Hu Xiaofeng, QiuXuwei, Yan Haoan, Wang Bo (TOD Chemical Industrial Co., Ltd., Fushan, Guangdong 528515, China)
Abstract : Waterborne metallic paint is a kind of novel environmental friendly paint and has drawn much attention due to its excellent decorative performance. In this article, a waterborne metallic clearcoat has been prepared by using acrylate as main film-forming substance and treated solvent -based aluminum powder as pigment. In addition, the acrylate emulsion having core -shell structure is introduced into the coating system. The rheological property and thixotropic index of coating system are modified. Meanwhile, PUA is incorporated into the coating system to improve stability and other properties.
The solvent volatility (VOC) of traditional solvent-based coatings is PM2.5. One of the main sources. Traditional solvent-based metallic metallic paints contain about 80% of VOCs and will be emitted into the atmosphere during the coating process, which is one of the most important sources of VOC emissions in the coating industry. One of the main base materials of water-based metallic glitter paint is water-based acrylic resin. There are still some problems in using water-based acrylic resins, and there is a large gap with solvent-based acrylic resins. By adjusting the rheological properties and the wetting properties of the main resin to the aluminum powder, the problems of sag, leveling, sedimentation, orientation, and alignment of the aluminum powder can be easily controlled, which greatly enhances the workability of the coating. It has narrowed the gap with solvent-based metallic flash coatings.
1 The basic principle of water-based glitter paint
The reason why water-based metallic glitter primer has extraordinary decorative effect is that this type of paint is added with effect pigments such as metal aluminum powder or pearl powder. This effect pigment is aligned in the coating film. After the light passes through it, it passes through various regular reflections, transmissions, or interferences. Finally, people will see a glittering effect with metallic luster and different colors. The reflection of light on the coating surface can be represented by Figures 1, 2, and 3.
But Figure 2 and Figure 3 are basically ideal orientation effects. For solvent-based coatings, due to the fast evaporation rate of the solvent, it is mainly due to the rapid shrinkage of the paint film that the metallic pigment can be oriented quickly to achieve better results, as shown in Figures 4 and 5.
There are many methods of solvent-based metallic paint to improve the orientation effect of aluminum powder and improve the metallic feel, such as using quick-drying solvents, adding CAB, adding nitrocellulose, adding directional wax, and so on. However, due to its own characteristics, water-based glitter paints cannot volatilize so quickly. None of these methods can make water-based metal glitter paints achieve the immediate effect of solvent-based.
2.1 Experimental materials and equipment
2AB903W core-shell acrylic emulsion, 3UA49W acrylic modified polyurethane dispersion, WD011 aluminum powder treatment agent: Tongde Chemical; aluminum powder: Japan Toyo Corporation; N, N-dimethylethanolamine (DMEA), amino resin: commercially available; B Glycol butyl ether: analytical grade; deionized water: homemade; NDJ-1 rotary viscometer, Tu-4 cup: Guangzhou Standard Geda; W-101 air spray gun: Iwata, Japan; rheometer: Anton Paar.
2.2 Preparation steps of water-based metallic paint
Firstly, the solvent ethylene glycol butyl ether is slowly added to a certain amount of aluminum powder in a certain ratio, and the mixture is stirred uniformly at a low shear rate to make it fully mixed uniformly and ready for use. Secondly, PUA resin and amino resin are added to the acrylic emulsion at a low shear rate, so that the paint liquid is fully mixed uniformly. Then add the prepared aluminum paste, disperse for 5 to 10 minutes under low speed stirring, slowly stir, and finally add 10% DMEA to adjust the pH to about 8.5. Finally, add water to adjust the viscosity of the paint solution (coated-4 cups) for 30 to 50 s , Filtration, water-based metallic glitter base paint is finished and ready to be sprayed. Whether it is the formulation of water-based aluminum powder slurry or water-based metallic glitter paint, it should be carried out at a low shear rate.
2.3 Test methods and test results
This paper mainly tests the final aluminum powder orientation performance of water-based metallic paint. Visually observe the arrangement of aluminum powder. The higher the whiteness of the aluminum powder when viewed from the front, the better the aluminum powder orientation effect; the lower the whiteness of the aluminum powder from the side, the better the aluminum powder alignment effect. Using 2AB903W core-shell acrylic emulsion and adding about 25% resin butylene glycol (BG) solvent, aluminum powder has the best orientation effect. Most of the water-based metallic paints need to be covered in actual application, and after the wet-on-wet process construction, the full performance is tested by baking at 140 ℃ for 20 minutes, as shown in Table 1.
3 Analysis and discussion
3.1 Technical route of water-based metallic paint
The current most demanding application area of water-based metallic paint is water-based automotive OEM original paint. At present, there are basically the following types that can be applied to the market, as shown in Table 2.
It can be seen from Table 2 that the main resins used as glitter paint in the market are mainly core-shell acrylic emulsions and acrylic modified polyurethane (PUA) or PUD. However, as a low cost, it is mainly core-shell acrylic emulsion. The other key point of water-based glitter paint is the thixotropic index Ti, or rheological performance. The rheological effect of Fig. 6 is achieved, and the orientation effect of aluminum powder is better.
At present, there are several directions that can adjust rheological properties in metallic paint, as shown in Table 3.
