The first packaging use of aluminum, closures on glass bottles, dates to the early 1900’s. This was a successful application primarily because the closure could be made to hold tightly to the somewhat irregular glass threads. Aluminum foil was adopted for packaging shortly after its initial production in 1913.
Candy-bar and chewing gum wraps took advantage of many of the desirable properties of aluminum foil. Impact-extruded collapsible tubes of aluminum were introduced in the United States in 1921. In the late 1950’s, there was a trend toward adoption of aluminum for a broad range of rigid containers. The fabricating characteristics of aluminum permitted production of such containers by impacting, drawing, and adhesive bonding, as well as by spiral winding of foil laminates.
Aluminum for packaging is preponderantly in the form of sheet or foil. Commercially pure aluminum is mainly employed, although non-heat-treatable alloys of the aluminum-magnesium and aluminum-manganese types are also used for higher strength. With both commercial-purity aluminum and alloys, broad ranges in mechanical properties are available through varying degrees of work hardening.
The uses of aluminum in packaging are identified by the industry in four categories:
- Flexible wraps and laminates, incorporating foil
- Rigid containers, made from sheet, foil laminates, or drawn heavy-gage foil, or impact-extruded from slugs
- Closures for bottles and jars, made from sheet or foil
- Collapsible tubes, fabricated from slugs by impact extrusion.
Flexible Packaging
In the glossary of packaging terms, flexible packaging is defined as involving the use of flexible materials such as foil, films, paper, and sheet, to form the container. Aluminum foil has become firmly established as one of the major flexible packaging materials. About 85% of all aluminum foil produced is used in some form of packaging. The three major packaging applications are household foil, 35%; laminated foil, 30%; and formed containers, 28.5%.Aluminum foil is sheet less than 0.0060 in. (0.15 mm) thick. It can be rolled commercially as thin as about 0.00017 in. (0.005 mm). An important characteristic of aluminum foil is its high covering area per unit of weight.
Aluminum foil is outstanding in its low permeability to water vapor and gases. Extremely small pinholes are unavoidable in thickness less than 0.001 in. (0.025 mm). Foil is tasteless, odorless, nontoxic, and hygienic. A special advantage of annealed foil is that it is substantially sterile because of the temperature (over 340°C or 650°F) of annealing. Foil is greaseproof and nonabsorptive to liquids, and hence is especially suited for packaging medicinal oils, ointments, grease-base cosmetics, and similar products. Foil remains dimensionally stable during exposure to oils and greases.
Foil is an effective barrier to light and is used extensively to package photographic materials and other light-sensitive products. Ultraviolet radiation accelerates the development of rancidity in certain foods; foil is a good barrier to this radiation, retarding loss in flavor and appearance, and inhibiting development of rancidity and staleness.
Because it is an efficient reflector and low emitter of radiant heat, aluminum foil is employed for packaging where the thermal insulating properties imparted by these characteristics are advantageous. Despite these insulating effects, the good thermal conductivity makes it possible to chill or heat aluminum packages more rapidly than those with nonmetallic covering.
Alloys used for foil in packaging applications include 1100 (99.0 to 99.3 Al), 1145 (99.45 to 99.60 Al), 3003 (Al-1.25 Mn), and 5052 (Al-2.5 Mg-0.25 Cr). Annealed or soft foil is identified as O-temper. Annealing is accomplished by heating the metal to above 340°C (650°F). This softens the foil, removes traces of lubricants left from rolling, and effectively sterilizes it.
For many years, all the aluminum foil marketed commercially was in either the annealed or the full-hard H19 temper. Foil gages above 0.002 in. (0.05 mm) are also available in the intermediate, partially annealed tempers H25 and H27. Lighter gages in intermediate tempers are available from a few foil sources. The hardest temper of foil, available in any gage and designated H19, is an extra-hard temper that has received a high degree of cold work.
Laminating, Coating, and Printing Materials. Unsupported foil in the light gages used for packaging often lacks the ruggedness to withstand abuse encountered during shipping and handling. For this reason, foil gages less than 0.001 in. (0.025 mm) are generally laminated to paper or plastic films. In many instances, the foil also is coated for protection, decoration, or heat sealing; usually, the laminate is printed for product identification and appeal.
Heat sealing unites two or more surfaces by fusion, either of the base materials or of coatings that have been applied, using controlled temperature, pressure, and dwell time. The bond between the heat-sealed surfaces may be one of complete fusion or may be partially fused to allow easy separation of the two surfaces. Laminated structures with various types and weights of paper or plastic film perform satisfactorily under a wide variety of conditions.
Plastic films laminated to foil for commercial use include cellophane, cellulose acetate, rubber hydrochloride, polyvinyl chloride, polyesters, and polyethylene.
