The specialty and usage of metal aluminum powder

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As aluminum powder has wide application, big demand and numerous varieties, it is one of  broad categories of metal pigment. The aluminum powder which is used on the pigment has metallic color and shielding function as it has scaly shape in its particle. Its industrial production was started long before. The early production method was using mashing and punching way, in which the aluminum debris was put into the groove of the punching machine and mashed continuously by the stamp pestle under the mechanical drive, thus the aluminum which has malleability will gradually change to lamina and crush. Screening the aluminum after it becomes meager and small and fetching the aluminum powders which meet the requirements as the products. The production efficiency of mashing and punching method is very low, in which its products quality are difficult to master. Besides, there is a lot of dust in the production process and it can easy to catch on fire and blast. In 1894, a German named Hamtag produced aluminum powder by using the ball mill, in which the steel ball, aluminum skimmings and lubricant were put into and the scaly aluminum powder was generated by crushing the aluminum skimmings with the flying steel balls, meanwhile, the inert gas was filled inside the ball mill and pipeline. This method is still used today which is called “dry process”. In 1910, an American called J. Hall invented a method that added petroleum solvent instead of inert gas into the ball mill, in which the aluminum powder was composited into slurry with the solvent and became slurry aluminum powder pigment. As this method requires simple equipment and has safe process, besides, its products are very easy to use, it is quickly adapted for use by countries around the world. It is also used in most of modern pigment, which is known as “wet process”. China’s aluminum industry has 50 years of history up to now since 1950’s. Subsonic air atomization method is also adapted in atomizing aluminum powder. Besides, the pressure water atomization method starts to enter the production sequence, in which its products both have irregular shape and spherical shape. The dry process and wet process are both used by the manufacturers to produce the aluminum powder and aluminum paste with ball-milling method, in which the products include leafing type aluminum powder and its aluminum paste as well as non-leafing type aluminum powder and its aluminum paste. Water dispersible aluminum paste which is applicable to water-based paint has already been available for a long time. In addition, the aluminum paste in the form of aluminum alloy such as zinc-aluminum paste is also produced.

Properties and classifications of aluminum powder and aluminum powder paste

The properties of aluminum powder

Aluminum is a kind of silver grey metal which has the relative molecular mass of 26.98, the relative density of 2.55, the purity of 99.5 percents, the melting point of 685 degrees Celsius, the boiling point of 2065 degrees Celsius and the fusion latent heat of 323kj/g. As aluminum has reducibility, it is easy to oxidize and will release heat in the oxidation process. When the oxidation is sharp, it will release 15.5KJ of heat per gram. As aluminum is a kind of ductile metal, it is easy to process. The oxidation film on the surface of aluminum is transparent and has excellent chemical stability. The aluminum powder used in the pigment refers to the particles that have scaly shape, coated with treating agent and is suitable for making pigment. Aluminum paste is the mixtures of aluminum powder pigment and solvent, which also has roughly the same properties and usage with aluminum powder. As it is easy to use, its output and dosage are larger than aluminum powder.

Aluminum powder that is used on the pigment has more specialties compared with other pigment, which can be presented in the following aspects:

Scaly cover specialty

Aluminum powder particle has scaly shape. The proportion between its sheet size and thickness is within the range of (40:1)-(100:1). After the aluminum powder disperses on the carrier, it can be parallel to the substrate, in which many of the aluminum powder will connect to each other and form a continuous metal film by the mutual filling of different sizes particles, thus will both cover the substrate and reflex the light ray of outside coating. This is called the covering power of aluminum powder which depends on the surface area size and can be also called radius-thickness ratio. Aluminum is extended in the grinding process, in which the radius-thickness ratio is increased as well as the covering power is enhanced.

The shielding property of aluminum powder

The aluminum powder dispersed in the carrier will float and parallel to the substrate which is coated with the carrier, thus will form continuous aluminum powder layers that are arranged in multilayer parallel in the carrier film. The pores between layers of the aluminum powder will be stagger to each other and cut off the capillary micropores of the carrier film so as to prevent the outside moisture and gas from reaching the substrate through the pores. This is called the excellent physical shielding of aluminum powder.

The optical property of aluminum powder

Aluminum powder, made from aluminum that has light color and high metal luster, has bright and clean surface and can reflect 60 percents to 90 percents of visible light, UV-light and infrared light. Besides, the object coated with coatings which contain aluminum powder has silver shiny surface. This is known as the reflection ray feature of aluminum powder.

The “contrast effect” property of aluminum powder

As aluminum powder has metal luster and can be parallel to the substrate, its glossiness and shade will be changed with the changes of incident Angle and visual angle of incident light in the carrier that contains transparent pigment, which can be called the “contrast effect”. Aluminum powder is arranged in different levels in the coating, in which the reflection light will have different brightness as the incident light will be weakened differently by passing through different thickness of coating when it exposures on the layers of aluminum flake. When the light is casted into the coating that contains transparent pigment and aluminum powder, the incident light will become colored light by penetrating the pigment particle and change its hue and metal luster after being reflected in different layers of aluminum powder. The incident light and visual angle will have angular displacement from the perpendicular state, while the light will be reflected with different hue and metal luster after penetrating into different quantity of pigment particles and different particle size of aluminum powder. Due to the above specialty of aluminum powder, it has been widely used as the hammer paint or metallic paint in the coatings.

The floating property of aluminum powder

One of large varieties of aluminum powder for sale and aluminum paste is the floating type, in which its scaly floats on the coating surface.