Arrangement for the electrophoretic dip coating of web material

Abstract

Arrangement for the electrophoretic dip coating of web material, comprising a dip coating bath through which the web material is passed at a continuous feed rate, wherein an electric field is formed between the web material and at least one counter electrode, which leads to the deposition of a paint film on the surface of the web material during the passage, characterized in that the dip-coat bath is shaped or electrically insulated from other bath areas by electrically insulating vessels, partitions, tubes or chambers, at least in part of its total volume, that this shaped or insulated part of the dip-coat bath forms an electrical ohmic volume resistance, which is passed through by the web material as an inlet section in its feed direction on the way from the immersion position to a remote position of the counter electrodes, and has such a large quotient of path length and cross-sectional area of ​​the dip coating bath in the area of ​​the inlet section that the electric field voltage, which builds up via this volume resistance in interaction with the temporally and spatially progressive film formation around the web material, forms a zero point at the immersion position if the operating parameters are selected appropriately and / or that the dip-coat bath is shaped or electrically insulated from other bath areas by electrically insulating vessels, partitions, tubes or chambers, that this shaped and insulated part of the dip-coat bath forms an electrical ohmic volume resistance, which is traversed over part of its length by the web material in its feed direction on the way from the position of the counter electrode to a distant emerging position as an outlet section and is connected to the inlet section over a further part of its length in such a way that it forms an electrical connection between the inlet and outlet sections.

Description

Area of ​​application and technical background

[0001] The invention relates to the electrophoretic lacquer coating of web material, preferably of wires, foils or sheets, wherein in particular the application of a permanent electrical surface insulation or an anti-corrosion coating can be intended.

[0002] A preferred area of ​​application is the production of enamel-insulated winding wires, such as those used in the manufacture of electrical coils. Such coils are used as electronic components, in transformers, or in electric 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 all types of electrical surface insulation is noteworthy, as the invention also enables the selective correction of any defects. State of the art

[0003] DE 10 2021 001 741 A1 shows how a coating on web material can be advantageously removed using cathodic dip coating (CED) methods and describes a suitable setup for this. 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 carried out on the same production line.

[0004] In practical application, however, the following problems arise which cannot be adequately solved with the means and methods known to date: 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. 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.

[0005] 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 lifted up. 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 lift 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.

[0006] Neither method is applicable when processing web material because it necessarily has a consistently uniform electrical potential, which cannot be interrupted and can be individually supplied with voltage. Instead, DE 10 2021 001 741 A1 proposes providing anode sections with different potentials. This is problematic in that sections with lower potentials become partial cathodes compared to those with higher potentials, thus causing cathodic reactions, including film formation on their surfaces. These reactions, however, must not occur there, but only on the substrate to be coated.

[0007] Dripping at drip edges and subsequent repeated rinsing after the coating process, as is known from piece goods, is also not feasible in the same way with web goods.

[0008] 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.

[0009] Furthermore, despite the many advantages of e-coating, it may be advisable to use anodic dip coating (ADL) 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.

[0010] US Pat. No. 3,813,327 A describes a reaction vessel for the electrophoretic coating of web material. It contains an inner vessel that is largely electrically and hydraulically insulated from an adjacent outer bath volume. It has an electrical and hydraulic connection to this outer vessel volume only at a bottom inlet for the passage of the web material. This connection is designed as a tunnel-shaped passage, so that the electric field between the electrodes and the substrate is dissipated in the opposite direction of the web material, so that no significant field strength occurs outside the inner vessel.This approach could solve the sub-problem mentioned under 1) if the partial current density at the substrate surface were to increase continuously as the substrate passed through the passage from bottom to top into the inner vessel, thus avoiding a spontaneous current flow at the substrate. Instead, this passage should be dimensioned such that it forms a seal for the electric field, so that it cannot have an effect in the outer bath volume. To this end, the distance between the passage and the substrate is intended to be kept as small as possible. From this it can be concluded that the achieved transition into the region of higher partial current densities is further away from the immersion point and the resist initially comes into contact with the substrate in a field-free area. However, this does not solve problem 1) because an uncontrolled high field increase and suddenly very high partial current densities must be expected behind the passage.This arrangement also fundamentally offers no solution to the sub-problem mentioned under 2). Another disadvantage is that an additional paint volume and an additional flow length through the paint bath are required without performing any other technically necessary functions. In this area, where the substrate flows from top to bottom into the reaction vessel, there is no bath circulation, making the bath parameters difficult to control. The formation of bubbles on the substrate surface is not actively prevented, so paint defects caused by them will continue to occur.

