Electrodeposition apparatus

Abstract: Electrodeposition apparatus comprises a bath (100) including a first, pendant electrode (1) comprising an article to be coated by electrodeposition and a second electrode (2) comprising a membrane (3) through which material attracted thereto can pass by osmosis; means for conveying out of the bath the material that has passed through the membrane; a detector (5) of current or electrical potential at the second electrode; an operator (7) for computing the electrical resistance or voltage drop of the membrane as a function of the current or electrical potential detected by the detector and the voltage applied to the membrane; and a data display (8) that displays the output of the operator.

Description: Field of the Invention This invention relates to electrodeposition apparatus, and more particularly to electrodeposition apparatus of the type comprising a first, pendant electrode comprising an article to be coated by electrodeposition, and a second electrode comprising a membrane. Background of the Invention Electrodeposition is widely used, e.g. for metal undercoating and automatic paint processing of automobile bodies, to give uniform, adherent paint films on a substrate. The paint or other coating material deposited from the electrolytic bath is generally anionic, e.g. a carboxylated resin, or cationic, e.g. an amino resin. These paints are water-soluble, but have a low degree of ionisation on dissolution in water. For this reason, an alkaline neutralising agent such as triethylamine is used with anionic paints and an acidic neutralising agent such as acetic acid is used with cationic paints, to achieve neutralisation and enhance the degree of ionisation in the water. During electrodeposition, further resin is supplied to the solution in order that its concentration should not decrease undesirably. Neutralising agent is also supplied, in order to maintain the degree of ionisation. The consequence is that the neutralising agent accumulates, leading to redissolution of the painted surface or the generation of pin-holes. With a view to avoiding this problem, Japanese Patent Publication No. 45-22231 discloses using an ion-exchange membrane for extracting the neutralising agent, by separating the coated article that is one of the electrodes and the aqueous solution from the other electrode. Through this membrane, the neutralising agent is extracted from the aqueous solution, thereby preventing the concentration of the neutralising agent in the electrolytic bath from increasing. The practical effect is pH control. A disadvantage associated with the use of a membrane is that, even when the ideal balance is achieved, the electrical resistance of the membrane progressively changes to 10-50 times its initial value. In these circumstances, the efficiency of, say, acid removal falls, and the efficiency of electrodeposition is also greatly reduced. By way of illustration, if the electrical resistance of the interior of the membrane is 1Ω and the electrical resistance of the aqueous solution between the exterior of the membrane and the coated electrode is 8Ω, when the resistance of the membrane becomes 11Ω, the electrical current flowing through the solution is halved. For this reason, the efficiency of electrodeposition is halved. When the resistance of the membrane becomes 31Ω, the current passing through the solution and the electrodeposition efficiency are decreased to 25% of their original values. EP-A-302 describes an electrodeposition coating system, wherein an article to be coated and forming one electrode and a plurality of membrane electrodes are provided in one and the same bath. Summary of the Invention Novel electrodeposition apparatus comprises: a bath including a first, pendant electrode comprising an article to be coated by electrodeposition and a plurality of second electrodes each comprising a membrane through which material attracted thereto can pass by osmosis; means for conveying out of the bath the material that has passed through the membrane; an electrode potential sensor arranged close to the outside of each membrane; a detector of current or electrical potential at the second electrode; an operator for computing the voltage drop of each membrane as a function of the current or electrical potential detected by the detector and the sensor; and a data display that displays the output of the operator. The novel apparatus overcomes the problems described above. In particular, by continuously observing the electrical resistance of the ion-exchange membrane that acts to extract the neutralising agent, the apparatus can be maintained in good working order, and the quality and efficiency of electrodeposition are, and remain good. Description of the Invention The invention will now be described by way of example only with reference to the accompanying drawings, and in particular to the embodiments of the invention that are fully or partially shown, schematically, in Figures 1 to 3. These embodiments will be described in connection with the use of a cationic paint. Fig. 2 shows a bath 100 including a pendant cathode 1 and an anode 2 comprising a membrane 3. There exist a