The application of electroplated silver special effect powder coatings on aluminum alloy wheels is introduced, and the influence of carboxyl polyester resin, epoxy resin, leveling agent, silver powder special effect arrangement aids, etc. in the formulation on the coating performance is analyzed. The surface is covered with acrylic transparent The powder conducts mechanical performance testing and coating film performance testing on the wheel hub, and solves the corresponding problems, and further explores the possibility of applying the powder coating to replace the automobile silver paint on the bottom surface.
Automobile aluminum alloy wheels are key parts of automobiles, and major wheel manufacturers are exploring new production processes. The current process flow of automobile wheels is: first spray powder coating primer (180℃/20min drying), then spray water-based topcoat, and finally spray water-based clear paint or acrylic clear powder coating. In order to simplify the process, this study changed the existing 3-layer coating process of aluminum alloy wheels to a 2-layer powder spraying process: the first layer, spray powder (the bottom surface is combined with a silver electroplating effect powder coating, 180℃/20min curing) ; Second layer: spraying acrylic overprint transparent powder coating. The intermediate painting links are reduced, and the coating performance has reached the performance of the original three coatings, ensuring the corrosion resistance and aesthetics of the aluminum alloy wheel, and greatly reducing the VOC.
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1.1 Main raw materials
Epoxy resin E-12 resin: domestic market; Polyester saturated resin, acrylic overprint powder coating A-CL0306A: Guangzhou Kinte Material Technology Co., Ltd.; Silver powder alignment agent, leveling agent, benzoin: Industrial grade, Ningbo Nanhai Chemical Co., Ltd. product; 2091 floating silver: industrial grade, Benda-Lutz company.
1.2 Coating formula
The formula of powder coating with silver electroplating effect on the bottom surface is shown in Table 1.
In order to make the powder coating coated on the aluminum alloy wheel have the effect of electroplating silver paint, the transparent base powder of Table 1 needs to be bonded according to the proportion (2091 floating silver accounts for 0.5% to 1.5% of the total), and the bonding process is set. The fixed speed of the machine is 200～1400 r/min, and the silver float and the base powder are fully mixed under high-speed rotation. After bonding, the gloss of the powder-coated aluminum alloy wheel reaches 400-450.
1.3 Performance characterization
Test the performance of electroplating powder coating of automobile aluminum alloy wheels according to the standards of wheel factories. Appearance standard: According to Q/DK WJ003—2014 test, it is required that the powder electroplating coating and electroplating silver paint have the same fullness and surface flatness; and passed the paint film performance test of major automobile assembly plants, mainly including electroplating primer and surface The adhesion, impact resistance, water resistance test of acrylic transparent powder, CASS (copper salt accelerated acetate spray) test, filiform corrosion test, etc.
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2.1 Selection of resin
According to the performance requirements of the aluminum alloy wheel coating, the powder coating suitable for the surface spraying of the automotive aluminum alloy wheel must have the characteristics of leveling, air permeability, and excellent chemical resistance. This test uses an epoxy/polyester system, mixes epoxy resins with different epoxy equivalent weights and polyester resins with different acid values, and cures at 180°C for 20 minutes to test its effect on the film properties.
Different polyester resins have different gel time and flow. In order to make powder coatings with leveling and fullness in line with the appearance of electroplating, a suitable polyester resin should be selected. Polyester resin is prepared by the polycondensation reaction of polyol and polyacid. The polyol TMP and polyacid TMA in the resin increase, the viscosity of the polyester resin increases, the relative molecular weight of the resin increases correspondingly, and the compatibility of the polyester resin with epoxy When the coating is cured, the resin system is not well dispersed, the surface leveling effect is poor, orange peel is produced, and it is not conducive to the migration of floating silver to the surface in the coating. However, the viscosity of polyester resin is too small, the gelation time of the powder coating is long, and the horizontal flow is large. The floating aluminum powder in the coating is evenly embedded in the coating, and it is too late to migrate to the surface, which will cause serious bottom shrinkage. The construction is unfavorable. From the data in Table 2, it can be seen that Resin D has suitable viscosity, gelation flow ratio and excellent leveling performance, which can meet the requirements of appearance effect and conventional performance of automobile wheels.
In this experiment, epoxy resin with low epoxy equivalent weight (epoxy value: 0.09～0.17 mol/100g; softening point: 86～105℃) is selected to balance the relationship with saturated carboxyl polyester resin and help epoxy and poly The compatibility of the ester resin makes the powder coating on the surface of the aluminum alloy wheel better level and fullness, and more beautiful. When an epoxy resin with a high epoxy equivalent is selected in the experiment, as the epoxy equivalent increases, the softening point and viscosity of the epoxy increase, and the brittleness of the coating increases correspondingly. The surface of the wheel hub sample is complicated and the coating is easy to appear. The cracking is not conducive to the overall performance of the powder coating.
2.2 Effect of leveling agent on performance
In this experiment, 0.5%～1.5% of the carrier silica leveling agent was added to the formula to reduce the melt viscosity of the coating, make the surface tension of the electroplated silver powder of aluminum alloy wheels more uniform, and obtain better leveling and fullness . According to the construction particularity of the wheel hub, the selection of leveling agent must solve the problem of adhesion to the surface finishing acrylic on the one hand, and at the same time meet the requirements of high leveling and high fullness of electroplated silver paint. Table 3 shows the effect of leveling agent dosage on coating performance.