[0011] The publication JP 2003 - 213 493 A proposes a similar passage, which, however, would result in not only electrical but also hydraulic sealing. Aside from justified doubts about the technical feasibility of such an arrangement, it would have the same disadvantages as already described. Technical task

[0012] The invention is based on the problems stated under 1) and 2). The technical task is to design an apparatus for electrophoretic dip coating that does not have the aforementioned disadvantages. The teachings of the invention are intended to be equally applicable to both cathodic dip coating and atomic dip coating processes.

[0013] 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.

[0014] 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.

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

[0016] The problem is solved by the invention characterized in the main claim. Advantageous embodiments are specified in the further claims. The following terms are agreed:

[0017] Web material is the term used to describe metallic or metallized starting material which is to be coated on its surface, which is so long that it cannot be coated across its entire length at once, and which is preferably flexible enough to be supplied to the coating process on unwinding spools or coils and removed again on take-up spools or coils. 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 build up an electrical field relative to the reference potential and are subjected to either a more positive electrical potential as an anode or a more negative electrical potential as a cathode than the substrate. The terms counter electrode or counter electrode can be used here.Anode or cathode can also mean that it is not just a mere electrode but a dialysis cell of a known design with a semi-permeable membrane that surrounds the counter electrode but builds up an electric field in the same way with respect to the substrate as a whole.

[0018] 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. The entire paint bath volume is contained 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. The entire passage route 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, which are referred to in the order in which they are passed through: firstly as the inlet section, secondly as the electrode chamber orSpecifically for e-coating, it is referred to as the anode compartment, and thirdly, as the outlet section. The electrode compartment 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 compartment before and after, respectively, in the feed direction.

[0019] 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.

[0020] As already explained, a key objective of the invention is to reduce the electrical currents acting on the substrate, both during immersion in the lacquer bath and upon exiting it. To achieve this, a suitable field distribution must be achieved along the path the substrate travels through the lacquer bath. The importance of current-free immersion has already been explained. However, a reduction in the electrical field before the substrate exits 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. This cohesion 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 resist solution upon leaving the resist bath. Therefore, the electric field at the exit point should be weakened to such an extent that no significant polarization occurs.

[0021] The electrochemical principle of film formation by electrophoresis in the paint bath, which is used in electrocoating, is described in the specialist literature and is assumed to be well known. The formation of the electric field in the paint bath is generally described there as a voltage- and field strength-oriented process. The spatial concept conveyed in this way, as is familiar, for example, from electrodes in a free vacuum, does not apply to an electrocoating bath and leads to general misunderstandings. In fact, the materials involved between the electrodes are not dielectrics, as is often misinterpreted, but layered volume conductors. Thus, the interactions between colloids, molecules, and electrons can be explained less by electric field strength than by partial current densities.The resist film deposited on the substrate has a much lower electrical conductivity than the resist bath surrounding it, but nevertheless cannot be considered a dielectric.

[0022] To specify the intensity of film deposition in different regions of the coating bath, a differentiation of the current density along the substrate passing through it is necessary. Only the substrate and the anode(s) can be used as electrical poles with defined potentials. The use of additional auxiliary electrodes must be ruled out because they would either have an intermediate potential that would play a cathode function relative to the anode, or, at a potential that exceeds the anode voltage, would displace the actual anode into the relative function of a cathode.

[0023] To circumvent this dilemma, the invention utilizes the electrical conductivity of the paint bath to create precisely definable resistance paths at which a voltage drop is generated by current flow of the required magnitude. This then leads to a specific potential profile within the paint bath along the direction of travel of the substrate. A key methodological approach to solving the problem therefore consists in not using additional electrodes to form the electric field and for a defined current density distribution along the substrate, but rather in deliberately forming ohmic volume resistances in the paint bath itself and technically utilizing the current flow between the counter electrode and the substrate, which is required anyway, to generate voltage differences at such volume resistances.

[0024] 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 µS / 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 or reduce the cross-sectional area to increase the resistance value.

[0025] 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.

[0026] 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.

[0027] 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.

[0028] 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.

[0029] 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.

[0030] According to the invention, two basic forms of electrical paint bath resistors are preferably used: 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. 2) Arrangement of two-pole, preferably low-ohm, electrical resistors in the form of vessels, tubes, or chambers filled with paint bath fluid. Their geometry ensures the required resistance values. These resistors are additionally integrated into the hydraulic system of the paint bath as an electrical bridge or bypass, without necessarily fulfilling a hydraulic function. Such regions will be referred to in the following text as "backflow devices."

[0031] It may also be useful to use a different electrolyte or dialysis fluid instead of the lacquer bath fluid in parts of such a system and to separate this from the lacquer bath by means of a semi-permeable membrane in such a way that an ion exchange takes place but no direct mass transport across these boundaries.

