig, hat aber sein Maximum am Anodenrand, um anschließend wieder kleiner zu werden. Ein solcher „harmonischer“ Verlauf mit allmählichem Start und langsamem Ausklingen des Dickenzuwachses wird als beste Ausgangsbedingung für einen homogenen Schichtaufbau gewertet.With the realization of an inlet section ES that satisfies the conditions shown in Figure 1, illustration a), an important sub-task has already been fulfilled.

[0104] The web material (the substrate S) could continue its passage in the same manner without any problems. However, it has already been explained why the electric field should be as small as possible when the substrate S emerges from the coating bath LB. If the outlet section AS is designed similarly to the inlet section ES, a high-ohm volume resistance is again created. To ensure an effective voltage drop across this section as well, an external electrical connection is created between the outlet and inlet sections using a yoke. This does impair the effectiveness of the above-described design of the inlet section ES because an additional current results from the yoke.However, there is sufficient design flexibility so that the entire return circuit together with the outlet section AS can be designed with a sufficiently high resistance and the unavoidable additional current can be taken into account without violating the voltage-free immersion condition.

[0105] Because of the remaining conduction path to the substrate S, the outlet section AS—unlike the inlet section ES—does not reach a zero point, which in practice is not necessary to sufficiently reduce unwanted cohesion. However, the aim is to make the outlet section AS as high-impedance as possible and the return section as low-impedance as possible so that the voltage divider formed by the two has a minimal output voltage.

[0106] There are two options for keeping the resulting backflow resistance as small as possible: In one embodiment according to the invention, the inlet and outlet areas of the substrate S in the paint bath LB are brought as close to each other as possible to keep the backflow as short as possible. One suitable measure for this is to fold the flow path into a U-shaped structure and guide the substrate S over deflection rollers in the middle of the flow direction. This results in a minimal distance between the inlet and outlet sections. This variant is particularly suitable for small systems and has the advantage that small installation sizes and fill volumes for the paint bath LB can be sufficient.

[0107] Particularly for the production of very large quantities and to avoid unnecessary deformation of the substrate S, another embodiment according to the invention with an elongated throughput path and with minimal necessary deflection of the web material (the substrate S) may be preferred. However, this arrangement results in an extension of the return path to approximately the entire throughput path of the substrate S. This must then be compensated for with an enlarged return path cross-section. A parallel arrangement of the return path is suitable for this purpose, extending on one or more sides along the outside of the insulated throughput path, where it finds sufficient space for cross-sectional areas that reach many times the cross-sections of the inlet and outlet paths.

[0108] Both options will be explained further later with the implementation examples.

[0109] In order to continuously supply the substrate surface with fresh reactants and counteract the accumulation of waste products and sediments, one embodiment provides for an intensive flow of the bath contents along the substrate S. For this purpose, the dip-coat bath vessel has at least one inlet opening (40, 41) and at least one outlet opening 15, both of which are connected to an external circulation circuit driven by a circulation pump (30), so that no external material inflow is required. Slender vessels, which are preferably used according to the invention due to their higher electrical resistance, allow for a constant fluid movement that encompasses almost the entire cross-section of the flow path and also the substrate surface.

[0110] Furthermore, such a flow in the paint bath LB offers the possibility, if necessary, of reducing the differential velocity between the substrate S and the paint bath LB to such an extent that a diffusion-controlled boundary layer can form on the substrate surface even at high feed rates, and film formation is still possible. For this purpose, it is sufficient to design the circulation system described above in such a way that a sufficiently fast flow is generated in the film formation area, parallel to the substrate S and in the direction of its feed, and the flow conditions between different areas of the paint bath LB are appropriately balanced.