If it is to make the original automotive paint, the current mainstream technology route is rheological modified wax additive and silicate. BASF and Kansai in Japan mainly use silicate to adjust the rheological effect. Other companies will also Use silicate and rheologically modified wax additives together. However, it is well known that the prices of raw materials purchased by these multinational corporations are much lower than the market price. Moreover, the cost pressure for the original automotive paint will not be as great as that of ordinary industrial paint.
3.2 Characteristics of core-shell acrylic emulsion
2AB903W (Tongde) is a typical core-shell acrylic emulsion. Its basic properties are shown in Table 4.
2AB903W has a core-shell structure. His hydrophobic core possesses most of the resin properties, and its carboxyl shell becomes neutral with amines and becomes water-soluble. After neutralization, the shell swells, and its polymer chains “overlap” and become entangled, forming a high viscosity. Under shear force, the polymer chains are untied and the viscosity decreases. This gives the resin pseudo-plasticity and therefore excellent workability (atomization) and excellent orientation properties of pigments (aluminum powder and mica powder), as shown in Figure 7.
2AB903W This type of core-shell emulsion only works if the pH is increased and the system contains a solvent. It is recommended to control the pH value between 8-9, and the solvent content accounts for 25% of 2AB903W. Figure 8 shows the effect of solvent content on viscosity and thixotropic index at pH = 8.
Based on these characteristics, the preliminary recommendations for water-based glitter paint formulations are shown in Table 5.
3.3 Performance improvement and cost reduction of glitter paint
According to GPC testing, the relative molecular mass of 2AB903W is about 2 million. According to the formula in Table 5, the paint film hardness is above 2H and the flexibility is poor. Some flexible resins need to be spelled in and the aluminum powder orientation cannot be affected. When the 3UA49W resin is incorporated, the flexibility and interlayer compatibility are significantly improved. 3UA49W is a PUA resin with good flexibility and fast drying speed. According to the adjustment, the more suitable formula is shown in Table 6.
It can be seen from Table 6 that after the performance problem is solved, the most expensive formula in the whole is water-based aluminum powder. There are many water-based aluminum powders on the market. The cheap ones may only cost about 100 yuan / kg, but they are better than solvent-based. It is much more expensive, and it needs good appearance, strong metal, and high whiteness, which will be more expensive. In addition, the choice of general water-based aluminum powder is not as wide as that of the solvent type. WD01 treatment agent is a phosphate polymer, which can better process most silver dollar solvent aluminum powder into aluminum powder that can be used in aqueous system. The processing method is simple and only requires low-speed stirring. This processing method has been maturely applied to major automotive paint companies. The flatulence detection equipment is shown in Figure 9.
Because not all aluminum powders are suitable for processing, it is recommended that each aluminum powder be tested for flatulence after processing. According to experience, after being placed in a 40 ℃ water bath, the water surface can be kept from rising 4 cm to meet the requirements for use. Ability to maintain a validity of 6 months. Generally, if the water is not treated properly, the water surface will rise by more than 4 cm after 1 d. See Table 7 for the formula after WD01 treatment.
In this way, the cost of this water-based glitter paint is relatively low, and the production process is simple, and the aluminum powder has a good orientation effect. It is recommended for some markets such as tricycles and four-wheel electric vehicles. The performance test results of the composite coatings made according to this formula are shown in Table 8.
Note: Performance testing conditions: Water-based glitter paint (12 ~ 18) μm is sprayed on the electrophoretic steel plate, and after baking at 80 ° C for 5 minutes, solvent-based varnish (35 ~ 45) μm is sprayed, and the final baking at 140 ° C for 20 minutes is tested.
In summary, from the perspective of raw materials, firstly, water-based aluminum powder was replaced with solvent-based aluminum powder, and the average cost was reduced by about 5 yuan / kg. Not only this, but also greatly expanded the range of aluminum powder selection. Secondly, the core-shell acrylic emulsion partially replaced the expensive water-based polyurethane dispersion (PUD), and the amount of rheological additives was reduced, which reduced the cost by about 8 yuan / kg on average. Although the cost of the formula may still be higher than that of the solvent type, the use of the solvent is reduced, and the use of the gun wash solvent is reduced. The overall cost can be basically the same as that of the solvent type.
Due to the deterioration of China’s climate and environment, the Chinese government has gradually strengthened the rectification of some polluting companies. Now the original automobile paint has been fully used in large automobile factories, such as BAIC, Great Wall, Changan, etc., but there are also many low-end Of customers, such as tricycles and scooters of the old age, these products have small profit margins and low performance requirements. The prices of coatings are basically around 30 yuan / kg, so most of them will not actively replace with higher cost. Water-based car paint. The low-cost metallic paint in this article gives a solution, and the core-shell acrylic emulsion is low in cost, easy to use and simple. It will also be a mainstream technical solution and route of water-based metallic paint. Of course, water-based paint still has its own shortcomings. For example, water-based glitter paint is more dependent on the temperature and humidity of the environment. That is to say, if there is no temperature and humidity control, spraying on the same day, due to rain, the temperature and humidity have changed. The appearance of the product may vary greatly. But if adding constant temperature and humidity equipment will greatly increase the cost of painting. However, I believe that with the increasing pressure on environmental protection and the continuous updating of technology, this problem will continue to weaken and even be effectively resolved.