Coatings used on aluminum foil for protection can be formulated for resistance to chemicals, heat, or scuffing. Resistance to chemical attack or to mechanical abuse can be provided either by protective coatings or by various plastic films. In many applications, a protective coating serves also as a heat-seal surface, as discussed subsequently.
Printing on aluminum foil can be accomplished by any commercial process, including rotogravure, flexography, lithography, and letterpress. The foil generally is coated with a washcoat or a primer prior to printing. Shellac washcoats normally are used for gravure or flexographic printing, which utilize rapid-drying organic-solvent-base inks.
Rigid Containers
Rigid containers are stiff packages, usually of metal sheet and usually round in shape. This type, known familiarly as "cans", originated in the days of Napoleon. Some rigid containers employ foil laminated to a stiff nonmetallic backing sheet. Others are smooth-wall containers drawn from heavy-gage foil. Rigid containers should protect the product and withstand reasonable handling without deformation or breakage.The can industry is the largest segment of the packaging complex. Cans are generally made from steel, tin-coated steel, aluminum, a combination of fiber and aluminum foil, or plastic. Most rigid containers are cylindrical. This shape utilizes the container material efficiently, but the greatest advantage is that it permits manufacturing, drilling, and closing at high speeds with exacting control. Almost all beer, beverage, vegetable, fruit, dog food, and motor-oil containers are cylindrical.
Aluminum Alloys and Tempers. The aluminum alloys commonly used are 1100, 3003, 5052, 5082, 5086, and 5154. The tempers employed vary from the annealed condition, used for impact can slugs, to the strain-hardened, extra-hard H19 temper for beer and oil can ends. Intermediate work-hardened tempers are used where appreciable forming is required. Gages range from 0.003 in. to 0.018 in. (0.075 to 0.45 mm), except for impact or drawn-and-ironed bodies, which start as slugs or disks up to 0.125 in. (3 mm) thick.
The alloys with low yield strength and high elongation are easy to form, whereas those with high yield strength and low elongation can be used only for simple forms such as can ends.
Closures
A glass container consists of a glass body and a closure. Many years ago the closures were cork, or a glass stopper, usually with a rubber-type gasket. Today, closures are made from aluminum, tin plate, steel, plastic, or, sometimes, paper. Ideally, a closure must be hermetically scalable, easily applied, and readily removable and recloseable. Attractive appearance is advantageous for sales appeal. The cost should be low, but more important in many applications is the need to protect the product. Furthermore, it is often desirable to make the closure tamperproof.Alloys and Characteristics. For many years, aluminum closures were manufactured principally from 1100 alloy. During the 1920’s, alloys 3003 and 3004 were adopted, to take advantage of their higher mechanical properties with satisfactory adhesion for the coatings then used. The H14 and H34 tempers have been used most frequently.
About 1950, the aluminum-magnesium alloys, such as 5050 and 5052 in H36 temper, were introduced for the larger-diameter closures, where higher strength is required. Stronger alloys in full-hard or extra-hard tempers allow a reduction in metal gage. Gages commonly used for closures are in the range of 0.006 to 0.012 in. (0.15 to 0.3 mm) for 3003 alloy and somewhat thinner for 5052. For example, a closure normally made in 0.0095-in. (0.25 mm) gage of 3003 may be satisfactory at 0.0085 in. (0.22 mm) with 5052-H19, providing a reduction in gage of 10% and almost as much in cost. Advances in drawing techniques are expected to promote greater use of the stronger tempers of 5052, including the extra-hard H19.
Types and Applications. The rolled-on closure is the most popular, and probably the most responsible for the extensive use of aluminum in closures. Many variations of closures employ the rolled-on principle.
In addition to the various rolled-on designs, aluminum closures are made as screw cap, hidden-thread screw cap, and tumbler cap.
Collapsible Tubes
A collapsible tube is defined as a cylinder of pliable metal that can be sealed in such a manner that its contents, although readily discharged in any desired quantity, are protected from contact with air or moisture. Products so packaged must flow under pressure low enough not to damage the tube, but must be sufficiently viscous not to spill out of the tube. Commercial production of aluminum collapsible tubes by impact extrusion began in Switzerland in 1914 and in the United States in 1921.After extrusion, the tubes are annealed to remove the work hardening and provide the softness or limpness needed for good collapsibility. The degree of hardness that remains in the annealed condition is needed for the tube to maintain its shape and to hold the crimped fold at the closed end.
Coatings are required inside some aluminum collapsible tubes to prevent corrosion by certain products. Even a superficial amount of corrosion, which might be tolerated for other applications, is objectionable in the tubes, because gas produced by the corrosion reaction causes the tube to swell. Consequently, coatings that provide a high degree of protection against corrosion are required.
