A standard soda can is made almost entirely of aluminum, but not the same aluminum throughout. The body and the lid are two distinct alloys, each chosen for different mechanical properties, and a thin polymer coating lines the interior to keep the drink from ever touching bare metal. That layered construction, along with some clever engineering of the can’s shape, is what lets a container weighing roughly 15 grams hold a pressurized, acidic liquid on a shelf for months without leaking or corroding.
Two Different Alloys in One Can
If you tear a soda can apart, you are handling two separate aluminum alloys. The body of the can is typically made from alloy 3104, a variant of the well-known 3004 alloy. Both belong to the 3000 series of aluminum alloys, which are alloyed primarily with manganese and a small amount of magnesium. These additions make the metal strong enough to hold carbonation pressure while remaining easy to form into the familiar cylindrical shape.1ASM International / CrossRef. 3004, Alclad 3004, and 3104
The lid, on the other hand, is stamped from alloy 5182, part of the 5000 series of aluminum alloys. These contain around four to five percent magnesium, which gives them a superior combination of strength and formability compared to other aluminum alloys.2SAE Technical Paper Series. A Highly Formable Aluminum Alloy-5182-SSF That extra strength matters because the lid has to resist the internal pressure of carbonation without bowing outward, and it also has to accommodate the pull-tab mechanism. The tab itself is usually the same 5182 alloy, riveted to the lid during assembly. Using a stronger alloy for the lid means it can be made thinner without sacrificing the ability to pop the tab cleanly.
This split-alloy design is a deliberate engineering trade-off. The body alloy needs to be soft enough to stretch during forming but stiff enough under pressure. The lid alloy needs to be harder and more resistant to deformation. Trying to do both jobs with a single alloy would mean compromising on one end or the other.
From Flat Sheet to Finished Can
A soda can starts life as a large coil of aluminum sheet, typically around 0.3 millimeters thick. The manufacturing process that turns this flat metal into a seamless cylinder is called deep drawing and ironing, which remains the dominant method for beverage can production today.3SAE Technical Paper Series. Process Simulation for Can Manufacturing by Deep Drawing and Ironing
In the first step, a circular blank is punched from the sheet and drawn into a shallow cup by a punch pushing the metal through a die. That cup then passes through a series of progressively tighter ironing rings, which thin the walls while stretching the cup taller. The bottom of the can stays close to the original sheet thickness, while the sidewalls are thinned to roughly 0.1 millimeters. That is thinner than a sheet of paper. The entire process happens in a single stroke and takes a fraction of a second, which is part of why modern can lines produce over two thousand cans per minute.
After forming, the can body gets trimmed to a uniform height, washed, and coated inside and out. The exterior receives a base coat of white paint followed by the printed label, then a clear overcoat. The interior gets a spray-applied polymer lining. The can body is then necked inward at the top to reduce the diameter of the opening, which allows a smaller, lighter lid and also adds structural rigidity. The lid is seamed on only after the can has been filled.
Why the Bottom Is Concave
Turn a soda can upside down and you will notice the bottom is not flat. It curves inward in a dome shape. That dome is one of the most critical structural features of the can, and it exists because of the internal pressure created by dissolved carbon dioxide. A flat bottom on such a thin-walled container would bulge outward under even modest pressure, making the can unable to stand upright.
The concave dome solves this by putting the metal in a state of compression rather than tension. As long as the internal pressure stays within design limits, the dome holds its shape. But if the can is overpressurized, whether from heat, shaking, or a manufacturing defect, the dome can snap through and invert outward in an event engineers call dome reversal. The pressure at which this happens is a function of the dome’s geometry and the metal’s thickness, and it represents a hard structural limit on can design.4Journal of Pressure Vessel Technology. Dome Reversal of Metal Beverage Containers Once the dome reverses, the can loses its ability to stand and may rupture. If you have ever seen a bulging can bottom at the back of a hot car, that is a dome approaching its limit pressure.
Designing the dome profile is a balancing act. A deeper dome resists higher pressure but uses more material and reduces the interior volume. A shallower dome saves metal but risks reversal at lower pressures. Can makers have spent decades refining the geometry, shaving fractions of a millimeter off the dome profile to reduce aluminum use without triggering failures on store shelves.