[0032] 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.

[0033] 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 conductivity. 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 conductivity 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.

[0034] 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.

[0035] 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.

[0036] If a resistance value in the paint bath is to be further increased under spatially restricted conditions, it is still advisable to insert a labyrinth of insulating discs and ribs so that the cross-sectional area is reduced in favor of the length.

[0037] 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, as shown below in a model.

[0038] Fig. Figure 1a shows a cross-section through the model with a round wire as substrate S, which passes through a cylindrically shaped paint bath LB with cross-sectional area A from left to right at a constant feed rate v. The immersion position of the substrate is located at x=0, while the anode An, in the form of a hollow cylinder, is reached and passed from the same substrate position only later at a distance x=1. The so-called inlet section is located in between.

[0039] The bath vessel is not shown here. The horizontal position of the paint bath could also be rotated 90° clockwise, so that the immersion position is then at the surface of the paint bath, which can be contained in a cylindrical, electrically non-conductive vessel. Further possibilities and details of the technical implementation can be found in the examples.

[0040] Simply because the film formation process must be progressive over time and can only begin after immersion at the earliest, an increasing film thickness d results along the inlet section of the substrate. F (x), which is exaggerated to scale in the model sketch for better visibility. In practice, the film thickness in the range of approximately 5...30 µm will 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 resist bath cross-section. It is also currently unknown where exactly film formation begins and what progression the film thickness takes along the path, which is why the image shows some increase in thickness along the entire path.

[0041] Both the cross-sectional shape of the substrate and that of the paint bath can deviate from a circle at will. 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 should always be adapted to that of the surrounding substrate whenever possible.

[0042] In the illustrated configuration, a long, slender structure, the paint bath acts electrically like a technical wire-wound resistor. For example, if its diameter is 14 mm, practical experience shows that resistances in the order of 60 kOhm per meter can be achieved. As with a wire-wound resistor, partial taps in the bath volume are also possible along its entire length.

[0043] The further mathematical-physical derivations and findings that were specifically developed for the present invention are not trivial and are not known from the previously known literature, which is why they are presented and explained in detail below.

[0044] As a result of all preliminary considerations, it is obvious to simulate the model with an electrical equivalent circuit according to Fig. 1b, which resembles the long telecommunication line known from telecommunications. The inlet section from the immersion position x=0 to the edge of the anode x=1 can be described as a chain circuit of individual voltage dividers G i and R, each of which occupies a length element LE with the constant partial length Δx. The series resistances R represent the longitudinal resistance of the paint bath, are the same everywhere for a constant assumed cross-sectional area A and are calculated according to R=ρBA⋅ΔxandR=RΔx=ρBA=const, with the specific bath resistance ρ B . Equation (1b) calculates a length-specific longitudinal resistance r for further model calculations.

[0045] Instead of the transverse resistances that form in the paint film, transverse conductances G are calculated in accordance with the conduction theory. i used because these, like the series resistances, are proportional to the length units considered. They are calculated according to Gi=κ⋅uCudFi⋅Δx and gi=GiΔx=κ⋅uCudFi≠const, with the specific conductivity κ of the deposited coagulated film layer, the circumferential length u Cu of the substrate and the thickness d Fi 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 g i .

[0046] The electrical resistance of a metallic substrate is negligible compared to the much higher resistivity of the paint bath, which is why a continuous line is drawn as the reference potential in the equivalent circuit. In the anode chamber, the resistivity is eliminated because the same anode potential is applied everywhere and no voltage gradient occurs. From the left edge of the anode toward the immersion position, it is assumed that, due to the slender shape of the paint bath, in each plane x i a constant potential U i trained.

[0047] For the voltages and potentials given in the equivalent circuit, for each line element, according to Kirchhoff’s theorems Ii+1=Ii+ΔIi=Ii+Ui⋅gi⋅Δx and Ui+1=Ui+ΔUi=Ui+Ii⋅r⋅Δx From this, the system of equations similar to the telegraph equation is derived ΔIiΔx=Ui⋅gi,ΔUiΔx=Ii⋅r which with Δx → 0 in the differential form dI(x)dx=U(x)⋅g(x) ,dU(x)dx=I(x)⋅r Furthermore, for any node i Ii=∑j=0i−1ΔIj.

[0048] 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.

[0049] In order to be able to capture these relationships analytically, the function of the thickness increase will now be derived.