Adding 1.5% leveling agent to the formula can obtain the required surface tension of the hub coating. When the surface tension is 31mN/m, the leveling and fullness of the electroplating coating meets the requirements of the hub, and the adhesion between the transparent acrylic layer The best effort. In addition, the overall performance of aluminum alloy wheels including CASS (Copper Salt Accelerated Acetate Spray Test), crushed stone impact and other properties also meet the coating performance requirements.
It can also be seen that when the amount of leveling agent is small, the leveling of the electroplated coating sprayed on the surface of the aluminum alloy wheel hub is insufficient, and electroplating black silver spots are prone to appear, which affects the appearance of the coating. With the increase of the dosage, when the leveling agent is used at 1%, the coating level is fully leveled, and the electroplated silver coating is fully leveled and coated on the surface of the wheel hub (gloss can reach 420), but the coating surface appears when excessive Wavy orange peel, while the mirror effect of the coating becomes turbid and the gloss decreases.
2.3 Selection of alignment agent
Achieving the appearance of electroplated silver paint is the key to this research. In order to achieve a high gloss of 400 to 500, a special aligning agent needs to be added to the formula. The specific indicators are shown in Table 4. When designing the formula, the wettability of floating silver and powder is poor. When solidified, the viscosity of the powder system increases, and the floating silver in the coating migrates upward under the action of the aligning agent, forming a uniform, dense and orderly electroplating coating on the surface of the aluminum alloy wheel. .
When the recommended amount of aligning agent is less than 0.5%, it cannot improve the gloss; when the amount is large, the coating surface will be dry and fogging will seriously affect the appearance of the aluminum alloy wheel. The addition amount is 0.7%～0.8%, the electroplating effect is the best, and it does not affect the recoating performance of the surface finishing acrylic transparent powder.
2.4 Overcoat transparent powder
The base powder in this experiment is floating aluminum silver powder. Its surface has not undergone any treatment and has poor weather resistance. It is necessary to coat the surface of the wheel with acrylic transparent powder coating A-CL0306A in order to pass CASS and meet water resistance and crush resistance. Performance requirements.
2.5 Influence of construction conditions
In this experiment, the temperature of Dicastal's furnace was first tracked on site, and the temperature of the wheel hub edge was in poor contact, which was the same as the air temperature. The results are shown in Figure 2. The curing condition of transparent powder wire is 160℃/20min, and the curing condition of Dicastal ordinary wire is 180℃/20min. According to the customer's on-site furnace temperature (Figure 2 and Figure 3), set the construction conditions, adjust the Golden Horse spray gun voltage 70kV, air pressure 65Nm3, electrode 5.2mg/m3, current 17μA, the spraying effect is the best under this parameter. It should also be noted that when spraying, ensure that the spray gun continuously discharges powder and the wheel hub rotates at a uniform speed. If there is powder breakage and the wheel hub does not rotate at a uniform speed, orange peel is easy to appear.
3. Key performance characterization test of aluminum alloy wheel coating for automobile
Except for the high humidity exposure test (water resistance test), the performance tests conducted in the wheel laboratory have basically reached the required standards for wheels, as shown in Table 5.
In the experiments conducted on the Dicastal site, the poor water resistance of the powder coating (condensed water constant temperature test) is mainly manifested in the blackening and poor adhesion of the silver powder layer between the powder electroplating coating and the acrylic clear coating. The carboxyl resin and ring The proportion of oxygen increases the overall viscosity of the powder during melting, while reducing the amount of wax powder additives in the formulation, adjusting the amount of bonding floating silver, and the water resistance of the coating is significantly improved. After adjustment, a single batch of wheel block condensate constant temperature test is qualified, but occasionally the problem of blackening and silver spots still occurs. By increasing the temperature of the acrylic spraying line, the curing temperature and curing time of the surface acrylic transparent powder will be increased. The adhesion between the coatings is better, which effectively solves the problem of coating blackening.
4. Comparison of powder and paint
The feature of the integrated silver electroplating powder coating on the bottom surface of automobile aluminum alloy wheels is that it eliminates the need to spray paint in the middle of the wheel. The wheels sprayed with two layers of powder coating can be used in the next line. Compared with the electroplated silver paint coating, it has the characteristics shown in Table 6. Through experiments and comparisons, the application of silver electroplating powder on aluminum alloy wheels can basically reach the corresponding standards of solvent-based coatings.
5. Summary
By using the spraying equipment of the existing wheel manufacturers to improve the on-site construction conditions, the electroplating effect powder spraying can achieve the appearance of automotive aluminum alloy wheels, achieve the spraying effect of electroplating silver paint, and meet the primary physical properties and secondary chemistry of automotive wheels Performance, meet the existing industrial production requirements. At the same time, powder coating can reduce VOC emissions and protect the environment. Although there are still some unstable factors in the production process of the product, it can be improved through subsequent further tests to solve the existing problems.
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