Aluminum foils are made by rolling of aluminum slabs. The slabs of aluminum are scalped before they are rolled to remove the oxide film from the surface. Then they are hot to reduce the thickness sufficiently for cooling (0.2 to 0.25 inch). Great care must be used to avoid overheating, which would produce oxides that might accumulate on the milt rolls and lead to perforations in the finished foil. When aluminum in annealed once at this stage, it is cold rolled to the required thickness without further annealing. Each pass through the cold rolling mill reduces the thickness by one-half. For thickness below 0.001 inch two layers are usually put through together for the final pass. This produces a shiny finish on one side and a satin finish on the other. This pairing is not only to increase production but also to help control the thickness. Since the foil is being reduced in thickness, it leaves the rolls faster than it enters and thus burnishes the surface to a mirror finish. A certain amount of lubrication is necessary, depending upon the degree of polish required. Kerosene blended with oleic acid, palm oil, or lauryl alcohol is cooled and sprayed generously over surface of the foil before it passes through the mill rolls. If the oil is not blended with sufficient slip characteristics, the foil will be marked with a herring bone pattern. On the other hand, if it is too slippery, the surface will not be as brightly polished as it should be.
Adhesive for Lamination
The least expensive material for handling aluminum foil to paper is sodium silicate solution, which is widely used for cigarette packages and soap wrappers. Other type of water soluble and water-emulsion adhesive can also be used. For special application casein-latex formulations and resin emulsions may be required, but these are more expensive materials. A dying tunnel must be used before the material can be rolled up, and since the foil is a perfect barrier for moisture, the amount of dryness must be carefully controlled before it is trapped by coiling. If a foil to foil lamination is sometimes used to provide a heat sealable material. In this case a light weight tissue of about 4 to 5 kgs. per ream is bonded to foil with upto 10 kg. of microcrystalline wax per ream. When heat is applied in fabricating a package, it drives off the wax through the tissue to form a bond with the adjacent surface.