[0050] The decisive factor for film formation on the substrate 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 applies when the colloids are deposited from the coating bath onto a substrate of any size. m=E∫0TI(d)dt or V=Eρ∫0TI(t)dt with the deposited mass m or the volume V with the substance density ρ 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 E=Mz⋅F 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 is therefore proportional to the total amount of charge that flowed over this section of the substrate, which generally applies to any substrate surface statically located in the coating solution. The electrochemical equivalent of electrodeposition coatings is not easy for the user to calculate from a molecular formula, but can be determined experimentally as a guideline for the particular coating used by solving (8a) for E and determining a deposited mass for a defined amount of charge. For a specific coating formulation, this value can be considered sufficiently constant with fixed bath parameters.

[0051] To derive the deposition equation for the continuously passing substrate, according to Fig. 2 whose annular partial surface A Cualong a length of Δx in the development, which gradually moves with an average speed ν=ΔtΔx=const The step size is therefore always Δx and the residence time in each step is Δt. The film volume above the partial area is calculated as V=u Cu · d Fi · Δx. Each step position now belongs to a line element according to Fig. 1b, so that an arbitrarily assumed current profile is passed through step by step as shown in the diagram. After reaching step i, the total charge acting on the substrate section in question is calculated from the sum of the partial charges. Using equations (8b) and (7), the film thickness is calculated according to dFi=EuCu⋅ρ⋅ν⋅∑j=0i−1ΔIj=E⋅IiuCu⋅ρ⋅ν.

[0052] Using equation (2), the length-specific transverse conductance is determined after passing through the positions 1 ... i as gi=κ⋅uCu2⋅ρ⋅νE⋅Ii.

[0053] 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, so that equation (11) applies simultaneously to any other positions adjacent to each other.

[0054] All derivations up to equation (11) are valid for arbitrarily small step sizes Δx, so that after the transition Δx → 0 for any position x on the substrate the continuous function g(x)=κ⋅uCu2⋅ρ⋅νE⋅I(x)=k1I(x), with k1=κ⋅uCu2⋅ρ⋅νE can be written as [ ]. For shortened notation, the process constant k1 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.

[0055] After insertion, the system of equations (6) is from which the differential equation d U(x)d I(x)=r⋅(I(x))2k1⋅U(x) From the general solution and the boundary condition U(I=0) = 0, which can be seen from the equivalent circuit diagram, the partial solution U=23rk1(I(x))3 From this, the further differential equation (13a) is derived d I(x)dx=23r⋅k1⋅I(x) for which the general solution for the current function I(x)=16⋅r⋅k1(x−C)2 It should be remembered that this function describes the current in the longitudinal direction along the inlet path and should therefore not be confused with the current density at the substrate.

[0056] Inserted into (15), (12) and (2), the voltage, conductance and film thickness curves are calculated according to U(x)=118⋅r2⋅k1(x−C)3 g(x)=6r⋅(x−C)2 dF(x)=κ⋅uCu⋅r6⋅(x−C)2

[0057] With the boundary condition that the node voltage at the transition to the anode is equal to the anode voltage U(x=l)=U A must be, is calculated according to (18) as the integration constant C=l−18⋅UAr2⋅k13.

[0058] 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 , i.e., before this zero, a renewed increase of the functions is physically impossible after the field voltage has already dropped to zero. The film deposition on the substrate therefore only begins when the considered substrate point has passed the coordinate x=C, regardless of how long the coating bath has already been passed through. From this, the important condition l≥18⋅UAr2⋅k13 This is the minimum distance between the immersion position and the edge of the anode to allow the substrate to be immersed in the coating bath 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, which should not be allowed.

[0059] The film thickness at the transition from the inlet section to the anode chamber is calculated from (20), (21) and (12b) with x=l to dFl=3⋅κ⋅E2⋅U2A2⋅uCu⋅r⋅ρ2⋅ν23.

[0060] 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 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 r(x)=R(x)Δx=ρBA(x2)≠const and therefore the equations derived from it must also be adjusted.

[0061] In the area under the anode there are no voltage drops in the direction of travel, which is why Fig. 1b no series resistances R are entered and from (6) follows dU(x)x=0

[0062] From this, in the same way as shown above with U=U A the differential equation dI(xI)dxi=UA⋅k1I(x1) with the general solution I(x1)=2⋅UA⋅k1⋅x1. (27) Inserted into (10), the following applies to the film thickness under the anode: dF(x1)=E⋅2⋅UA⋅k1uCu⋅ρ⋅ν⋅x1

[0063] While the film thickness along the inlet section in front of the anode chamber increases quadratically according to (20), it decreases below the anode only with x to.