[0111] Immersed substrate surfaces must be wetted quickly and thoroughly with the electrocoating paint to prevent paint defects. In one embodiment, the flow of the bath contents is additionally used to generate an intensive surface pressure on the substrate S at the immersion position so that possible air pockets are displaced and good mechanical contact is established between the immersion paint and the substrate S right at the start of the immersion coating. For this purpose, at least one inlet nozzle (40) is arranged near the immersion position, preferably just below the bath surface, the flow direction of which is aligned towards the substrate S and preferably slightly inclined against the feed direction. Alternatively, disruptive surfaces which have a similar effect can be inserted into the flow channel. In this way, a constant liquid surge orbuild up turbulence, which simultaneously also transports air bubbles out of the paint bath LB.

[0112] In a further embodiment, the substrate S is mechanically vibrated immediately before or during immersion in order to shake off any air bubbles from its surface. Electromechanical vibrators, rotating eccentric shafts, or similar devices are suitable for this purpose. These devices are preferably mechanically coupled to the substrate S via bearing-mounted deflection pulleys or in direct sliding contact. The use of a sound or ultrasonic generator that vibrates the substrate S or the entire paint bath LB can also be provided for this purpose.

[0113] In order to break the cohesion between the film layer and the uncrosslinked paint molecules at the end of the outlet section when leaving the paint bath, one embodiment uses a paint flow that flushes the free paint components away from the film layer when the substrate S emerges from the paint bath LB. For this purpose, a paint flow is formed near the exit point of the substrate S with at least one diffuser (41) directed towards it, so that all loose paint components are entrained by the flow movement against the conveying direction and by the simultaneous effect of gravity in the direction of the paint bath LB and flow back into the paint bath LB. This paint flow, like the one used for the immersion position EP, is also preferably branched off as a partial flow from the above-mentioned circulation flow of the paint bath LB.Because a constant flow is maintained and the substrate S is constantly moved, deposition of solid paint components by drying is largely avoided.

[0114] In a further development, additional filters (see Figure 5) are provided which are inserted into the circulation flow and remove disturbing particles from the paint bath LB in order to keep it in a clean condition and to supply the film formation with a constantly fresh paint solution, free from contamination or conglomerates.

[0115] Because the LB paint bath must be stabilized at a specific temperature, a further development involves inserting heating or cooling units into the circulation flow and allowing the circulating fluid to flow directly through them. The resulting turbulence in the electrocoating coating ensures rapid and thorough mixing of the LB paint bath components at different temperatures. Depending on the system size, Peltier elements or heat pumps, for example, can be considered as heating and cooling units.

[0116] Before the coated substrate S emerging from the coating bath LB is conveyed over a first roller

[0117] 22, it should be at least superficially dry. In an advantageous embodiment, this is achieved by heating it and simultaneously blowing fresh air onto it.

[0118] In order to continuously remove chemical reaction products that hinder the film formation process from the coating bath LB, one embodiment envisages the use of conventional dialysis cells instead of simple anodes An. Such dialysis cells consist of an anode An surrounded by an anolyte chamber. A semipermeable membrane at the interface to the coating bath LB prevents direct hydraulic contact but allows electrolytic current flow. The anolyte chamber is fed with fresh deionized water continuously or at controlled intervals via an inlet, while the reaction products that diffuse through the membrane, such as acetic acid, are removed via an outlet. To control the exchange volume of the anolyte liquid based on the degree of contamination, conductometric control is very well suited for continuous process monitoring.

[0119] As the equations of the analytical derivations showed, the potential distribution in the electric field of the dip-coat bath reflects a large amount of operating data. Therefore, the invention provides that at least one electrical measuring electrode (M) for measuring the electric field potential is located in the paint bath (LB). This electrode is connected to the current-compensated input of a signal amplifier and is used as a reference variable for adjusting operating parameters or for obtaining data for maintaining suitable bath parameters. Three categories of information can be obtained from this:

[0120] 1) Consumption status of the paint bath LB

[0121] 2) Monitoring of film thickness, e.g., to adjust the feed speed

[0122] 3) Target potentials for tracking auxiliary voltages such as the anode voltage, potentials of a pump housing of the circulation pump (30) or other metallic fittings in the paint bath (LB) or its circulation circuit

[0123] This is explained below.

