Aluminum cans are a ubiquitous form of packaging, designed to safely hold pressurized liquids while maintaining product quality and ensuring long shelf life. The construction of a modern beverage can relies on specialized materials, each selected for its precise chemical and mechanical properties. The final product is a lightweight, durable, and highly recyclable vessel that requires different material compositions for its body, lid, and protective interior.
The Core Material: Aluminum Alloys
Beverage cans are not made from pure aluminum but from specific aluminum alloys tailored for high-speed manufacturing. The main body of the can, which accounts for the majority of the material, is typically formed from the 3004 series alloy. This alloy is primarily strengthened with manganese and a small amount of magnesium, granting the metal excellent formability and ductility. This composition allows the material to be drawn and ironed into its thin, seamless shape at rapid production speeds, enabling the can walls to be made extremely thin while maintaining structural integrity.
The can body must be strong enough to withstand the internal pressure of carbonated drinks and the external stresses of stacking and shipping. The 3004 alloy balances this strength with the malleability necessary for the deep drawing process, which converts a flat disc into the can’s cup shape. This process work-hardens the metal, increasing its final strength. The alloy also offers good corrosion resistance.
The lid, or end, of the can requires a different alloy because it must withstand the highest stress points. The material used is generally a stronger alloy, often from the 5182 series, which contains a higher percentage of magnesium. This higher magnesium content provides greater tensile strength, necessary to contain the significant internal pressure generated by carbonation. The lid does not require the same deep-drawing properties as the body, but it must resist deformation when the pull tab mechanism is actuated.
The Critical Role of the Interior Lining
While the metal provides the structure, the contents are separated from the aluminum by a thin, non-metallic interior lining. Aluminum is a chemically active metal, and without this barrier, acidic beverages would rapidly corrode the can, leading to leaks and a metallic flavor in the drink. This polymer-based coating, often referred to as a lacquer, is spray-applied to the interior of the finished can. Its primary functions are to prevent the metal from reacting with the liquid and to stop the leaching of aluminum into the beverage.
For many years, the most common lining material was an epoxy resin, which often contained the chemical Bisphenol A (BPA). This epoxy creates a robust, flexible, and highly adhesive film effective against corrosive contents. Due to public health concerns, the industry has transitioned toward alternatives.
Today, many cans use newer formulations known as BPANI (Bisphenol A non-intent) coatings. These alternatives include materials like acrylic, polyester, vinyl, or plant-based oleoresinous coatings. The lining’s effectiveness depends on the beverage; different formulas are used for beer versus highly acidic fruit juices. Regardless of the specific chemical composition, this ultra-thin film, typically just a few microns thick, is necessary for product safety and quality.
Essential Materials for Sealing and Opening
The final components involve the materials that seal the contents and allow for convenient opening. The pull tab mechanism, which is riveted to the lid, is typically made from the high-strength 5182 aluminum alloy. This material choice ensures the tab is strong enough to withstand the stress of being pulled without failing, while also being flexible enough to shear the metal of the lid. The small rivet that attaches the tab is usually an aluminum alloy chosen to create a durable hinge point.
The air- and liquid-tight seal between the can body and the lid is achieved through a process called the double seam. This process involves rolling the edges of the can body and the lid together to form five layers of tightly interlocked metal. A non-metallic element, known as the seaming compound or gasket, is applied to the end curl of the lid before the seaming process.
This compound is often a water-based dispersion of compounded rubber or a similar polymer sealant. When the double seam is formed, the compound fills any microscopic gaps in the interlocked metal layers, creating a hermetic seal. This gasket prevents the escape of carbonation pressure and blocks contaminants from entering the can.