[0064] In developing (26) to (28), it was initially assumed that x1=0 is located at the left edge of the anode and the substrate is just being immersed there and film formation is starting - hence the use of x1 instead of x. In fact, x1 must be shifted along x such that this current is as large as that from (17) and (21) with x=l. After equalizing the currents, an offset for x1 at the left anode edge, i.e. for x=l, is calculated as x11=9⋅UA32⋅r2⋅k13.

[0065] Inserted into (27), the current at the anode edge is I(x11)=3⋅k12⋅r⋅U2A3 and is therefore as large as the equation (17) gives for this position.

[0066] In summary, for arrangements according to Fig. 1, in which the substrate is pulled through the coating bath as a web in the manner shown at a constant speed, the following conclusions can be drawn on the basis of the developed equations: 1) The coating bath and the film layer deposited on the substrate form, in their joint interaction, an electrical conduction system distributed over the length of the pass, which can be described as a discretized model according to Fig. 1b can be divided into finite length elements. The voltage dividers cascaded in this way can be considered a line network. 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 and its resistivity, the conductance and specific mass of the film layer, the circumferential length of the substrate and its feed rate, and the anode voltage were derived as conditions influencing the equilibrium. 3) The electric field is formed along the inlet section 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 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. 4) If the calculated film formation length with C<0 is greater than the inlet section, i.e., the distance l between the immersion position 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 appropriately selecting the process parameters and the inlet section length. 5) If, however, the calculated length for film formation with C>0 is shorter than the inlet distance, there is an area in the travel direction behind the immersion position 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 if the substrate is fed into the paint bath more slowly according to equation (21), while it would shift increasingly towards the anode as the feed speed of the substrate increases. As the feed speed increases, the achievable film thickness that is deposited under otherwise unchanged conditions also decreases, which 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. 6) Along the inlet section, 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 can be designed to create a conical cross-sectional constriction along the inlet section.

[0067] Fig. Figure 3 shows the variation of stress and film thickness using an example with real parameters. The section through which the film passes is divided into three parts: 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 that the . Fig. 1a meets the above conditions, an important subtask has already been fulfilled.

[0068] The web material could continue to pass through in the same way without any problems. However, it has already been explained why the electric field when the substrate emerges from the coating bath should be as small as possible. If the outlet section is designed similarly to the inlet section, a high-ohm volume resistance is again created. To ensure that an effective voltage drop can also be achieved across this section, an external electrical connection is created between the outlet and inlet sections using a yoke. This does impair the effectiveness of the inlet section design explained above because an additional current results from the yoke. However, there is sufficient design flexibility so that the entire yoke circuit, together with the outlet section, 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.

[0069] Because of the remaining conduction path to the substrate, the outgoing path—unlike the incoming path—does not reach a zero point, which in practice is not necessary to sufficiently reduce unwanted cohesion. However, the aim is to make the outgoing path as high-impedance as possible and the return path as low-impedance as possible so that the voltage divider formed by the two has a minimal output voltage.

[0070] There are two options to keep the resulting resistance as small as possible: In one embodiment of the invention, the substrate inlet and outlet are placed as close together as possible to keep the return path as short as possible. A suitable measure for this is to fold the throughput section into a U-shaped configuration and guide the substrate over deflection rollers in the center of the throughput 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 can be sufficient.

[0071] Particularly for the production of very large quantities and to avoid unnecessary deformation of the substrate, another embodiment according to the invention with an elongated throughput path and minimally necessary deflection of the web material may be preferred. However, this arrangement results in an extension of the return path to approximately the entire throughput path of the substrate. 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 outside along the insulated throughput path, where it finds sufficient space for cross-sectional areas that are many times the cross-sectional areas of the inlet and outlet paths. Both possibilities will be explained in more detail later with the exemplary embodiments.

[0072] To continuously supply the substrate surface with fresh reactants and prevent the accumulation of waste products and sediments, one embodiment provides for an intensive flow of the bath contents along the substrate. This flow is generated by a pump as a circulating flow, thus eliminating the need for external material input. Slender vessels, which are preferred 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.

[0073] Immersed substrate surfaces must be quickly and thoroughly wetted with the electrocoat to prevent puddle defects. In one embodiment, the flow of the bath contents is additionally used to generate intense surface pressure on the substrate at the immersion position, so that possible air pockets are displaced and good mechanical contact is established between the immersion paint and the substrate right from the start of the immersion coating process. For this purpose, nozzles or diffusers are arranged near the immersion position, preferably just below the bath surface. Their flow direction is directed towards the substrate and preferably slightly inclined against the feed direction. This allows a constant surge of liquid to build up at the immersion position, which simultaneously expels air bubbles from the paint bath.