[0124] To maintain consistently high paint quality, constant monitoring and adjustment of bath parameters is necessary. As a clear measure of the wear status of electrocoating, the MEQ value is determined by chemical titration, as is common practice with other paints and is specified by standards. Such an analysis relies on the taking, preparation, and subsequent discarding of samples and is therefore not easily integrated into a consistent manufacturing process. In a continuous production process, the bath parameters should be constantly monitored by central process control, just like all other process parameters. It has been shown that, because the processes always run in the same way and under the same conditions, substitute parameters can be determined that correlate with the MEQ value with sufficient accuracy.These include the pH value and the electrical conductivity, which is determined by conductometry. Conductometry offers a particularly simple measurement method that can be performed regularly during the ongoing process without effort, wear, or waste. The use of measuring electrodes directly in the reaction vessel or externally in the circulation system is suitable for this purpose. However, if a working current flows in the paint bath during electrocoating, such measurements are not easily feasible, at least not in this range. However, the bath resistance can be determined by other methods.

[0125] As can be seen from equations (18) with (12b) and (21), the specific bath resistance can be determined from the voltage curve along the inlet section ES, because all other components of the equation are known or can be determined experimentally. Thus, the voltage value of a single measuring electrode outside the anode chamber, together with the anode voltage known by default, is sufficient to draw conclusions about the voltage curve and, based on previously determined empirical values, about the condition of the dip coating bath / paint bath LB.

[0126] It has already been shown how the degree of film deposition depends on the feed rate of the substrate S or its residence time in the coating bath LB, and that the spatial formation of the electric field and the current density distribution also depend on the feed rate according to equations (20) and (28). To compensate for these dependencies, one embodiment provides at least one measuring electrode whose electrical potential serves as a criterion for the field profile and thus for regulating the anode voltage. Alternatively, it is also possible to fix the anode voltage and control the feed rate based on the voltage of the measuring electrode.

[0127] For the current- and electrolysis-free derivation of field voltages via measuring electrodes, suitable amplifier arrangements are provided, the inputs of which are current-compensated via negative feedback circuits.

[0128] Another design stabilizes the anode current by adjusting the anode voltage. The anode current is measured in a known, simple manner. Because the anode current, together with the feed rate, is a direct indicator of the strength of the ion and thus material transport to the substrate S, it can be set proportionally to the substrate speed to achieve a stabilized film formation rate.

[0129] All metallic parts of the circulation system that come into contact with the electrocoat must be maintained at an electrical potential that is no more negative than the surrounding paint bath (LB). Otherwise, they would act as a cathode and deposit a film layer. Likewise, their potential should not be higher to avoid an additional, uncontrollable anode current.

[0130] If such parts have no electrical connection – even a high-resistance one – to other parts of the assembly, they could assume a floating potential via the paint bath LB and would only need to be reliably electrically insulated from the outside. In practice, however, residual currents occur which, if constantly present, can nevertheless lead to unwanted film formation. Larger units and those with electrical connections, such as circulation pumps or thermostats, must therefore be biased with a suitable potential, for which the anode potential could be a suitable option. However, this could inject additional currents into the paint bath LB via the pipe connections, which would then unintentionally bias the area around the immersion position of the substrate S in particular.

[0131] Therefore, in one embodiment, at least one additional measuring electrode is provided in the paint bath LB, preferably near the connection points of the circulation system. The electrical potential at these measuring electrodes is used to generate an automatically adjusted potential equalization voltage as a reference voltage via a suitable amplifier arrangement, so that no current flows through the paint bath LB to these metallic parts – neither in one direction nor the other.

[0132] The arrangement and method according to the invention are suitable not only for large-scale processes but also for small-scale plants. This also enables application in special productions with lower production volumes.