The Coating You Cannot See
Bare aluminum reacts readily with acids, and soda is quite acidic. A typical cola has a pH around 2.5, which would corrode unprotected aluminum in a matter of days. The solution is a thin polymer lining sprayed onto the can’s interior immediately after forming. This coating, usually an epoxy-based or polyester-based resin, creates a barrier between the beverage and the metal.
The lining also serves a taste function. Without it, aluminum ions would leach into the drink, giving it a metallic flavor. Corrosion studies have shown that the acidic components of soft drinks, along with dissolved ions like phosphates, benzoates, and chlorides, aggressively attack uncoated aluminum. Higher temperatures and higher chloride concentrations make the attack worse.5Scientific Electronic Archives. Corrosion study of cola soft drink cans Even among different beverages, corrosion rates vary. Research comparing soft drinks and energy drinks found that the main solution of a cola was more corrosive than acetic acid alone, while an energy drink corroded uncoated aluminum at a lower rate.6PubMed Central. Corrosion Behavior of Aluminium-Coated Cans
The internal coating is extremely thin, typically only a few micrometers, but it has to be flawless. Even a tiny pinhole can become a corrosion site where the beverage eats through the aluminum wall. Can manufacturers test their coatings rigorously, often using an electrical conductivity test where current passed through the filled can reveals any breaks in the lining.
The BPA Question
For decades, the most common interior coating was an epoxy resin made with bisphenol A, widely known as BPA. The concern is that BPA can migrate from the lining into the beverage, especially under heat. Laboratory measurements have detected BPA leaching from aluminum cans with epoxy resin linings at levels around 68 nanograms per container under controlled conditions.7Letters in Applied NanoBioScience. Leaching of BPA from Baby Feeding Bottle, Aluminium Can and Thermal Printed Paper That amount is far below the levels most regulatory agencies consider harmful, but it has been enough to push the industry toward alternatives.
Many can makers have transitioned to BPA-free coatings, typically based on acrylic, polyester, or vinyl resins. Whether the replacements are truly safer is still debated. Some substitute chemicals have less long-term safety data than BPA itself, which has been studied extensively. The practical takeaway for consumers is that the amount of any chemical leaching from a modern can lining is extremely small, and the lining itself is non-negotiable. Without it, the can would corrode, the drink would taste metallic, and the shelf life would drop from months to days.
Where the Aluminum Comes From
Primary aluminum starts as bauxite ore, which is refined into alumina (aluminum oxide) and then smelted into metal using the Hall-Héroult process. This electrolytic process dissolves alumina in a molten mineral bath and uses large electric currents to separate out pure aluminum, producing metal that is about 99 to 99.5 percent pure.8Encyclopedia of Aluminum and Its Alloys. Hall–Heroult Process The principle has been in use for over a century and remains the only commercially viable way to make aluminum from ore.
The process is notoriously energy-intensive. Producing one ton of primary aluminum requires roughly 14,000 kilowatt-hours of electricity, which is why smelters tend to cluster around cheap hydroelectric power. That energy cost is the single biggest reason recycling aluminum is so valuable: remelting scrap aluminum requires only a fraction of the energy, somewhere in the range of five to ten percent of what primary smelting demands.9ScienceDirect. Recycling aluminium for sustainable development: A review of different processing technologies in green manufacturing
Once the pure aluminum is produced, it is alloyed by adding manganese, magnesium, silicon, and trace amounts of other elements to hit the specific compositions needed for can bodies and lids. The alloy is cast into large ingots, hot rolled into thick slabs, then cold rolled down to the thin gauge needed for can stock. The entire supply chain, from mine to sheet coil, is global and tightly optimized. A single large can plant might consume hundreds of tons of aluminum sheet per day.
Recycling and Its Limits
Aluminum cans are one of the most recycled consumer products in the world. The appeal is straightforward: melting down a used can and rolling it into new sheet uses a small fraction of the energy needed to produce virgin aluminum, and the metal does not degrade with repeated recycling. A can collected today can be back on the shelf as a new can within about sixty days.
