Study and Application of Water-based Metallic Flash Coatings for Aluminum Profiles

【ALUMINIUM NETWORK】At present, most of the aluminum coating is using the powder coating method. The cost of this process is relatively low, the powder coating has a high solid content, does not contain organic volatiles, and is in line with the trend of industrial environmental protection. However, there is a defect in the powder coating application: the decorative effect is poor, so liquid paints such as metallic glitter paint are still selected for products with higher decorative effect requirements. Solvent-based metallic flash coatings can achieve good metal arrangement, excellent metal texture and good workability. However, due to high solvent content, VOC emissions are not up to standard, and there are potential safety hazards in production storage, etc. Aqueous metallic flash coating to solve the above problem.

1. Formulation Design of Aqueous Metallic Flash Coatings for Aluminum Profiles 1.1 Formulation Design Ideas The main factors affecting the arrangement of aluminum powders are: the solid content of the wet film, the thickness of the wet film, the drying speed, the role of additives, and the acid value of the resin. In addition to low-acid resin and fast-drying solvent, solvent-based metallic flash coating can also add CAB to reduce the wet film solid content, improve the workability, and improve the arrangement of aluminum powder. The raw materials used in waterborne coatings, because the diluent is water, determines that it cannot compete with organic solvents at the rate of volatilization. Most of the water-soluble resins have a high acid value, which is disadvantageous for the storage of aluminum powder. Some emulsions have low solids and low gloss after film formation, failing to meet gloss requirements. Considering the use environment of aluminum profiles, a cross-linked curing type coating system must be adopted. From the viewpoint of product performance, workability, and cost, a water-based acrylic amino curing system is used, and a 40% hydroxyl ester emulsion and dimethyl ether are selected as final choices. Waterborne amino resins are used as film-forming substances.

1.2 Effect of rheological additives on the arrangement of aluminum powders In aqueous metallic flash coatings, rheological additives have an extremely important position, affecting all aspects of the performance of the coating. An ideal and effective rheological additive needs to have two functions:

1) Thixotropy. Good thixotropy can play a role in anti-sinking and anti-sagging. More importantly, good thixotropy allows the paint to be sprayed onto the plate, and under the effect of pressure, the aluminum powder maintains a parallel arrangement.

2) Late liquidity. After spraying, the paint needs a certain thixotropy to prevent sagging and maintain the arrangement of aluminum powder. After that, the shrinkage of the coating causes the aluminum powder to be further aligned, and the lowest possible viscosity helps the aluminum powder to align. The influence of rheological agents on the arrangement of aluminum powders is shown in Table 1.

铝型材用水性金属闪光涂料的研究与应用


It can be seen from the above results that the use of an inorganic thickener (thixotropic agent D) has a better arrangement of aluminum powder.

1.3 Influence of film-forming additives on the arrangement of aluminum powder For water-based aluminum powder paint systems, the choice of film-forming aids is mainly based on the following considerations.

1) Good film-forming properties. The use of filming aids can be reduced and the formulation is more flexible.

2) Fast drying. This point is of importance to the orientation and early performance of silver powder.

3) Appropriate volatilization gradient. A suitable amount of slow volatilization film-forming aid has a great effect on the film forming compactness and the prevention of shrinkage.

At present, the common filming aids on the market are DPM, PM, DPnB, TEXANOL and so on. Special occasions will even choose fast volatile solvents such as ethanol, acetone and isopropanol.

In alcohol ether solvents, the choice of PM is mainly due to its rapid volatilization ability and can only be used as a cosolvent. The choice of DPM, DPnB, and TEXANOL is mainly to establish a suitable volatilization gradient. The test results show that the use of coalescents can achieve very good alignment.

1.4 Appearance improvement of water-based paint electrostatic spray coating The construction of water-based paint, compared with solvent-based paint, the larger difference is easy to sag, while affecting the leveling, the appearance of the coating becomes worse. This problem can be well solved by controlling the thixotropic value. Experimental studies have found that when the TI value is between 2.8 and 3.4, the sprayed product has a good leveling and anti-sag balance. The effect of different thixotropic agents on the TI value, appearance, moist heat resistance, and water resistance of the system was investigated in Table 2.

铝型材用水性金属闪光涂料的研究与应用


The test results show that the H15 thixotropic agent can provide a suitable TI value, and has little effect on the appearance, and has almost no effect on the film performance.

2·Aluminum Profiles Waterborne Metal Flash Coatings Performance Specifications Aluminum Profiles Waterborne Metal Flash Coatings The main performance indicators are shown in Table 3.

铝型材用水性金属闪光涂料的研究与应用


3 · Application of aluminum water-based metal flash coating 3.1 Process Flow The process flow is as follows: Upper → Degreasing the substrate → Washing → Passivation washing → Drying → Primer spraying → Primer flash dry (80 ~ 100 °C, 10min) → Top coat spraying → Leveling (10~15min) → Baking (160°C, 10min) → Schematic representation of the next 3.2 production line.

铝型材用水性金属闪光涂料的研究与应用


3.3 Aluminum alloy waterborne flash paint construction common problems and solutions 3.3.1 Coating equipment Because water is a good conductor, so common spraying equipment can not be applied to waterborne electrostatic spraying. The special water-based electrostatic spray systems have all undergone major innovations in the insulation class and are able to fully satisfy the existing water-based automatic spray production lines. Currently, the most famous companies in the industry include ABB and Lanshi. Common waterborne electrostatic spraying systems generally include side spray (commonly known as cobra) spray systems, rotary cup electrostatic spray systems, and DISK dish spray systems.

3.3.2 Recommended Coating Methods for Waterborne Coatings Baking Methods Coatings for Waterborne Coatings Because the volatility of water is not the same as solvent volatility, a pre-drying process (also known as flash drying) is required. The pre-baking process is more important in the spraying process. Only through pre-drying, the moisture in the primer can be volatilized in time, so that the appearance of a good topcoat or varnish can be obtained, which also helps the performance of the paint film. Therefore, most of the water-based coating is recommended to block the gradient temperature baking method, see Figure 2.

铝型材用水性金属闪光涂料的研究与应用


3.3.3 Construction of waterborne coatings under low temperature and high humidity conditions Waterborne coatings are difficult to evaporate under high humidity and low temperature conditions. If they are handled in accordance with normal construction methods, they can easily cause paint film sagging and aluminum powder alignment to deteriorate. The poor appearance of the paint and its degraded performance. In response to this phenomenon, it can be improved from the following aspects:

1) Add the air conditioning and heating to the booth room and install dehumidifiers to reduce the air humidity.

2) The spray booth solves the problem of low temperature and high humidity by transporting hot air.

3) Make sure that the workpiece in the flash dry area reaches the desired effect.

4) Increase the viscosity of the construction and properly adjust the amount of paint applied by the sprayer.

4. Conclusion After researching the relationship between rheological additives and thixotropy, viscosity, anti-settlement, aluminum powder arrangement and product physicochemical properties of aqueous industrial coatings, we have developed water-based coatings that are suitable for both electrostatic spraying and good aluminum powder alignment. Metal flash coating, through the debugging of the customer's production line, can fully meet the electrostatic spraying construction of aluminum profiles, and has excellent coating performance. Now it has been approved by a certain aluminum profile processing factory in Guangdong.

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