[0133] Compared to other common painting processes, a significantly better energy balance is achieved because the process itself requires only minimal energy and only a small amount of energy is released. For the remaining heat sources and heat sinks, a substantial energy coupling is possible – even when temporally changing ambient temperatures lead to fluctuating energy balances.

[0134] The environmental impact of the material can also be described as low, as the process allows for very thin coating layers and essentially generates no waste. Apart from acetic acid, which is naturally biodegradable or could even be added to the new coating formulation, only hydrogen and oxygen are formed. On an industrial scale, the hydrogen can be reused for energy. The LB coating bath also produces no waste, apart from a small amount of filter sediment, because it needs to be regularly refreshed by adding transparent paste but does not need to be replaced.

[0135] Two preferred design principles were specified for the return path design: one with the shortest possible return path length, and another requiring a longer return path but compensating for this with a larger return path cross-section. An example of each of these principles is provided below.

[0136] In a first preferred embodiment, which is particularly suitable for small systems, the dip-coating bath vessel has a U-shape with two openings at the top, wherein a first opening for the substrate (S) to enter the paint bath (LB) with a vertically downward direction of travel, a second opening for its exit from the paint bath (LB) with a vertically upward direction of travel, and a deflection device for the substrate (S) are arranged between the downward and the upward direction of travel. One or more counterelectrodes are preferably located in the central region of the path of travel of the substrate (S) through the dip-coating bath vessel. Figure 4 shows such a reaction vessel 100 in an external view according to the invention.A housing 10 is connected to a double tube 12a, 12b made of electrically insulating material in such a way that a bath vessel with a continuous, elongated U-shaped cavity is formed. The housing contains deflection rollers 14, which are accessible via a removable lid 11. To insert the substrate S, the reaction vessel 100 can be completely emptied via the pipe socket, so that the deflection rollers 14 become accessible after opening the lid 11. By means of gravity and a pull wire (not shown), possibly also with the aid of a magnet, the beginning of the substrate S can be inserted into the track without having to open the lid 11.

[0137] Between the two legs of the double tube 12a, 12b is a pipe section 13, which creates a hydraulic and electrical cross connection between the two. The U-shape has the advantage that the web of substrate S is not subject to a preferred direction due to gravity when passing through the two vertical main paths, and that the desired proximity between the inlet and outlet of the substrate S is easily achieved, allowing the pipe section 13 to be kept short as a return path.

[0138] The substrate S, in the example a flat wire, enters the left-hand tube 12a at the feed rate v and thus into the paint bath therein downwards in the direction of the arrow, is reversed in its direction of travel at the deflection rollers 14 in the housing 10, and leaves the reaction vessel 100 upwards after passing through the right-hand tube 12b. Additional deflection rollers 21, 22 serve to feed and remove the substrate S from the outside. For orientation, the approximate positions of the inlet section ES and the outlet section AS have been noted. However, the outlet section AS begins already after passing through the anode chamber, thus including the travel section over the deflection rollers 14 in the housing 10.

[0139] The entire vessel system is filled with the coating bath (not visible here) up to above the cross connection 13. At the pipe socket 15, the bath liquid is sucked off by means of a circulation circuit in order to return it in the same total amount to the reaction vessel via the hoses 16 and 17. These hoses 16, 17 are connected to inlet nozzles 40 and 41 according to Figure 6. This creates a continuous flow of the bath liquid from top to bottom in a branch over both legs with an unchanged fill level. This is an open system, allowing the passage of the substrate S without the need to seal the reaction vessel 100. However, the cross-sections of the upper openings can be reduced by means of additional lids to limit the evaporation of the liquid.

[0140] On the shaft of a motor 18, there is an impeller with an oval cross-section or an eccentric shape relative to the shaft, which contacts the substrate S. Due to the rotation of this impeller, the substrate S is set into mechanical vibrations as it enters the reaction vessel 100 due to its radius, which varies over the circumference.

