The water-soluble resin for anodic electrophoresis is a high acid value carboxylate. It is dissolved in water and exists in a DC electric field in a molecular and ion equilibrium state. After energization, the ions move in an orientated direction due to the potential difference between the two electrodes, and the anion is deposited on the anode surface. While the cation is electron-reduced to an amine on the surface of the cathode, it is an electrochemical reaction involving four simultaneous processes of electrophoresis, electrolysis, electrodeposition, and electroosmosis.
1. Electrophoresis: Under the action of DC voltage, the charged colloidal particles dispersed in the medium move under the action of an electric field to the electrode opposite to the charge it carries, called electrophoresis.
2. Electrodeposition: An anion resin emits electrons deposited on the surface of the anode to form a water-insoluble paint film. This process is called electrodeposition.
3. Electroosmosis: Electrophoresis reverse process, when the anion resin is on the anode, the medium adsorbed on the anode is under the action of the internal osmotic force, and the electroless osmosis is called from the anode through the deposited paint film into the paint liquid.
4. Electrolysis: When the current passes through the paint liquid, the electrolyte will discharge hydrogen gas, and the anode will release oxygen. This process is electrolysis.
Electrophoretic coating
It is said that electrophoretic coatings can be divided into three generations, the first generation is epoxy resin coating, the second generation is acrylic resin coating, and the third generation is polyurethane coating. Since epoxy coatings are mainly used in automotive chassis, the third generation is mainly used for cathodic electrophoretic paints, which are applied to the surface of jewelry. Therefore, the second generation, namely acrylic resin coating, is mainly introduced. The resin is in a mess, the carboxyl group is trapped therein, and the amine group is externally attached. Among them, 70% of the first carboxyl group is substituted by an amine group, and the resin is made water-soluble due to the presence of -COONHR in the resin. Acrylic resins coated on the surface of aluminum profiles are mostly crosslinked and cured by using an amine resin as a curing agent. At the same time, the uniformity of the coating molecules has a great influence on the process operation. Generally speaking, the better the emulsification, the more uniform the molecules.
Painting process
1 Degreasing: If there is an acid recovery device, it is recommended to use alkaline degreasing. After alkaline degreasing, the surface of the aluminum profile is brighter and will not cause side effects with the subsequent alkali corrosion. For example, alkaline degreasing is the main The ingredients are Na 2 CO 3 and NaOH.
2 Washing: Tap water to wash away the acid or alkali in the previous process.
3 Eclipse: the alkali etching process of adding alkali etchant will reduce the surface brightness of the profile, but the effect is not very obvious. It should be noted that the content of Al 3 in the tank should not be too large, and the temperature is too high, otherwise it will easily produce a washable spot. After the paint is dried, it is yellow.
Two washes: It is best to have a spray or increase the overflow to ensure thorough cleaning.
Ash removal: HNO 3 works well, but pay attention to strengthen the water wash (minimum two + spray).
Washing: The tap water is degreased with H 2 SO 4 and washed with water. The ash is removed with HNO 3 and needs to be washed twice.
Oxidation: H 2 SO 4 oxidation is generally 20min, the oxide film reaches 9u, the so-called high-temperature oxidant promoted by some companies, the main component is a mixed acid, it is recommended not to use, the color, hardness, colorability of the oxide film Nothing is good.
Washing: The tap water increases the overflow.
Coloring: Single tin salt, single nickel salt, tin nickel composite salt can be used, be careful not to have color difference, because the color difference will increase after electrophoretic painting.
Washing: It is best to add spray to minimize the amount of acid in the subsequent process.
Hot pure water washing: required conductivity < 100us / cm, temperature 70-80 ° C, PH = 4-6, especially silver-white painted profiles or large voltage profiles in oxidation should be treated in this tank for a long time, PH The value can be adjusted with triethylamine.
Pure water washing: required conductivity <100us / cm, room temperature, PH = 4-6, can be treated for 2-4 minutes. The pH can be adjusted with triethylamine.
Five electrophoresis coating equipment
Electrophoresis tank and its auxiliary equipment
Electrophoresis tank: The tank body is usually lined with hard PVC plastic or epoxy glass reinforced plastic.
Overflow tank: Its function is to control the height of the paint in the electrophoresis tank and remove the foam on the surface of the paint. Its capacity is usually 1/5 of the capacity of the electrophoresis tank.
Stirring cycle system: Its main function is to ensure uniform composition and concentration of paint liquid. The flow rate of the circulation pump should be such that the paint solution of the entire electrophoresis tank can be circulated 4-6 times in one hour. When the temperature is high or continuous production, there should be a cooling device in the circulation system due to the temperature rise. In the winter, if the temperature of the bath drops below 10 °C, it must be heated up for production. It can be heated by the heat exchanger or directly into the overflow channel of the heating pipe. The latter method must ensure that the heating pipe does not leak. The heat exchangers used for heating and cooling are generally turbulent-promoted, in the form of plate and frame and tubular (generally without plate and frame).
Electrode: A plate: usually 316 stainless steel. It should be noted that the anode area cannot exceed 2 times the cathode plate anyway, and for safety reasons, the area ratio is generally 1:1. Therefore, the manufacturer must clarify the area of ​​the cathode plate from the designer. B pole cover: mainly used to collect the H 2 generated by the reaction on the cathode. Of course, some manufacturers use semi-permeable membranes or 1 # industrial canvas to make bags with epoxy adhesive, in addition to collecting H 2 , The pH in the tank can be adjusted. When it is used, it is filled with deionized water, and the NH 4 + ions generated during electrophoretic coating enter the bag through the semi-permeable membrane under the action of an electric field, and are regularly excluded to keep the pH value within a certain range. Of course, it also has a part of the action of removing impurities.
