Some knowledge of aluminum and silver paste (1)

As a kind of metal pigment, aluminum-silver paste can make the coating produce special effect which is different from other ordinary pigments. In the process of use, it is different from other ordinary pigments in terms of coating formula, coating production mode and construction mode. This paper briefly describes some skills in the use of aluminum-silver paste.
1. Orientation of pigments:
When the flake pigment is parallel to the surface of the film, the best effect is achieved. The difference in parallel orientation can lead to "turbidity" or diffuse reflection. The orientation of flake pigments is related to the formula and construction conditions. The volatilization of the solvent causes the shrinkage of the wet coating film, and the aluminum pigment is finally pressed into a horizontal directional position, the stronger this effect is, the better the arrangement effect is, so the more fast volatile strong solvents in the paint formula and the lower the solid content during construction, the better the effect is, but make sure that the film just sprayed is slightly moist and smooth. The higher the solvent content in the coating, the stronger the effect. This explains the orientation of flake pigments. Therefore, the optical properties of low solid coatings are better than those of high solid coatings. The volatilization of the solvent will cause a strong eddy current inside the wet film, but if the solvent volatilizes too slowly, it will form the so-called Bernard vortex, which will cause the aluminum sheet to roll up and down with the Bernard vortex, flow at will and arrange in disorder, resulting in black and white film and a large area of speckles. Hinders the parallel orientation of aluminum pigments. Resins can be used to promote the rapid volatilization of solvents (such as ester butyl cellulose cab), and some auxiliaries are also used. They can fix flakes of paint. It has been reported that wax dispersions have "spacer effect", and surfactants have similar functions. However, its function should be tested before use.
2. Dispersion of pigments:
Aluminum-silver paste contains a certain amount of slow volatile mineral oil and additives to ensure its good storage stability and dispersibility, so the storage of aluminum paste is required to be airtight, otherwise it will cause air-drying oxidation and lead to poor dispersion, paint film granulation, blackening and coarsening. In general, intact airtight packaging can be preserved for more than a year. In order to achieve a satisfactory special effect, metal pigments must be completely dispersed in the coating system, and the coating should be in a uniform state without fine particles. Aluminum scale is easy to bend and break, if it is stirred at high speed or processed continuously and violently in the process of coating production, its geometric structure is easily destroyed, resulting in bad phenomena such as coarse grain, dark color, reduced covering power, metal migration and so on. Therefore, the dispersion method of high shear force should not be adopted. Suggestion: adopt the method of pre-dispersion; first select the appropriate solvent or the mixture of several solvents, add the solvent to the aluminum-silver slurry with the ratio of aluminum-silver paste to solvent at 1:1 or 1:2, and slowly stir until uniform (about 10~20min). Add wetting agent or dispersant to the system. In general, aluminum-silver paste is soaked in solvent for several hours before stirring.
3. Selection of solvents:
The choice of solvent is mainly determined by the paint determined by the formula. Non-floating aluminum-silver paste can widely use polar and non-polar solvents, such as aliphatic or aromatics, esters (such as butyl acetate), ketones (such as methyl ethyl ketone, methyl isobutyl ketone), alcohols (such as ethanol), ether alcohols, among which esters have the best effect. Moisture will react with aluminum to form hydrogen and cause listening effect, so the moisture in the coating should be strictly controlled below 0.15%. It is suggested that dehydrating agent should be used to remove too much water in the paint solvent or diluent. Chlorinated solvents (halogenated hydrocarbons.). It is not suitable for any aluminum-silver paste pigment. Chlorinated solvents release HCI, which reacts with fine aluminum pigments.