[0074] In a further embodiment, the substrate 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 are preferably mechanically coupled to the substrate via bearing-mounted deflection pulleys or by direct sliding contact. The use of a sound or ultrasonic generator that vibrates the substrate or the entire paint bath can also be provided for this purpose.

[0075] 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 as the substrate emerges from the paint bath. For this purpose, a paint flow is directed near the substrate exit point, preferably towards the substrate in such a way that the flow movement against the conveying direction and the effect of gravity towards the paint bath entrain all loose paint components and flow back into the bath. This paint flow, like the one used for the immersion position, is also preferably branched off as a partial flow of the above-mentioned circulation flow of the paint bath. Because a constant flow is maintained and the substrate is constantly moved, deposition of solid paint components through drying is largely prevented.

[0076] In a further development, additional filters are provided which are inserted into the circulation flow and remove disturbing particles from the paint bath in order to keep it in a clean condition and to supply the film formation with a constantly fresh paint solution, free from contaminants or conglomerates.

[0077] Because the bath must be stabilized at a specific temperature, one 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 ensures rapid and thorough mixing of components of the paint bath at different temperatures. Depending on the system size, Peltier elements or heat pumps, for example, can be considered as heating and cooling units.

[0078] Before the coated substrate emerging from the coating bath is guided over a first roller, it should be at least superficially dry. In an advantageous embodiment, this is achieved by heating it and simultaneously blowing fresh air onto it.

[0079] In order to continuously remove chemical reaction products that hinder the film formation process from the coating bath, one embodiment envisages the use of conventional dialysis cells instead of simple anodes. Such dialysis cells consist of an anode surrounded by an anolyte chamber. A semipermeable membrane at the interface to the coating bath 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.

[0080] 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 proposes to position measuring electrodes in the dip-coat bath and evaluate the electric potential at specific locations. From this, three categories of information can be obtained in particular: 1) Consumption status of the dipping paint bath 2) Monitoring film thickness 3) Target potentials for tracking auxiliary voltages

[0081] This is explained below.

[0082] To maintain consistently high paint quality, constant monitoring and adjustment of the 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 collection, 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 continuously monitored by central process control, just like all other process parameters.

[0083] It was shown that, because the processes always proceed in a similar manner 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 carried out regularly during an 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 in this range. However, the bath resistance can be determined by other methods.

[0084] 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, 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.

[0085] It has already been shown how the degree of film deposition depends on the feed rate of the substrate or its residence time in the bath, 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.

[0086] 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.

[0087] 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 ion and thus material transport to the substrate, it can be set proportionally to the substrate speed to achieve a stabilized film formation rate.

[0088] 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. 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.

[0089] 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 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 via the pipe connections, which would then unintentionally bias the area around the substrate immersion position in particular.

[0090] Therefore, in one embodiment, at least one additional measuring electrode is provided in the paint bath, 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 to these metallic parts – neither in one direction nor the other.

[0091] 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.

[0092] 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.

[0093] The environmental impact of the materials 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 degradable 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 coating bath also produces no waste, apart from a small amount of filter sediment, because it is regularly refreshed by adding transparent paste but does not need to be replaced. Examples of implementation

[0094] Two preferred design principles were specified for the design of the return path: 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.

[0095] The Fig. Figure 4 shows an external view of a reaction vessel in accordance with the invention, which is particularly suitable for small-scale systems. A housing 10 is connected to a double tube 12a, 12b made of electrically insulating material to form a bath vessel with a continuous, elongated U-shaped cavity. The housing contains deflection rollers 14, which are accessible via a removable lid 11. To insert the substrate, the reaction vessel can be completely emptied via the pipe socket, so that the deflection rollers become accessible after opening the lid. Using gravity and a pull wire, possibly also with the aid of a magnet, the beginning of the substrate can be relocated without having to open the lid.

[0096] Between the two legs of the double tube is a pipe section 13, which creates a hydraulic and electrical cross connection between the two pipes. The U-shape has the advantage that the web material is not subject to a preferred direction by gravity when passing through the two vertical main paths, and that the desired proximity between the substrate inlet and outlet is easily achieved, allowing pipe section 13 to be kept short as a return path.

[0097] The substrate S, in the example a flat wire, enters the left tube 12a downwards in the direction of the arrow at the feed rate v, is reversed in its direction of travel by the deflection rollers in the housing 10, and exits the reaction vessel upwards after passing through the right tube 12b. Additional deflection rollers 21, 22 serve to feed and remove the substrate externally.

[0098] The entire vessel system is filled with the paint bath up to the level 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 quantity to the reaction vessel via the hoses 16 and 17. These hoses are equipped with inlet nozzles according to Fig. 6. This creates a steady flow of the bath liquid from top to bottom in a branch across both legs while maintaining a constant fill level. This is an open system, allowing the substrate to flow through without the need to seal the reaction vessel. However, the cross-sections of the upper openings can be reduced using additional lids to limit liquid evaporation.

