Characteristics and Properties
Aluminum foil is a solid sheet of aluminum, or of an appropriate aluminum alloy, rolled very thin, varying from a minimum thickness of about 0.00017 in. to a maximum of about 0.0059 in. (aluminum 0.006 in. is sheet). From the standpoint of packaging and other principal applications one of the most important characteristics of aluminum foil is its impermeability to water vapor and gases. Bare foil one mil and thicker is completely impermeable; much thinner gauges laminated to an appropriate film form impervious composite materials, making them ideal for packaging and general insulation/barrier applications which, with solid foil semi-rigid containers, account for most foil consumption.
While most aluminum foil is employed in packaging, its many unique properties also account for its use in literally hundreds of other applications.
Chemical Characteristics of Aluminum Foil
Resistance of aluminum foil to chemical attacks depends upon the specific compound or agent, but it has excellent and good compatibility with most products. Even some of the compounds classified as only fair in this respect are frequently packaged in coated or film-laminated foils.
Aluminum has high resistance to most fats, petroleum greases, and organic solvents. Intermittent contact with water generally has no visible effect on aluminum otherwise exposed to clean air. However, standing water in the presence of certain salts and caustics can be corrosive. For example, some hygroscopic products packaged in aluminum foil may cause some reaction, particularly if the product contains salt, or salt and some mild organic acid. In these or any other applications which may subject the aluminum to mild attack, coating or lamination protection is employed on the foil surface next to the product.
In general, such food products as candies, milk, unsalted meats, butter and margarine are compatible with bare aluminum. They also greatly benefit from its opacity, which retards deterioration from exposure to light. Similarly, many drug and cosmetic products are compatible with aluminum foil and also must be protected from light.
Aluminum resists mildly acidic products better than it does mild alkaline compounds, such as soaps and detergents. While use with the stronger concentrations of mineral acids is not recommended without proper protection because of possible severe corrosion, weak organic acids, such as those found in foods generally, have little or no effect on aluminum. Reliable information and suitable tests are always recommended.
Mechanical Properties of Aluminum Foil
Mechanical properties of popular aluminum foil alloys are given in the Appendix, Table 2A.
Because these foils are made of aluminum and aluminum alloys, their compositions are exactly the same as any aluminum products made of those same alloys. Nothing is added to the metal for foil stock. Each wrought alloy has its own four-digit number, which indicates its composition; even the various high-purity, unalloyed aluminums, which are used for some foils, have their specific alloy numbers.
Mechanical properties of any given aluminum foil, therefore, are directly related and similar to those of sheet or plate made of the same alloy in comparable temper or metallurgical condition. The only differences arise from the extreme thinness of the foil gauges of the metal.
The Metallurgy of aluminum is a technical subject beyond the scope of this book. However, an understanding of the mechanical-property-related terms used for aluminum, hence for foil, requires a general familiarity with some of aluminum’s metallurgical properties and their related terminology. It is of particular importance to be familiar with how aluminum alloys are tempered and annealed, or hardened and softened.
Tempering Aluminum-As earlier implied, adding certain other elements strengthens aluminum; the specific alloys thus produced can be further strengthened by mechanical and/or thermal treatments of varying degree and combinations.
The lowest or basic strength of aluminum and each of its alloys is determined when the metal is in the annealed or soft condition. This is designated as the O (letter O) temper. For example, the annealed condition for alloy 1145 is written 1145-O. Annealing consists of heating the metal to the appropriate temperature for the correct period of time. The reroll stock, from which foil gauges are produced, for example, is annealed prior to the rolling operations, so that it will be of maximum workability. All alloys can be annealed.
All alloys also are strain-hardened and strengthened when cold worked, as in foil rolling. When the final product is wanted in the soft condition, it is given a final anneal.
Because aluminum and certain of its alloys are strengthened beyond their basic strengths only by strain hardening, they are non-heat-treatable. Nearly all of the foil presently produced is rolled from non-heat-treatable alloys, which are given H number temper designations for the various specific strain-hardened conditions in which they are made. For example, a common foil availability is alloy 1145-H19; the H1 means strain hardened, and the next digit indicates the degree of hardening, the 9 meaning full hard.