plurality of such membrane electrodes. On the application of a direct current from a voltage source 200, electrodeposition occurs, and the resin component and the colloidal pigment molecules that have a positive charge in the aqueous solution W migrate towards the cathode 1. After discharge on the surface of the cathode 1, the solids cohere to form a paint film. A neutralising agent such as acetic acid is also present in the bath. This anionic species moves towards the tubular electrode 3A, and is easily able to pass a membrane 3B, to discharge at the tubular electrode 3A. Since the discharged neutralising agent in low concentration substantially completely ionises, it is attracted towards the anode when an electrical current is running, and will accumulate between the tubular electrode 3A and the membrane 3B. By forcefully circulating a carrier liquid such as pure water in this space, the accumulated acetic acid is continuously discharged out of the bath. During the electrostatic coating process, the total of electrical resistances based on a current value detected by an electrical current detector 5 and a voltage applied to the membrane electrode 3 is continuously computed and outputted by an operator 7. Also, as necessary, the electrical resistance of membrane 3B is outputted from operator 7 in a different form, that is, the change in voltage drop computed based on information from a sensor 4 that detects the electrical potential of the exterior side of membrane 3B and a voltage value applied to the membrane electrode device 3. Furthermore, as necessary, the electrical resistance of membrane 3B is specifically computed and outputted by operator 7 based on the change in voltage drop of membrane 3B and an electrical current value detected by electrical current detector 5. The second electrode 2 preferably comprises multiple membrane electrodes 3. The tubular electrode 3A is made from non-corrosive material. A bottom cover 3D is provided for the device 3, and membrane support material 3E for the upper portion of the device. A cap 3F is provided that can be freely attached or removed. The bottom cover 3D, support material 3E and cap 3F should all be made from insulating material. On the exterior part of the membrane 3B is arranged an electrical potential sensor 4 made up of sensor electrode 4A that is arranged facing the exterior of membrane 3B, tubular support material 4B that is made of insulating material fitted on one end of this sensor electrode 4A, high-resistance material 4C fitted in the tubular support material 4B, and signal line 4E that transmits to sensor output terminal 4D on the other end of tubular support material 4B electrical potential information detected at sensor electrode 4A through this high-resistance material 4C. When the whole apparatus is under operating conditions, the electrical potential on the exterior of membrane 3B is effectively captured by sensor electrode 4A, and through high-resistance material 4C is transmitted to sensor output terminal 4D. Even if for some reason the sensor output terminal 4D is shorted to other material or ground, high-resistance material 4C operates to effectively prevent high current flow to the outside. The sensor 4 is shown as fixed. However, it can be freely attached and removed with respect to the bath 100, or it can be integrated with the membrane electrode device 3. Each of the membrane electrode devices 3 is equipped with a current detector 5 that individually detects current flow between each membrane electrode device 3 and coated substrate. An applied voltage detector 6 detects voltage E applied commonly to each membrane electrode device 3. From the current value detected by the detector device 6 and each electrical detector 5 and also from inputs such as from the sensor 4, and by performing given operations, operator 7 is adapted to compute the electrical resistance of membrane 3B, and the corresponding drop in electrical potential. The operator 7 preferably has a first computing function that operates when current value (Ix) detected by the detector 5 and voltage (E), i.e. the output of detector 6, applied to membrane electrode device 3, is inputted, and based on each of these input data computes the resistance between electrode and coated electrode. A second computing function operates when electrical potential value (Ex) detected by sensor 4 and voltage E are inputted; the operator thus computes the drop in voltage (ΔE) of membrane 3B. A third computing function operates when Ix, E and Ex are inputted; the operator thus computes electrical resistance RK of membrane 3B. The first and second computing functions have, in this example, been set by outside commands from an operating mode-setting device 7A, but they can also be made to work by automatic setting. Examples of the three functions will now be given, respectively. Because the electrical resistance R 1 of the anolyte in membrane electrode device 3 and the electrical resistance R2 of the aqueous solution in the electrostatic bath 100 are known, then based on current I that flows between each membrane electrode device 3 and electrode 1 (in reality each membrane electrode device 3 differs) and