[0141] After passing through the reaction vessel 100, the substrate S passes a high-frequency inductor 19 connected to a fan 20, where it is heated by eddy current and dried in an air stream. Figure 5 shows a schematic representation of the hydraulic system of the circulation circuit with the associated fittings. The reaction vessel 100 from Figure 4 can be seen again with its housing 10, the double tubes 12a, 12b, and the pipe section 13. Via a hose system, the pipe socket 15 is connected successively in the flow direction to a filter 31, a circulation pump 30, and a thermostat 33, before subsequently branching into the two hoses 16 and 17 and flowing back into the paint bath (not shown here). A hydraulic equalizer (not shown) for distributing the flows is provided.The thermostat 33 is formed by a Peltier element, which acts as a heat pump in relation to a radiator 34, heating or cooling in one or the other energetic direction as required. The circulation pump 30 ensures a constant flow of the paint liquid in the direction of the arrows. With the aid of an expansion tank 32, the fill level of the open system can be adjusted using a piston 32.1. In addition, the entire contents of the paint bath can be drained for maintenance purposes in a short time with almost no loss, stored under an airtight seal, and later refilled. A control circuit (not shown) can measure the fill level in the bath tank and, with the aid of the expansion tank 32, adjust it to a desired level using a motor. Furthermore, the position of a measuring electrode M is marked. This electrode is located at some distance from the substrate and scans the field voltage according to Figure 3 as a function of the distance to the anode chamber.

[0142] Before entering the reaction vessel 100, the substrate S is provided to pass through suitable preparation, cleaning, and rinsing baths (not shown), in particular for degreasing and etching the surface and for pickling. Furthermore, after passing through the reaction vessel 100, it is provided to pass through a kiln for curing the lacquer layer, preferably with high-frequency inductors for generating eddy currents or with infrared radiators.

[0143] Figure 6 shows an example of the geometric design of the inlet nozzles - in a) for tube 12a of the double tube for immersing the substrate S into the paint bath and in b) for exiting the paint bath in tube 12b. Again, S denotes the substrate and 16, 17 the inlet hoses. The fill level of the paint bath is marked with the dashed line 42. In case a), the outlet opening of the circulating paint liquid is below the fill level and the inlet nozzle 40 is directed slightly upwards onto the substrate S. This creates a pressure on the substrate surface that flows firmly onto the paint directly at the immersion position and displaces any air inclusions upwards. In contrast, in b), the outlet opening is located above the fill level in order to achieve a rinsing effect on the film surface with the increased pressure of the diffuser 41, thus overcoming the cohesion of the entrained liquid and flushing it back into the paint bath.

[0144] Figure 7 shows the exemplary structure of a dialysis cell 50 functioning as the anode An, as inserted into the left-hand tube 12a in Figure 4, in a cut-away view from two perspectives. Due to its smaller diameter, it reduces the cross-section of the resist bath and thus fulfills the function of an artificial aperture to increase resistance. For this purpose, it has sealing rings 51 to separate the space between the two tube walls from the resist bath and thus prevent current flow to the substrate. The inner wall of the tube 12a is therefore directly adjacent to the outer diameter of the sealing rings 51 and is not shown here for reasons of clarity. In the lower part of the tube 12a there are a plurality of bores 52 covered by a semi-permeable membrane 53.At a radial distance from the membrane 53 and centered by means of spacers 55 is the anode 54, which is shaped like a tube and which on the outside is almost flush with the inner diameter of the tube 12a. Thus, there is a space between the tube 12a and the anode tube 50, which is closed at the top and bottom with the sealing rings 51 and forms the anolyte chamber of a dialysis cell. Fresh demineralized water is fed in from above via the hose 56, while the anolyte solution, enriched with acetic acid and sinking due to its somewhat higher density, is discharged below via another hose 57. Both hoses, as well as the electrical anode lead 58, are led through the anode tube 50, sealed there, and then continue upwards along the inner wall of the anode tube 50.In this way, a dialysis cell is created which, according to the known principle, only makes electrical contact with the paint bath via a semi-permeable membrane 53 and extracts the acetic acid that forms from it. In contrast to known designs that form anodic surfaces, in this dialysis cell the substrate S is enclosed in a tubular shape. Furthermore, this illustration makes it clear that the anode can only electrically act on the paint bath through the holes 52 in the lower region of the tube, whereby the remaining length of the tube creates exactly the effect of the elongated volume conductor already described. In order to achieve a larger anode surface and thus higher anode efficiency, the tubular anode 54 shown could also be extended upwards without changing anything in the volume conductor of the inlet section - provided that this does not extend the area in which the holes 52 are located.

