Principle of electrophoresis
Electrophoresis is basically the same as traditional light and transparent electrophoresis. Anodic electrophoresis is performed under the action of direct current. Under the action of the electric field, the coated particles with charge move to the coated workpiece, so that the electrophoretic coating deposited on the surface of the aluminum profile precipitates, forming a film. The difference is in the type of coating and slightly different equipment.
Electrophoretic coatings include acrylic resins, melamine resins, pigments, solvents, neutralizers, additives and other ingredients. By thermal fixation in the viaduct layer of particles, the shape of the concave coating is essentially unchanged during baking and the appearance of the film is uniformly formed. Dimming can be adjusted by the ratio of high gloss paint to low gloss paint.
Electrophoretic coating has the following characteristics compared with other coating methods:
(1) the use of water-soluble coating, water as the solution medium, save a lot of organic solvents, greatly reduce the air pollution and environmental hazards, safety and health, and avoid the hidden danger of fire;
(2) high coating efficiency, coating loss is small, the utilization rate of coating can reach 90 percent 95 percent ;
(3) Uniform film thickness, strong adhesion, good coating quality, all parts of the workpiece such as the inner layer, sag, weld and other places can be uniform, smooth film, solve the coating problem of other coating methods for complex shape workpiece;
(4) High production efficiency, construction can realize automatic continuous production, greatly improve labor efficiency;
(5) the equipment is complex, the investment cost is high, the power consumption is large, the drying and curing temperature is high, the coating and coating management is complex, the construction conditions are strict, and the wastewater treatment is needed;
(6) only water-soluble paint can be used, and the color can not be changed in the painting process. The stability of the paint is not easy to control if it is stored for a long time.
One, electrophoresis coating equipment
Electrophoretic coating equipment is composed of electrophoresis tank, mixing device, coating filter device, temperature regulation device, coating management device, DC power supply device, washing device after electrophoretic coating, ultrafiltration device, baking device, spare tank and so on.
The size and shape of the electrophoresis tank should be determined according to the size, shape and construction process of the workpiece. Under the condition of ensuring a certain distance between poles, it should be designed as small as possible. The tank is equipped with a filter device and a temperature regulating device to ensure a certain temperature of the paint and remove impurities and bubbles in the circulating paint. The stirring device can make the working paint keep uniform and consistent, and the circulating pump is mostly used. The circulation of paint is generally 4-6 times per hour. When the circulating pump starts, the surface of the paint in the tank should be turned evenly. The function of the coating management device is to supplement and adjust the coating composition, control the PH value of the tank liquid, remove the neutralizer with diaphragm electrode and eliminate the low molecular weight components with ultrafiltration device. Dc power supply is generally used for selection of electrophoresis power supply. Rectifier equipment can be silicon rectifier or silicon controlled.
Two, the main process parameters affecting electrophoretic coating
1, the voltage
Electrophoretic coating is a constant voltage method, the equipment is relatively simple, easy to control. Voltage has great influence on paint film; The higher the voltage is, the thicker the electrophoretic paint film is, and the coating ability can be improved accordingly for the parts that are difficult to be coated and the construction time can be shortened. But voltage is too high, can cause paint film surface coarse, easy generation after drying "orange peel" phenomenon. Low voltage, slow electrolytic reaction, thin and uniform paint film, poor penetration force. The choice of voltage is determined by coating type and construction requirement. Under normal circumstances, voltage and coating solid cent and paint temperature into inverse ratio, proportional to the distance between the two poles. Steel surface is 40 70V, aluminum and aluminum alloy surface can use 60 100V, galvanized parts use 70 85V.
2. Electrophoresis time
The film thickness increases with the extension of electrophoresis time, but when the film reaches a certain thickness, it can not increase the thickness, but will aggravate the side reaction. On the contrary, the electrophoresis time is too short, the coating is too thin. The electrophoresis time should be as short as possible according to the voltage used to ensure the quality of the coating. General workpiece electrophoresis time is 1 to 3 minutes, large workpiece 3 to 4 minutes. If the surface geometry of the coated object is complex, the voltage can be appropriately increased and the time can be prolonged.
