Virtually all aluminum beverage cans contain an internal coating, which is a thin polymer lining applied to the interior before filling. This coating acts as a seamless barrier, completely separating the liquid contents from the aluminum wall of the can. Its purpose is to ensure the integrity of both the packaging and the beverage over time.
Why Cans Need an Internal Barrier
The polymer film is an engineering necessity driven by the chemical nature of the beverages. Most sodas and carbonated drinks are acidic, having a low pH due to ingredients like phosphoric or citric acid. Without a protective layer, these acids would quickly react with the exposed aluminum metal.
This reaction would cause the can to corrode, potentially leading to pinhole leaks and a reduced shelf life. The chemical interaction would also impart an unpleasant metallic flavor to the liquid. The internal barrier prevents both corrosion and the degradation of the drink’s intended taste profile.
The Materials Used in Can Linings
For decades, the industry standard for can linings was a type of epoxy resin synthesized using Bisphenol A (BPA). These BPA-based epoxy coatings were favored for their stability, flexibility, and strong adhesion to the aluminum. They were successful because they could withstand the high-temperature sterilization process required for many canned goods.
In response to consumer and regulatory concerns, the industry has shifted toward alternative coatings. Modern linings frequently utilize non-BPA acrylics, polyester resins, or modified oleoresins. Finding a replacement with the same durability and performance as the traditional epoxy has been a material science challenge. These newer coatings must maintain integrity despite the mechanical stresses of manufacturing and chemical exposure from acidic beverages.
Safety Concerns and Regulatory Oversight
The primary concern regarding can linings centers on the potential for chemical migration, or leaching, of coating components into the packaged food or drink. This issue gained public attention due to the presence of Bisphenol A (BPA) in traditional epoxy liners. Scientific studies have shown that BPA can act as an endocrine-disrupting chemical, prompting a re-evaluation of its safety in food contact materials.
Governmental bodies have responded with varying approaches. The European Food Safety Authority (EFSA), for example, reduced its acceptable daily intake level for BPA in 2023 based on new research. The EFSA’s findings led the European Commission to ban the use of BPA in food contact materials by late 2024.
In the United States, the Food and Drug Administration (FDA) maintains that BPA is safe at the low levels found in food, but has agreed to reconsider its assessment based on evolving scientific evidence. Many manufacturers now market products as “BPA-free.” However, some replacement materials, such as Bisphenol S (BPS) or Bisphenol F (BPF), are structurally similar to BPA. These alternatives are currently under increased scrutiny by researchers and regulators to determine if they pose similar health concerns.
The Role of Linings in Recycling
The polymer lining is a factor in the highly efficient process of aluminum recycling. When cans are transported to a recycling facility, they are subjected to extremely high temperatures in a smelting furnace. The melting point of aluminum is around 660 degrees Celsius (1,220 degrees Fahrenheit).
The organic polymer coating is completely burned off during this intense heat treatment. This process, known as pyrolysis, converts the lining into gases and ash. High temperatures ensure the organic material is removed without contaminating the resulting molten aluminum.
The lining does not compromise the recyclability of the aluminum itself, which remains a valuable material to recycle. The organic residue is managed by the furnace’s environmental control systems, allowing the pure aluminum to be recast into new products.