[0145] In a second preferred embodiment, particularly suitable for large-scale systems, the lacquer bath (Lß) is located in a vessel in the form of a trough (71), from which parts of the passage of the substrate (S) are separated by means of electrically insulating chambers (72, 73). The initial and final regions of these chambers are electrically and hydraulically connected to the remaining bath volume, with the counter electrode located in the central region of the passage through these chambers (72, 73).

[0146] Figure 8 presents a further exemplary embodiment of a reaction vessel 100 in this second preferred design. In the front of the drawing, a cutout is shown to show details inside. In this example, the paint bath LB is located in an elongated trough 71, through which the substrate S is pulled horizontally and fed in and out again via deflection rollers 21 and 22. In this example, three webs are coated independently of one another in the same paint bath LB. Here, too, inlet nozzles 16 and outlet diffusers 17 are provided in the already known manner. In order to reduce the cross-sectional areas of the inlet and outlet sections in order to increase resistance, chambers 72 made of electrically insulating material with a U-shaped cross-section are introduced, which form an elongated electrical resistor from the enclosed paint bath LB in the manner described.These chambers 72 are enclosed by further chambers 73, which are connected to the chambers 72 at their ends in such a way that a closed cavity 74, open only at the top, is formed between the two. Anode plates 75 are inserted into this cavity 74 approximately halfway along the length of the chambers. These plates have an electrical connection to the anode chamber containing the substrate S via bores 76 covered with semi-permeable membranes. This cavity between the chambers 73 is filled with anolyte fluid and, together with the anode plates, forms a dialysis cell. Further details correspond to those already described for Figures 4 to 6. For example, in the center of the trough 71 there is a pipe socket (not shown here) for discharging the lacquer fluid for the circulation circuit.

[0147] The space 77 beneath the chambers 73, like the entire trough 71, is filled with the paint bath liquid and, with its large cross-section, forms the low-resistance return path between the beginning of the inlet and the end of the outlet sections. Although this embodiment requires a large paint volume, it is advantageous for large throughput quantities in mass production. Of course, the trough 71 can be covered from above to prevent evaporation of the paint bath LB.

[0148] The upper section of Figure 9 shows a schematic example of the wiring of a measuring electrode arranged close to the immersion position. A voltage follower circuit 61 with a very high-impedance, i.e., virtually current-free, input is connected to the measuring electrode M and tracks this potential in a voltage-correct manner, in order to then feed it to a low-impedance driver circuit 62, whose output P is electrically connected to all metallic parts of the circulation circuit, in particular to the pump 30 and the thermostat 33. In this way, a current flow to or from these parts is prevented, thereby preventing their undesirable effect as anode or—more importantly—as cathode in the electrolytic bath. The output and input of the circuit are electrically connected via the lacquer bath, resulting in positive feedback.However, since no voltage amplification occurs and the loop gain is less than one, the resulting control loop remains stable.