3, coating temperature
Coating temperature is high, film forming rate is fast, but the paint film appearance is rough, will also cause coating deterioration; Low temperature, less electrodeposition, slow film formation, thin and dense film. In the process of construction, due to electrical deposition part of the electrical energy into heat energy, mechanical friction in the circulation system to generate heat, will lead to coating temperature rise. General paint temperature control in some aspects 15 30 degree .
4, paint solid points and yan base ratio
The solid cent of the electrophoretic coating that sells on the market is generally about 50 percent , when construction, need to use distilled water to control the coating solid cent in 10 percent 15 percent . The solid content is too low, the covering power of the paint film is not good, the pigment is easy to precipitate, and the stability of the paint is poor. Solid points too high, viscosity, will cause the paint film rough loose, poor adhesion. The face base ratio of high gloss electrophoretic coating can be controlled at 1 to 4. As a result of actual operation in, the pigment quantity of coating can drop gradually, must add pigment to cent tall coating to adjust at any time.
5, the PH value of the coating
The PH value of electrophoretic coating directly affects the stability of bath. If PH value is too high, the newly deposited film will dissolve again, the film will become thin, and the film will be removed after washing after electrophoresis. PH value is too low, the workpiece surface gloss is inconsistent, the stability of the paint is not good, the dissolved resin will be precipitated, the paint film surface is rough, adhesion is reduced. Generally, the PH value should be between 7.5 and 8.5 during construction. During construction, cationic ammonium compounds accumulated in the paint due to continuous electrophoresis, resulting in an increase in PH value. The solution with low PH value can be added, the distilled water of the cathode cover can be replaced, ammonium ion can be removed by ion exchange resin, and the PH value can be reduced by anodic cover. If the PH value is too low, can be added to adjust the ammonium ethanol.
6. Coating resistance
The impurities ions brought into the electrophoretic tank by the previous process of the coated objects cause the decrease of resistance value of the paint, resulting in uneven roughness and pinholes and other defects of the paint film. In painting construction, the coating should be purified. In order to obtain high quality coating, cathode cover equipment can be used to remove positive ions such as ammonium and calcium and magnesium impurities.
7. Distance between the workpiece and the cathode
Short distance, high deposition efficiency. But the distance is too close, can make paint film is too thick and generation flow hangs, orange skin to wait for malady. Generally, the distance is not less than 20cm. For large and complex shape of the workpiece, when the external has been deposited very thick film, and the internal film is still thin, should be far from the cathode part, increase the auxiliary cathode.
Methods and skills of electrophoretic painting
(1) Electrophoretic coating of metal surface, its technological process is as follows:
Pre-cleaning → on-line → degreasing → washing → rust removal → washing → neutralization → washing → phosphating → washing → passivation → electrophoretic painting → in-tank cleaning → ultrafiltration washing → drying → offline.
(2) The substrate and pretreatment of the coating have a great influence on the electrophoretic coating film. Castings generally use sandblasting or shot blasting for rust removal, with cotton yarn to remove floating dust on the surface of the workpiece, with 80 120 sand paper to remove residual steel shot and other sundries on the surface. The surface of steel is treated with oil removal and rust removal. When the surface requirements are too high, phosphating and passivation surface treatment can be carried out. Ferrous metal workpiece must be phosphating before anodic electrophoresis, otherwise the corrosion resistance of the paint film is poor. Phosphating treatment, generally choose zinc salt phosphating film, thickness of about 1 2μm, requires fine and uniform crystallization of phosphating film.
(3) The circulation quantity of electrophoretic coating system directly affects the stability of bath liquid and the quality of paint film. With the increase of circulation, the precipitation and bubble in the tank decrease. However, the aging of the tank is accelerated, the energy consumption increases, and the stability of the tank becomes worse. It is ideal to control the circulation number of tank liquid 6 8 times /h, not only to ensure the quality of paint film, but also to ensure the stable operation of tank liquid.