[0099] An oval-shaped impeller is mounted on the shaft of a motor 18, which contacts the incoming substrate and, as a vibration generator, excites it to continuous mechanical vibrations as it immerses itself in the reaction vessel.

[0100] After passing through the reaction vessel, the substrate passes a high-frequency inductor 19 in conjunction with a fan 20, where it is heated by means of eddy current and dried in the air stream.

[0101] In the Fig. Figure 5 shows the hydraulic system of the circulation circuit with the associated fittings. The reaction vessel can be seen from Fig. 4 with its housing 10, the double tube 12, and the pipe section 13. Via a hose system, the pipe socket 15 is connected in the flow direction to a filter 31, a circulation pump 30, and a thermostat 33, in order to then branch into the two hoses 16 and 17 and flow back into the paint bath. A hydraulic equalizer (not shown) is provided to distribute the flows. The thermostat 33 is formed by a Peltier element, which acts as a heat pump relative to a radiator 34, heating or cooling in one or the other energetic direction as required. The circulation pump ensures a constant flow of the paint fluid 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. Furthermore, the entire contents of the paint bath can be drained quickly and with virtually no loss for maintenance purposes, 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, 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 can provide a field potential that increases the field voltage according to... Fig. 3 depending on the distance to the anode chamber.

[0102] Before entering the reaction vessel, the substrate is intended to pass through suitable preparation, cleaning, and rinsing baths, particularly for degreasing and etching the surface and for pickling. Furthermore, after passing through the reaction vessel, it is intended to pass through a kiln to cure the lacquer layer, preferably with high-frequency inductors for generating eddy currents or with infrared radiators.

[0103] The fig .Figure 6 shows an example of the geometric design of the inlet nozzles - in a) for the left tube for immersing the substrate in the paint bath and in b) for exiting the paint bath. 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 nozzle 40 is directed slightly upwards onto the substrate. This creates pressure on the substrate surface, which firmly applies the paint directly at the immersion position and displaces any air pockets upwards. In contrast, in case 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 forcing it back into the paint bath.

[0104] The Fig. Figure 7 shows the exemplary structure of an anode tube 50 as it is inserted into the left tube 12a in Fig. 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 exclude the flow of current to the substrate. The inner wall of the tube 12a is therefore directly adjacent to the outer diameter of the sealing rings and is not shown here for reasons of clarity. In the lower part of the tube there are a plurality of bores 52 which are covered with a semi-permeable membrane 53. At a radial distance from the membrane and centered by means of spacers 55 is the anode 54, which is shaped like a tube and which is almost flush with the inner diameter of the tube 12a on the outside.Thus, there is a space between the tube 12a and the anode tube 50, which is closed at the top and bottom with sealing rings and forms the anolyte chamber of a dialysis cell. Fresh deionized water is supplied from above via hose 56, while the anolyte solution enriched with acetic acid, which sinks due to its somewhat higher density, is discharged below via another hose 57. Both hoses, as well as the electrical anode lead 58, are passed through the anode tube, sealed there, and then continue upwards along the inner wall of the anode tube. This creates a dialysis cell which, according to the known principle, makes electrical contact with the lacquer bath only via a semi-permeable membrane and extracts the acetic acid that forms from it. In contrast to known designs that form anodic surfaces, the substrate in this dialysis cell is enclosed in a tubular manner.Furthermore, this illustration clearly shows that the anode can only electrically act on the paint bath through the holes 52 in the lower section of the tube, whereby the remaining length of the tube creates precisely the effect of the elongated volume conductor described above. To obtain a larger anode surface and thus higher anode efficiency, the tubular anode shown could also be extended upwards without changing the volume conductor of the inlet section – provided that this does not entail an extension of the area where the holes 52 are located.

[0105] The Fig. Figure 8 presents, as a further exemplary embodiment, a reaction vessel of alternative design, which is more preferable for large-scale plants. A cutout is shown at the front to show details inside. In this example, the paint bath 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 bath. 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 in the manner described.These chambers are enclosed by further chambers 73, which are connected at their ends to chambers 72 in such a way that a closed cavity 74, open only at the top, forms between them. Anode plates 75 are inserted into this cavity approximately halfway along the length of the chambers. These plates are electrically connected to the anode chamber and the substrate via holes 76 covered with semipermeable membranes. This cavity between the chambers is filled with anolyte fluid and, together with the anode plates, forms a dialysis cell. Further details correspond to those already described for . Fig. 4 to Fig. 6. For example, in the middle of the trough there is a pipe socket (not shown here) for discharging the paint liquid for the circulation circuit.