Power supply: At present, there is almost no gap between the electrophoretic paint power supply made in China and the internationally advanced power supply (such as ELCA, Japanese tricolor, etc.). In general, the ripple factor is <6%. Of course, if the customer needs it, it can be made <3% and <1%, but it doesn't make much sense. There are two control methods in use, namely, constant voltage and constant current.
Constant voltage: This is the most commonly used method in domestic production. The magnitude of the voltage setting is mainly determined by the anode workpiece area, the bath temperature, and the molecular weight of the coating.
Constant current: generally set at 10A/m 2 , the voltage can be slowly increased, when it rises to the set value, or when the set time (usually 2-3 min) is cut off.
Deionized water production equipment; generally requires the purity of water to be more than 1MΩ. The specific equipment should be determined according to local water quality report and water demand. At present, the most widely used configuration among domestic manufacturers is electrodialysis and ion exchange. Of course, RO is also used abroad, but the content of SO 4 2- and Cl - must be strictly controlled regardless of the equipment used.
Paint refining equipment: cation exchange resin tower and anion exchange resin tower, the resin capacity is generally yang resin: anion resin = 1:3. The cation resin is a macroporous if acidic resin, the anion resin is a strong basic resin or a large hole Weakly basic resin, the operation process is as follows:
refined:
Anion exchange resin column: The water in the column is discharged to a height of about 50 to 200 mm higher than the resin, and then the electrophoretic coating is fed from the top of the column.
Cation exchange resin column: The water in the column is discharged to a height of about 50 to 200 mm higher than that of the resin, and then the electrophoretic coating is fed from the top of the column.
Double tower linkage: follow the above steps to operate the twin towers.
regeneration:
Anion exchange resin tower: use compressed air to discharge part of the paint, so that the liquid level is about 50-200mm higher than the resin, discharge the paint to the overflow tank with deionized water, then deionized water enters from the bottom of the tower, rinses the floating resin bed, and then rinses the floating resin bed. It is discharged from the top of the tower, and then stopped for a while, so that the resin bed floating due to the countercurrent feed is settled by gravity, and then 25% of the NaOH solution is fed, enters from the middle of the tower, and is discharged from the bottom of the tower for 30 minutes. In order to fully and effectively utilize the remaining NaOH solution in the tower to regenerate the resin, slowly enter the deionized water from the top of the tower, discharge from the bottom of the tower for 30 minutes, and finally, enter the top of the tower with a large flow of deionized water. The bottom is drained until the outlet has a pH of <8 and lasts for about 50 minutes.
Cation exchange resin tower: Therefore, tower regeneration is more complicated, so it is listed step by step.
Compressed air is introduced from the top of the tower, and the paint is discharged from the bottom of the tower to a height of 50 to 200 mm above the liquid level. The electrophoretic coating is discharged from the top into the deionized water to the overflow tank. The deionized water was entered from the bottom of the column, and the floating resin bed was rinsed and then discharged from the top of the column for 30 minutes. Compressed air is introduced from the top of the tower, and the water in the tower is discharged to 50 mm above the resin. Inject 25% ammonia water through the ammonia nozzle with deionized water, and thus dilute 5%, then input the diluted ammonia water from the bottom of the column to make the liquid surface 100-200 mm higher than the resin. Air enters the bottom of the tower and is stirred with resin and ammonia for 5 min. Compressed air enters from the top of the tower, and the aqueous ammonia solution is completely discharged to the wastewater from the bottom of the tower. From the bottom of the tower, enter the deionized water, flow through the floating resin bed, thoroughly rinse the remaining ammonia water in the tower, and then discharge it from the top of the tower for 30 minutes. At rest for 5 min, the resin floating in the countercurrent flushing settled by gravity. 35% hydrochloric acid was inhaled with deionized water to dilute its concentration to about 3%, then passed through the middle of the column, passed through the resin bed, and discharged from the bottom of the column for 30 minutes. Deionized water was slowly introduced from the column, and the bottom of the column was discharged to fully utilize the remaining hydrochloric acid regeneration resin for 30 minutes. The compressed air is introduced from the top of the tower, and the water in the tower is discharged to a height of 50-100 mm above the resin. Compressed air was introduced from the bottom of the tower, and the top of the tower was discharged, and the resin was stirred for 5 minutes. Deionized water enters the top of the column, washes the resin, and is then discharged from the bottom of the column for 50 min.
Paint recycling equipment: The recovery of electrophoretic coatings generally uses ultrafiltration or reverse osmosis, but its working principle is fundamentally different. There are generally two kinds of use processes, which are described as follows:
mechanism:
Ultrafiltration UF: Separation of polymer and electrolyte, it can also be said that the paint water is separated, the working pressure is about 1Kg, about three times the osmotic pressure, and the membrane pore is larger.
Reverse osmosis RO: Purify or concentrate the solution, that is, to separate the organic matter from the inorganic matter, the working pressure is about 25Kg, about 50 times the osmotic pressure, and the membrane pore is small.
Operating Process: The characteristics of this process are that it must operate with production, otherwise it is meaningless. The characteristics of this process are quick results, especially when using RO, the water can be drained to reduce the burden on the ED tank refining equipment.
Oven: There are three main heating methods, namely convection heating, infrared heating, and far infrared radiant heating. The most widely used one is convection heating. The temperature difference between the points in the oven should be within 5 °C, and the temperature recovery speed is fast.
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