voltage E, at the time just after the start of electrodeposition (when the electrical resistance r of paint film on electrode 1 is zero) the electrical resistance R K of membrane 3B is computed as follows: R K = [E-I(R 1 + R 2 )]/I [Ω] Depending on the value of R K , the degree of degradation of membrane 3B can be determined, based on a fixed standard, as the first function. Based on electrical potential Ex and voltage value E, the voltage drop ΔE of membrane 3B is computed as follows: ΔE = E-Ex [V] Depending on the size of ΔE, the degree of degradation of membrane 3B can be determined, based on a fixed standard, or through experience, as the second function. With the inputs of current value Ix, voltage value E and electrical potential value Ex, then based on ΔE = E-Ex [V] R K = Δ/Ix [Ω] the voltage drop value ΔE of membrane 3B, along with electrical resistance R K of membrane 3B, are computed in real time, as the third function. The operation results of operator 7, with the input data, are displayed in a data display 8. Operator 7 also includes an operating mode-setting device 9 that sets externally each operating mode of operator 7, a circuit 10 that judges the degree of degradation of the membrane 3B based on the operation results of operator 7, and a register 11 that records output data 8 of this circuit 10 and the operator 7. The circuit 10 includes a second circuit 10A that judges the degree of degradation of membrane 3B based on the reference value already set corresponding to each operating mode of operator 7, and a display 10B that displays the output of the circuit 10A. Circuit 10A, when putting into action the operating functions of operator 7, provides for operating references. For example, when the electrical resistance R K of membrane 3B surpasses a reference, e.g. 10 times its initial value, then use is disabled (generation of degradation conditions). As a further example, related to the second operating function, when the voltage drop in membrane 3B surpasses a reference, for example 5 times its initial value, then use is disabled (generation of degradation condition). This avoids loss of electrodeposition efficiency. Between the operator 7 and each electrical current detector 5 is a first switching circuit 12 that sends, with a fixed timing, to operator 7, the output of detector 5. Also, between the operator 7 and each electrical potential sensor 4 is a second switching circuit 13 that is a sensor signal switching input that sends, with a fixed timing, to operator 7, the output of each sensor 4. These first and second circuits 12 and 13, as shown in Fig. 3, perform switching that is synchronised with a fixed time frame outputted from operator 7. For the same membrane electrode device 3, they can synchronously input to operator 7 the detected fixed information. For the sake of convenience in illustration, let the electrical resistance R 1 of anolyte Ko in the membrane be 1Ω and the electrical resistance R 2 of the aqueous solution W in the electrostatic bath 100 be 8Ω. The results, from the start of electrodeposition (when the electrical resistance r of a paint film on electrode 1 is zero) are shown in Figs. 4 and 5. As shown in Figs. 4 and 5, as the degradation of electrical resistance R K of membrane 3B advances to R K =5Ω, the electrical current, from an initial 10A, becomes 7.1A and the voltage between membrane 3B in aqueous solution and coated electrode 1, from an initial 80V, drops to 56.8V (drop in voltage of membrane 3B is ΔE=35.5V or an increase of around 3.5 times its initial value). Furthermore, as the degradation of electrical resistance R K of membrane 3B advances and R K =11Ω, the electrical current becomes 5A and the voltage between membrane 3B in aqueous solution and coated electrode 1, from an initial 80V, drops to 40V (the drop in voltage of membrane 3B is ΔE=55V or an increase of 5 times its initial value). In such conditions, clearly, the efficiency of electrostatic painting is halved. For this reason, in such a case (just after the start of electrostatic painting), the degradation judging reference value of circuit 10A is set at, for example, R K =10Ω (a fixed value of less than 5 times the initial voltage drop of membrane 3B). This avoids problems when the efficiency of electrostatic painting drops below 50%. Next, as electrostatic painting advances, the case when the electrical resistance r becomes 10Ω can be investigated. These results are shown in Figs. 6 and 7. As shown in Figs. 6 and 7, when the degradation of electrical resistance R K of membrane 3B advances to R K =5Ω, the electrical current from an initial 5A becomes 4.2A and the voltage between membrane 3B in aqueous solution and coated electrode 1, from an initial 90V, drops to 7.8V (the voltage drop of membrane 3B ΔE=21V increases to around 4.2 times its initial value). In this case, the drop in potential of the coated film is 42V. Furthermore, as the degradation of electrical resistance R K of membrane 3B advances to R K =11Ω, the electrical current becomes 3.33A and the voltage between membrane 3B in aqueous solution and coated electrode 1, from an initial 90V, drops to 59.4V (the voltage drop of membrane 3B