But the system is not as seamless as it sounds. The two different alloys in a can, 3104 in the body and 5182 in the lid, get mixed together during collection and shredding. When they are melted as a combined stream, the resulting alloy has a composition somewhere between the two, which has to be adjusted with fresh metal or additional alloying elements to bring it back to specification. Projections suggest that by 2030, roughly six megatons of aluminum scrap per year could go unrecycled globally because the accumulated mix of alloying elements becomes too difficult to sort or dilute economically.9ScienceDirect. Recycling aluminium for sustainable development: A review of different processing technologies in green manufacturing
There is also the problem of oxidation. When aluminum chips and scraps are melted, a percentage of the metal oxidizes and is lost as dross. This loss is not catastrophic, with recovery rates around 95 percent reported for well-run processes, but it adds up across millions of tons. Research into solid-state recycling methods, which reshape scrap without fully melting it, aims to reduce these losses and cut energy use even further.
How Cans Compare to Other Packaging
The environmental footprint of a soda can depends heavily on whether the aluminum in it is virgin or recycled. A can made from 100 percent recycled aluminum has a dramatically lower carbon footprint than one made from primary metal. Life cycle assessments of beverage packaging have found that for pressurized beverages like soda and beer, a fully recycled aluminum can is the least impactful option when compared against plastic bottles and other common alternatives.10Detritus. Life Cycle Assessment of Beverage Packaging
That finding comes with a significant caveat: “100 percent recycled” is a theoretical benchmark, not the current industry average. In practice, most cans contain a mix of recycled and virgin aluminum. The recycling rate for cans varies widely by country, from above 90 percent in some European countries and Japan to well below 50 percent in others. Every can that goes to landfill instead of a recycling bin shifts the overall calculation toward a higher environmental impact.
Glass bottles, by comparison, are heavier and more energy-intensive to transport, but they do not require an interior lining and can be reused rather than recycled. Plastic bottles are lighter than glass but face growing concern over microplastics and low recycling rates. Carton-based packaging, like the kind used for juice, performs well in assessments for non-carbonated drinks but cannot handle the pressures that carbonation demands. There is no single “greenest” package for all beverages. The answer shifts depending on what is inside and how the container is handled after use.
Why Your Can Gets Warm So Fast
Anyone who has taken a cold can out of the fridge and set it on a counter knows it warms up quickly. That is not your imagination. Aluminum is an excellent thermal conductor, with a thermal conductivity more than 200 times that of glass. Combined with the extremely thin walls of a can, this means heat flows through the container almost as fast as the surrounding air can deliver it.
Research comparing aluminum and glass containers confirmed that liquid in aluminum containers both chills and warms faster than liquid in glass ones. The thin, highly conductive aluminum wall transmits heat readily in both directions, so the same property that lets you cool a can quickly in ice also means it loses its chill faster at room temperature.11Heat Transfer Engineering. Thermal Performance of Aluminum and Glass Beer Bottles
This is why insulated can holders, or koozies, exist. The aluminum wall itself provides essentially zero insulation. A foam sleeve adds a layer of low-conductivity material on the outside, slowing the transfer of ambient heat into the drink. Without one, a can of soda can warm by several degrees in under ten minutes on a warm day. That rapid heat exchange is also why cans feel colder to the touch than a glass bottle at the same temperature: the metal pulls heat from your hand faster than glass does, creating the sensation of colder contact even when the liquid inside is identical.
What the Ink and Paint Are Made Of
The exterior of a soda can goes through its own layered coating process. After forming, the bare aluminum body receives a thin wash coat, then a white base coat that provides an opaque background for printing. The label graphics are applied using a dry-offset printing process, where multiple ink rollers transfer the design in a single pass. Modern can printers apply up to eight colors simultaneously, producing the sharp, photographic-quality labels you see on shelves.
The inks are typically UV-cured or heat-cured formulations designed to bond tightly to the base coat and resist abrasion during shipping and handling. After printing, a clear overcoat is applied to protect the graphics and reduce friction between cans during high-speed filling and packaging. The exterior coatings also serve a mild protective function, slowing oxidation of the outer surface, though exterior corrosion is rarely a concern for cans that spend their lives indoors on dry shelves.
All of these coatings, interior lining, exterior base coat, inks, and overcoat, add up to only a few percent of the can’s total weight. The overwhelming majority of the mass is aluminum, which is why the recycling process can handle printed cans without issue. The organic coatings simply burn off during melting, leaving clean metal behind. That simplicity is another reason aluminum cans are easier to recycle than multi-material packaging like lined cartons or laminated pouches, where separating the layers is a significant challenge.