Research on the application of electroplating effect powder coating on automotive aluminum wheels

Abstract
Introduces the application of electroplating silver special effects powder coating on aluminum wheels, analyzes the effects of mixed polyester pyrethroid resin, epoxy resin, leveling agent, silver powder special effect arrangement aid, etc. on the coating, performs the electrometric performance test and coating performance test on the wheel through the surface cover acrylic transparent powder, and solves the corresponding problems, and further explores the possibility of the substrate combined electroplating effect powder instead of the car silver paint in the construction.
1 Introduction
China's auto industry has developed rapidly in recent years, with China's auto production exceeding 3 million units in 2018 and sales of 2.95 million units, China has gradually become a big country for automobile sales and use, the entire industrial chain for automobile production is becoming more and more perfect in China, the 13th National Plan for the development of the automobile industry, increase the intensity of auto industry, explore and develop into the field of new energy vehicles, and foresee the future development of automobiles in a blue sea.
powder coating applications have been radiated to all areas of automotive coating, by the environmental protection policy "paint powder" industry, combined with the automotive aluminum wheel industry's own characteristics, powder coating will gradually replace the original automotive parts paint coating, the choice of non-volatile organic compound coatings, will be rapid development in the automotive industry
aluminum wheels as a key part of the automobile, the major wheel manufacturers are looking for new production processes. Now the process of automotive wheels is: first, spray powder coating primer (20min@180 degrees C drying); This paper introduces the three-layer coating process of the existing aluminum wheels into two layers of powder spraying process: first, the spray powder (the bottom surface is combined with a silver plating effect powder coating, 20min@180 degrees C curing); Reducing the intermediate painting link, powder coating is to achieve the performance of automotive silver paint, to ensure the corrosion resistance and aesthetics of aluminum wheels, but also greatly reduce VOC, in line with the strict requirements of national environmental protection policy, aluminum wheel coating production industry new trends.
Figure 1 Coating Schematic
2 Results and Discussion
2.1 Resin Selection
According to the characteristics of aluminum wheel paint requirements, suitable for automotive aluminum hub surface spray powder coating must have high level, good breathability, good chemical resistance and so on, which put forward high requirements for powder formulation. The test powder formula selected epoxy/polyester system, different epoxy equivalent epoxy resin and different acid polyester resin combination, cured at 180 degrees C 20min, coating film will appear different properties, coating performance results as shown in Table 1.
Table 1 resin performance comparison
According to the experimental comparison, first of all, different polyester resin adhesion time and flow are different, in order to make a qualified leveling and fullness of the electroplating appearance, to choose the appropriate polyester resin. Polyester resin is made of polyols and polyic acids for polymerization reaction, when the polyester resin viscosity is large, the polyol TMP and polyic acid TMA in the resin increase, the resin relative to molecular mass increases correspondingly, polyester resin and epoxy compatible difference, the resin system is not dispersed when the coating is cured, the surface leveling effect is poor, resulting in orange peel, while not conducive to floating silver in the coating to the surface migration. However, the choice of polyester resin stick through small, powder coating adhesion time is long, horizontal flow is large, the coating floating aluminum powder is evenly embedded in the coating, too late to move to the surface, will produce serious outcry shrink holes, adverse to construction. In order to meet the needs of the final aluminum wheels, the choice of resin D, its viscosity of 3200mPa.s, softening point 105 degrees C, glass temperature of 55 degrees C, with a suitable viscosity, adhesive flow ratio and excellent leveling performance, can ensure the general performance of the product, to achieve the appearance of automotive wheels requirements.
, a suitable epoxy resin should be selected to balance the relationship with the saturated pyride polyester resin. The test selected epoxy resin with low epoxy equivalent (epoxy value 0.09 to 0.17eq/mg; softening point 86 to 105 degrees C) to help the compatibility of epoxy and polyester resin, so that the powder coating on the surface of aluminum hub leveling and satiety is better, more beautiful. When the experiment selected epoxy resin with high epoxy equivalent, epoxy resin with the increase of epoxy equivalent, softening point, viscosity increased, the brittleness of the coating increased accordingly, the surface of the wheel hub sample is complex, it is easy to crack the coating, which is not conducive to the overall performance of the powder.
2.2 leveling agent on performance
leveling agent is an essential additive in powder coating formulation, in the formulation to add 0.5% to 1.5% leveling agent, can reduce the molten viscosity of the coating, so that aluminum hub electroplating silver powder surface pressure more uniform, to obtain better leveling and fullness. Powder coatings with leveling agents are mostly 100% solid leveling agents, different leveling agent carriers are different, generally silicon dioxide carriers. According to the construction specificity of the wheels, the difficulty of selecting leveling agent is to overcome the problem of acrylic adhesion with the surface cover, followed by the characteristics of high leveling and high fullness of electroplating silver paint. After analysis, the leveling agent A carrier is silicon dioxide, and different amounts of leveling agent are tested to have different effects on the electroplating coating of the bottom surface. Add 1.5% leveling agent to the design to obtain the surface tension required for the hub coating, the surface tension at 31mN/m, the plating coating leveling, fullness meets the hub requirements, and the acrylic transparent layer adhesion is the best. In addition, the overall performance of aluminum wheel CASS, gravel impact and other more sensitive factors also meet the film performance requirements, can meet the car wheel beautiful powder coating appearance requirements.
