Aluminum powder in the form of atomized powder, flakes and paste is widely used in the manufacture of paints, explosives, printing inks, fire works, welding electrodes, brake linings, etc. It is manufactured by melting aluminum and atomized followed by further processing of atomized aluminum powder into flakes or paste. It is generally graded according to particle size for different and use applications. For use in the paint industry its particle size or fineness, leafing or non-leafing property and degree of reflectivity are prime considerations. The atomization process yields dull grey powder which finds application in the metallurgical chemical and pyrotechnic industries.
Indian Standards Institution has brought out the following specifications on aluminum powder and paste:
IS: 289-1963; Aluminum Paste for Paints
IS: 438-1972; Aluminum Powder for Explosives and Pyrotechnic composition.
Aluminum powder should contain not less than 95 percent metallic aluminum. Metallic copper should be less than 0.1 percent. Aluminum paste should contain 60-65 percent aluminum and high surface tension thinner.
Pigment grade aluminum powder and flakes contain high proportion of powder passing through 325 mesh screens. The coarsest grades show 6.2 percent retention on 325 mesh, the lining grades about 1.0 percent and the extra fine linings about 0.1 percent. Beside particles size other characteristics used for grading of aluminum powder and flakes for surface coating applications are water covering, specular gloss, apparent reflectance and total reflectance. The specular gloss is measured with standard 60o gloss meter (ASTMD 523-49T); apparent reflectance with 0-45 reflectometer (ASTMD 771-47T) and the total reflectance with a Taylor Baumgartner reflectometer. The above characteristics of various grades of aluminum powders and flakes for applications in surface coating industry. The general range of specific gravity and bulking value for aluminum powder pigments are:
Specific gravity 2.50 2.55
Pound per gallon 20.82 21.24
Gallon per 100 lbs 4.70 4.80
Aluminum pigments consist of relatively pure aluminum. The surface of the particles carries a thin film of aluminum oxide and also a thin layer of stearic acid lubricant. When the flakes of aluminum are small enough and they carry a proper coating of lubricant, they posses the ability to 'leaf' when incorporated in suitable film forming liquids such as certain varnishes. Upon application of such an aluminum paint, most of the pigment particles near the surface of the film are oriented in layers of overlapping flakes parallel to the surface. Very fine grades of pigment are used to produce attractive finishes with excellent metallic sheen. Aluminum pigments are also characterized by high opacity to light.