The familiar beverage container, often called an aluminum can, is a sophisticated, multi-layered packaging system engineered for strength and preservation. Pure aluminum is too soft and reactive, so the finished product uses specialized metal alloys and non-metallic coatings. This complexity allows the can to be manufactured with ultra-thin walls, withstand internal pressure, and safely store acidic liquids. The precise composition of the metal and protective layers dictates the can’s performance, durability, and recyclability.
The Primary Alloy of the Can Body
The main cylindrical section of the can is formed from a 3000 series aluminum alloy, typically AA3004 or AA3104, optimized for the intense manufacturing process. The primary alloying element is manganese, usually present between 1.0% and 1.5%. This manganese addition enhances the metal’s strength and corrosion resistance while maintaining necessary formability.
The can body is created using draw and wall-ironing (DWI), where a flat disk of aluminum is rapidly pressed and stretched into a tall, thin-walled cylinder. The specific chemistry of the 3000 series alloy allows it to undergo this severe cold-working without cracking or tearing. This process results in finished can walls often only about 100 microns thick, thinner than a human hair. The manganese content ensures the can retains sufficient structural integrity to withstand the pressures of filling, shipping, and stacking.
Composition of the Can Lid
The lid, or end, of the can uses a 5000 series aluminum alloy, such as AA5182 or AA5052. This composition differs from the body because the lid must meet specific functional demands. It must be strong enough to contain the high internal pressure of carbonated beverages and resist the tearing forces applied by the pull tab.
The 5000 series alloys incorporate magnesium as the main alloying element, with grades like 5182 containing up to 4.5% magnesium. Magnesium significantly increases the alloy’s yield strength, making the lid harder and more rigid than the body metal. This rigidity is crucial for maintaining a tight seal and ensuring the easy-open tab mechanism functions correctly.
Essential Internal and External Coatings
The metallic structure must be isolated from the contents by a protective internal lining. This internal coating, typically an epoxy lacquer or polymer spray, acts as a barrier to prevent the beverage, especially acidic sodas, from reacting with the aluminum. Without this lining, the metal would corrode, affecting the drink’s flavor and compromising the container’s integrity.
Historically, coatings often relied on Bisphenol A (BPA), but the industry has widely adopted non-BPA alternatives, including various polyesters and water-based coatings. These polymer layers are applied thinly to the inside of the can, ensuring long-term product safety and flavor stability.
On the exterior, a separate set of coatings, inks, and varnishes are applied. These external layers provide scuff resistance, protect against external corrosion, and create the smooth, glossy surface necessary for branding and decoration.