(4) Ultrafiltration system controls the concentration of impurity ions brought into the workpiece to ensure the coating quality. In the operation of this system, attention should be paid to the continuous operation of the system after operation, discontinuous operation is strictly prohibited to prevent the drying of the ultrafiltration membrane. The dried resin and pigment adhere to the ultrafiltration membrane and cannot be thoroughly cleaned, which will seriously affect the permeability and service life of the ultrafiltration membrane. The effluent rate of ultrafiltration membrane decreases with the running time, and it should be cleaned once for 30 to 40 days in order to ensure the ultrafiltration water required for leaching and washing.
4. Excellent product performance
(1) High transparency and full size, with special gloss and transparency, the coating has a high three-dimensional effect, and the color of the substrate is obvious.
(2) high hardness, when baked at 180 degree for 30 minutes, the hardness can reach 4 to 5 hours.
(3) Good fluidity, firm film and smooth film.
(4) Strong binding force and permeability, coating is everywhere, wet film and dry film have a strong binding force.
(5) good performance, the workpiece does not change color at the baking temperature of 180 degree 200 degree , resist discoloration and corrosion, the product color and luster is bright and lasting, the corrosion effect is ideal.
(6) The average electrophoretic effect is good, the paint film is even and smooth, at 90-200V can reach 10-16um, high efficiency, low power consumption.
(7) Good impact resistance and artificial sweat resistance.
(8) It can be used for aluminum, metal plating, precious metal accessories, lamps and high-end furniture surface protection.
(9) It can be electrophoretic into a variety of color paste: titanium, ordinary gray, pure black and other colors. Because of the high screening rate of matte electrophoretic coating film, the extrusion die pattern on the surface of aluminum has a certain hiding effect, and also has a certain compensation effect on the color difference of dyeing process.
Five, the main characteristics of high-grade electrophoretic aluminum:
1, with strong film hardness, strong impact resistance.
2, with high adhesion film, not easy to fall off aging.
3. It has better wear resistance, weather resistance and alkali resistance than aluminum oxide.
4, the surface color is rich, beautiful, with mirror-like luster effect.
Six, the principle and application of extinction paint
With the development of electrophoretic coating technology, in China's coating industry, it has become the world's top coating manufacturers, formed relevant industrial groups, and actively promote the technological revolution and development of traditional coating field. During this period, bright and bright paint with its bright colors and bright colors and welcomed by consumers, and in a long period of time by the paint market. However, with the improvement of people's living standard, on the one hand, consumers feel that high gloss coating is more serious than reflective coating, which is harmful to people's eyes. On the other hand, consumers have an increasingly casual, fashionable and personalized aesthetic concept. Coupled with the booming demand of China's electronics and electrical industry. These factors have led to a sharp increase in the demand for low-gloss coatings with a soft appearance. At the same time, how to produce matte coatings must be considered.
Seven, gloss factors
Light projected onto the surface of the object will reflect light, and the surface of the object will reflect in light. Different surfaces have different glossiness. Measuring the surface size of light reflected by light is called glossiness. Glossiness is expressed as a percentage. The higher the glossiness of a surface, the greater its ability to reflect light and its brightness. Glossiness is measured by a photoelectric gloss meter. According to the size of gloss, coatings can be divided into light, matte and weak light. Coating gloss classification (60 reflections) :
(1) Low light: gloss less than 10 percent ;
(2) matt: gloss 10 60 percent ;
(3) light: gloss GT; 60 percent .
Eight, the influence of gloss factors
Surface gloss of an object is closely related to surface roughness. When light is on the surface of an object, some of it is absorbed by the object, some is reflected and scattered, and some is refracted. The smaller the surface roughness, the more light reflected and the higher the gloss. Conversely, if the surface of the object is not flat, the scattered light increases, causing the luster to decrease. The surface roughness (H) of the polished object can be perceived by human vision, and the theoretical knowledge of the microscopic surface can be calculated according to the following formula: H = λ/ cos a, where λ is the wavelength of incident light; One is the incident Angle. If the incident Angle is 60 degree , h =1.1μm is calculated. When the surface roughness is greater than 1.1μm, it will show uneven and glossiness decreases.
