Among the flexible packaging materials, metal foil has the most outstanding position. It is the perfect barrier for moisture and gases, for surpassing any paper or plastic material. In appearance it has a luster and color for the most discriminating task, and it is widely used for packaging luxury items because of it glamorous appeal. It fairly high in cost, compared with other flexible materials. A good definition of foil would be: any rolled section of metal less than 0.006 inch and in various tempers and surface finishes. It is possible to make a 99.99 percent pure aluminum foil; such foil is used for capacitors in the electronics industry. The hardness of softness is dependent upon the composition as well as the treatment of the aluminum foil. In the cold rolling process it becomes work-hardened and must be annealed for most purposes. The rolls of foil are heated to 650 oF for about 12 hrs, or until they reach the degree of softness desired. As a result of this heating, the rolling oils are burned off, and the foil is said to be "dry". If half hard or full hard foil is to be used, it should be remembered that not all the lubricant may be completely burned off and that consequently coating or printing may be more difficult.
As it comes off the rolling mill, the foil is in extra hard temper (H=19) because of the strain hardening that takes place when it is reduced in thickness, and it is necessary to anneal it before it can be used. Heavy foils above 0.002 inch are sometimes only partially softened to an intermediate temper (H = 25 or 27). For most applications, however, it is necessary to have good dead-folding properties, and a dead soft temper, sometimes called O-temper, is used. The tensile strength of aluminum foil is highest when it is in the hardened state. When it is annealed, it loses more than half its strength.