[0149] Another subcircuit in the lower section shows the regulation of the anode voltage as a function of the anode current. A clocked power supply 63, constructed in a known manner and shown schematically here, provides an anode voltage at terminal A. At the ground base of the anode circuit, which is also electrically connected to the substrate, is a shunt resistor 64, at which a current-proportional measurement voltage is obtained, amplified by a voltage amplifier 65, then compared with a target value in an evaluation circuit 66, and fed as a pulse width to the anode voltage generator. In this way, the anode voltage is adjusted so that a constant anode current is achieved even with changing feed speeds of the substrate S.

[0150] Using a pilot plant for coating copper electrical wires with a cathodic dip coating based on epoxy resin, the principles of this publication were technologically tested using practical examples. Micro-flat wires of various dimensions, e.g., 0.1 mm x 3.5 mm, as well as round wires, e.g., with a diameter of 0.28 mm, were coated and cured as substrates. In contrast to previously known and common wire coating processes, the coating film exhibited extremely good homogeneity, with no edge misalignment and no other surface defects such as bubbles or craters. It was also possible to achieve extremely thin coatings with film thicknesses down to a few micrometers, with excellent uniformity and complete electrical insulation.

[0151] List of reference symbols

[0152] Housing 55 spacer

[0153] Cover 56, 57 hoses

[0154] Double tube 58 Anode leada, 12b Individual tubes of the double tube 61 Voltage follower circuit

[0155] Pipe section as cross connection 62 Driver circuit , 21, 22 Deflection pulleys 63 Switched power supply

[0156] Pipe socket (outlet opening) 64 Shunt resistor , 17 Hoses 65 Voltage amplifier

[0157] Motor 66 Evaluation circuit.1 Impeller 71 Trough

[0158] High frequency inductor 72, 73 chambers

[0159] Blower 74 cavity

[0160] Circulation pump 75 anode plates

[0161] Filter 76 holes

[0162] Expansion vessel 77 Return chamber.1 Flask 100 Reaction vessel

[0163] Thermostat A anode connection

[0164] Radiator An Anode (abstracted)

[0165] Inlet nozzle (inlet opening) AS outlet section

[0166] Diffuser (inlet opening) EP immersion position

[0167] Filling level of the paint bath ES inlet section

[0168] Anode tube as assembly LB paint bath

[0169] Sealing rings LE length element

[0170] Holes M Measuring electrode semipermeable membrane P Amplifier output

[0171] Anode S Substrat

Claims

Patent claims 1. An arrangement for the electrophoretic dip coating of web material, comprising a coating bath (LB) through which the web material in the form of a substrate (S) to be coated passes at a continuous feed rate, wherein an electric field is formed between the web material / substrate (S) and at least one counter electrode, which leads to the deposition of a coating film on the surface of the web material / substrate (S) during the pass, characterized in that the coating bath (LB) is shaped or electrically insulated from other bath areas by electrically insulating vessels, partitions, tubes or chambers, at least to a part of its total volume, such that the shaped or insulated part of the coating bath (LB) forms an electrical ohmic volume resistance,which is traversed by the web material / substrate (S) in its feed direction on the way from an immersion position (EP) to a position of the counter electrode remote therefrom as an inlet section (ES), and has such a large quotient of path length and cross-sectional area of ​​the paint bath (LB) in the region of an inlet section (ES) that the electric field voltage, which builds up across this volume resistance in interaction with the temporally and spatially progressive film formation around the substrate (S), forms at least one zero point along the inlet section (ES) and / or that the paint bath (LB) is shaped or electrically insulated from other bath regions by electrically insulating vessels, partitions, tubes or chambers, for a further part of its total volume, such that this shaped or insulated part of the paint bath (LB) forms an electrical ohmic volume resistance,which is traversed over part of its length by the substrate (S) in its feed direction on the way from the position of the counter electrode to a distant emerging position as an outlet section (AS) and is connected over another part of its length to the inlet section (ES) in such a way that it forms an electrical return between the inlet and outlet sections (ES, AS).