Table 2 leveling agent dosage on the appearance of the coating
in a more detailed analysis, in Figure 2, we studied the leveling agent dosage and spray hub coating fullness and gloss of two important indicators, leveling agent dosage is small, sprayed on the surface of aluminum hub plating coating level is not sufficient, easy to appear electroplating black silver dot, affecting the beauty of the coating. With the increase in dosing, the coating level is sufficient, the electroplating silver coating film is fully flattened on the hub surface (gloss can reach 420), but when the addition of excessive coating surface appears wave orange peel, while the coating mirror effect becomes cloudy, the gloss is reduced to less than 200. At the same time, the use of leveling agents should also pay special attention to the compatibility with the customer spray site has powder.
Figure 2 leveling agent dosage on the effect of coating orange peel and electroplating gloss
2.3 directional arranger selection
to achieve the appearance of electroplating silver paint effect of the powder is the difficulty of design formula, dynamite plating sample powder can do 400 to 500 gloss, need to add a special directional arrangement agent in the formula, the specific indicators see Table 3. The design and formulation use of floating silver and powder wetting difference, when curing the powder system viscosity increased, the coating of floating silver by the directional arrangement agent action upward migration, in the aluminum hub surface to form a uniform, dense, orderly plating coating.
Table 3 Basic Characteristics of Directional Arrangers
Figure 3 shows the relationship between the coating gloss of directional arrangers, in the formulation debugging process, the recommended addition of directional arrangers 0.3% to 1.5%, the dose is less than 0.5%, the addition does not enhance the role of gloss. When the amount of addition is large, the coating surface will be dry, fog seriously affect the beauty of aluminum wheels, the amount of addition in 0.7% to 0.8%, the best electroplating effect, without affecting the surface cover acrylic transparent powder re-painting performance.
Figure 3 Effect of directional arranger dosage on coating film
2.4 cover light transparent powder
This base powder product belongs to floating aluminum silver powder, floating aluminum silver powder surface has not been treated, poor weather resistance, wheel spray plating powder curing Acrylic transparent powder needs to be applied to the surface of the wheel, surface spraying Guangzhou Dynasky Materials Technology Co., Ltd. A-CL0306A (to industrial mass production supply market), in order to achieve CASS, water resistance and gravel blow performance, in line with the industrial production needs of aluminum wheels (samples see Figure 4).
Figure 4 Automotive aluminum hub sample
2.5 Impact of construction conditions
in line with the automotive electroplating powder coating also requires a high spraying process, first of all, the furnace temperature of Decca on-site tracking test, transparent powder line temperature of 160 degrees C (20min), Decca ordinary line The temperature is 180 degrees C (20min), according to the customer's on-site furnace temperature, set the construction conditions, adjust the Golden Horse gun voltage 70kV, air pressure 65Nm3, electrode 5.2mg/m3, current 17μA parameters spraying, spraying effect is the best. Also pay attention to spray should ensure that the spray gun continuous powder, the hub rotation, such as broken powder and hub uneven rotation, it is very easy to appear orange peel. When spraying aluminum wheels, special attention should be paid to the construction conditions, according to the customer's on-site conditions, to make appropriate adjustments.
3 Automotive aluminum hub coating key performance characteristic test
aluminum wheels as an important part of the car, the coating paint film performance and secondary performance also have strict requirements, must meet the performance test of auto parts manufacturers. Performance tests carried out in the CITIC Dai Truck Wheel Laboratory (as shown in Table 4) except for the high humidity exposure test (water resistance test) are slightly worse, and the rest have basically met the standards required for wheels.
Table 4 Coating Secondary Performance Test Results
For experiments conducted at the Decca site, the lack of powder water resistance (condensate thermostat test) is mainly manifested in the powder plating coating and acrylic transparent coating in the middle of the silver powder layer blackening and adhesion difference, by adjusting the ratio of carboxyl resin, epoxy, improve the overall viscosity of the powder melting time, while reducing the amount of wax powder additives in the formula, adjust the amount of silver in the formulation, adjust the amount of silver The water resistance has the problem of insufficient water resistance performance (condensate constant temperature test) for the experiments carried out at the Daika site, mainly manifested in the powder plating coating and acrylic transparent coating intermediate silver powder layer blackening and poor adhesion, by adjusting the ratio of carboxyl tree fat and epoxy in the formula, improve the overall viscosity of the powder when melting, while reducing the amount of wax powder additives in the formula, regulating the amount of silver in bonding. After adjustment, a single batch of wheel block condensate thermostat temperature test passed, but occasionally there will be black silver point problem, by raising the temperature of acrylic spray wire furnace, improve the curing temperature and curing time of acrylic transparent powder on the surface, will make the adhesion between the coating better, effectively solve the problem of coating blackening.
4 Conclusion
Automotive aluminum hub bottom surface combined with an electroplating silver effect powder coating is characterized by the need to avoid the middle of the wheel painting link, spraying two layers of powder coating wheels can be used in the following lines, compared with paint with Table 5 characteristics. Table
Table 5 powder and paint advantages comparison
the application of an electroplating silver powder on aluminum wheels can meet the corresponding standards of paint, while playing the advantages of replacing paint products, reduce VOC emissions, protect the environment, in line with the national "paint powder" policy standards, although the product still has some performance instability factors exist, can be further tested and improved, to solve existing problems.
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