ΔE=36.6V increases to 7.3 times its initial value). In this case, the potential drop of the film of the coated electrode 1 is 33.3V. Then, as the degradation of electrical resistance R K of membrane 3B advances to R K =21Ω, the electrical current becomes 2.5A and the voltage between membrane 3B in aqueous solution and coated electrode 1, from an initial 90V, drops to 45V (the voltage drop of membrane 3B is ΔE=52.5V or an increase of 10.5 times its initial value). In this case, the potential drop of the film of the coated electrode 1 is 25V. In these conditions, clearly, the efficiency of electrostatic painting is halved. In consequence, for example, by setting the degradation judging reference value of the circuit 10A to R K =10Ω (a fixed value of less than 5 times the initial voltage drop of membrane 3B), the problem of having the efficiency of electrostatic painting drop below 50% can be avoided. The multiplying factor of voltage drop ΔE of membrane 3B in the case where a paint film of 10Ω is formed on electrode 1 is larger than for the case where no film is formed. Therefore, even by using as a reference the case where no paint film is formed, a degradation judging reference value of the circuit 10A can be set without any particular inconvenience. In this way, circuit 10 can be used to judge the output information of operator 7 based on a fixed degradation judging reference value and is capable of outputting the existence of degradation. This is true when using multiple membrane electrode devices 3, but satisfactory application is possible when using a single membrane electrode device 3. In this case, first and second switching circuits 12 and 13 are not needed. Operator 7 as described above has a structure providing for 3 operating functions, but the apparatus can also be structured to utilise three operators. Further, if desired, the output information of the operator can be judged by a human operator, with experience; in such a case, no circuit 10 is needed. Also, if operator 7 utilises only the first operation function, no electrical potential sensor 4 is required.

Claims: Electrodeposition apparatus comprising: a bath (100) including a first, pendant electrode (1) comprising an article to be coated by electrodeposition, and a plurality of second electrodes (2) each comprising a membrane (3) through which material attracted thereto can pass by osmosis; means for conveying out of the bath the material that has passed through the membrane; an electrical potential sensor (4) arranged close to the outside of each membrane; a detector (5) of current or electrical potential at each second electrode; an operator (7) for computing the voltage drop of each membrane as a function of the current or electrical potential detected by the detector (5) and the sensor (4); and a data display (8) that displays the output of the operator. Apparatus according to claim 1, wherein the detector is a current detector, and which additionally comprises a current switching device that sends to the operator the output of each detector at or over predetermined intervals. Apparatus according to claim 1, wherein the detector is an electrical potential detector, and which additionally comprises a sensor signal switching device that sends to the operator the output of each detector at or over predetermined intervals. Apparatus according to claim 1 or claim 2, wherein the operator has a first operating function that operates when a current value detected by the detectors and a voltage value applied to the membranes are inputted, and based on these input data computes the electrical resistance of the membranes, a second operating function that operates when an electrical potential value detected by the sensors and a voltage value applied to the membranes are inputted, and based on these input data computes the potential drop of the membranes, and a third operating function that operates when a current value detected by the detectors, a voltage value applied to the membranes and a electrical potential value detected by the sensors are inputted, and based on these input data computes the electrical resistance of the membranes. Apparatus according to claim 4, which additionally comprises a current switching input that sends to the operator the output of each detector at or over predetermined intervals, and a sensor signal switching input that sends to the operator the output of each sensor at or over predetermined intervals. Apparatus according to any preceding claim, wherein each sensor comprises an electrode that is arranged opposite to the exterior of the corresponding membrane, an insulating tubular support, fitted on one end of the electrode, including high-resistance material, and a signal-transmitting line passing through the high resistance material. Apparatus according to any preceding claim, which additionally comprises a circuit (10) adapted to output a given signal when the electrical potential of the membrane exceeds a predetermined level.

Inventors: INOUE AKITO, INOUE, AKITO, Inoue, Akito, c/o Polytechs Corporation
Applicants: POLYTECHS CORP, POLYTECHS CORPORATION
Application year: 1993
Main classification: C25D13
IPC codes: C25D13