2. Arrangement according to claim 1, characterized in that the dip paint bath vessel has at least one inflow opening (40, 41) and at least one outflow opening (15) for the dip paint, both of which are connected to an external circulation circuit driven by a circulation pump (30) in such a way that at least one flow of the paint bath (LB) is formed along the passage path of the substrate (S) and that at least one of the said inflow openings is designed as an inlet nozzle (40) directed towards the substrate (S) or a directed diffuser (41).

3. Arrangement according to claim 1, characterized in that an electromechanical vibration generator is arranged near the immersion position (EP) of the substrate (S), for which purpose an impeller which is oval in cross-section or eccentric to the shaft is located on the shaft of a motor (18), which impeller contacts the substrate (S) so that the rotation of this impeller, due to its radius which is variable over the circumference, causes the substrate (S) to vibrate mechanically as it enters the reaction vessel (100), or that the vibration generator is designed as an electroacoustic transducer which either contacts the substrate (S) directly or acts on it via further guide or coupling elements.

4. Arrangement according to claim 1, characterized in that in the paint bath (LB) there is at least one electrical measuring electrode (M) for deriving the electrical field potential, which is connected to the current-compensated input of a signal amplifier and is used as a reference variable for readjusting operating parameters such as the anode voltage, potentials of a pump housing of the circulation pump (30) or other metallic fittings in the paint bath (LB) or its circulation circuit and / or for readjusting the feed rate or obtains data for maintaining the suitable bath parameters.

5. Arrangement according to claim 1, characterized in that the dip-coating bath vessel has a U-shape with two openings at the top, a first opening for the entry of the substrate (S) into the paint bath (LB) with a vertically downward direction of travel, a second opening for its exit from the paint bath (LB) with a vertically upward direction of travel and a deflection device for the substrate (S) between the downward and the upward direction of travel and that one or more existing counter electrodes are preferably located in the middle region of the path of travel of the substrate (S) through the dip-coating bath vessel.

6. Arrangement according to claim 1, characterized in that the lacquer bath (LB) is located in a vessel in the manner of a trough (71), from which parts of the passage path of the substrate (S) are separated by means of electrically insulating chambers (72, 73), the start and end regions of which are electrically and hydraulically connected to the remaining bath volume, the counter electrode being located in the middle region of the passage path through these chambers (72, 73).

7. Arrangement according to claims 1 and 2, characterized in that a thermostat (33), preferably based on a Peltier element or a heat pump, is arranged in the external circulation circuit.

8. Arrangement according to claims 1 and 2, characterized in that an expansion vessel (32) with adjustable volume is arranged in the external circulation circuit.

9. Arrangement according to claim 1, characterized in that Parts of the passage path of the substrate (S) are flanked by at least one further tubular or chamber-shaped insert, which preferably contains a dialysis cell for removing reaction products with at least one counter electrode.

10. A method for the electrophoretic dip coating of web material, in a system according to claim 1, comprising a paint bath (Lß) through which the web material in the form of a substrate (S) to be coated passes at a continuous feed rate, wherein an electric field is formed between the substrate (S) and at least one counter electrode, which leads to the deposition of a paint film on the surface of the substrate (S) during the pass, characterized in that at least a part of the paint bath (Lß) forms an electrical ohmic volume resistance through which the substrate (S) passes in its feed direction on the way from an immersion position (EP) to a position of the counter electrode remote therefrom as an inlet section (ES), - wherein an electric field voltage builds up across this volume resistance in interaction with a temporally and spatially progressive film formation around the substrate (S), which forms at least one zero point along the inlet section (ES) and / or - wherein the part of the lacquer bath (LB) which forms an electrical ohmic volume resistance is traversed over part of its length by the substrate (S) in its feed direction on the way from the position of the counter electrode to an emergence position remote therefrom as an outlet section (AS), wherein the lacquer bath (LB) is connected over a further part of its length to the inlet section (ES) in such a way that an electrical return connection between the inlet and outlet sections (ES, AS) takes place via this further part of the lacquer bath (LB).

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