[0106] Space 77 beneath the chambers, like the entire trough, is filled with the paint bath liquid and, with its large cross-section, forms the low-resistance return line between the beginning of the inlet and the end of the outlet sections. Although this design requires a large paint volume, it is advantageous for large throughput quantities in mass production. Of course, the trough can be covered from above to prevent evaporation of the paint bath.

[0107] In Fig.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., practically current-free, input is connected to the measuring electrode M and tracks this potential with the correct voltage, then feeds 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.

[0108] 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 substrate feed speeds.

Claims

[1] Arrangement for electrophoretic dip coating of web material, comprising a dip coating bath through which the web material is passed at a continuous feed rate, wherein an electric field is formed between the web material and at least one counter electrode, which leads to the deposition of a coating film on the surface of the web material during the passage, characterized by , that the immersion lacquer bath is shaped or electrically isolated from other bath areas, at least to a portion of its total volume, by electrically insulating vessels, partitions, tubes or chambers, that this shaped or insulated part of the dipping lacquer bath forms an electrical ohmic volume resistance, which is traversed by the web material as an inlet section in the direction of its feed, on the way from the immersion position to a position of the counter electrodes further away from it, and thereby exhibits such a large quotient of path length and cross-sectional area of ​​the dipping bath in the area of ​​the inlet section that the electric field voltage, which builds up around the web material via this volume resistance in interaction with the temporally and spatially progressive film formation, forms a zero point at the immersion position with a suitable choice of operating parameters. and / or that the immersion lacquer bath is electrically isolated from other bath areas to a further part of its total volume by electrically insulating vessels, partitions, tubes or chambers, shaped in such a way as to be electrically insulated. that this shaped and insulated part of the dipping lacquer bath forms an electrical ohmic volume resistance, which is traversed over part of its length by the web material in its feed direction on the way from the position of the counter electrode to a distant emergence position as a run-out section and is connected to the inlet section over a further part of its length in such a way that it forms an electrical return path between the inlet and outlet sections. [2] Arrangement according to claim 1, characterized by that the dipping varnish bath vessel has at least one inlet and at least one outlet opening for the dipping varnish, both of which are connected to an external circulation circuit driven by a circulation pump in such a way that at least one flow of the varnish bath is formed along the passage of the web material, and that at least one of the said inlet openings is designed as a nozzle directed towards the substrate or as a diffuser directed towards the substrate. [3] Arrangement according to claim 1, characterized by, that an electromechanical vibration generator is arranged near the immersion position of the web material, which is preferably designed either as a rotating or oscillating eccentric or camshaft, or as an electroacoustic transducer, which either directly touches the web material or acts on it via further guide or coupling elements. [4] Arrangement according to claim 1, characterized by , that at least one electrical measuring electrode for deriving the electric field potential is located in the immersion paint bath, which is connected to the current-compensated input of a signal amplifier and is used as a control variable for adjusting operating parameters such as the anode voltage, potentials of the pump housing or other metallic fittings in the paint bath or its circulation circuit and / or the feed rate, or obtains data for maintaining the appropriate bath parameters. [5] Arrangement according to claim 1, characterized by , that the dipping varnish bath vessel has a U-shape with two openings at the top, wherein a first opening for the entry of the web material into the dipping varnish bath in a vertical downward direction, a second opening for its exit from the dipping varnish bath in a vertical upward direction, and a deflecting device for the web material between the downward and upward directions of travel are arranged, and that one or more existing counter electrodes are preferably located in the middle area of ​​the passage of the web material through the dipping varnish bath vessel. [6] Arrangement according to claim 1, characterized by, that the dipping lacquer bath is located in a trough-like vessel, from which parts of the throughput of the web material are separated by means of electrically insulating chambers, wherein the counter electrode is located in the middle area of ​​the throughput through these chambers and its beginning and end areas are electrically and hydraulically connected to the remaining bath volume. [7] Arrangement according to claims 1 and 2, characterized by that a thermostat, 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 by , that an expansion vessel with adjustable volume is arranged in the external circulation circuit. [9] Arrangement according to claim 1, characterized by, that parts of the substrate flow path are further narrowed in their cross-section by at least one further tube- or chamber-shaped insert, which preferably contains a dialysis cell for the removal of reaction products with at least one counter electrode. [10] Arrangement according to claim 1, characterized by , that a dryer, preferably designed as an eddy current inductor or hot air generator in combination with a fan, is arranged near the point where the substrate exits the paint bath.

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