The modern aerosol can is a pressurized dispensing system designed to safely store a product and a driving force, known as a propellant. These components work together to release the contents as a fine mist, spray, or foam. The can is a complex assembly of metal, plastics, and specialized gases, with each component selected for its strength, chemical compatibility, and ability to withstand internal pressure.
The Outer Shell Composition
The main body of an aerosol can is typically constructed from aluminum or tin-plated steel. Aluminum cans are generally manufactured using impact extrusion, which forms a seamless, monobloc container. This construction provides superior pressure resistance and makes the can lighter, often preferred for personal care products like deodorants and hairsprays.
Tin-plated steel consists of low-carbon steel coated with a thin layer of tin. Steel provides exceptional mechanical strength and rigidity, allowing these cans to withstand higher internal pressures than aluminum. This makes them common for larger or industrial products. The tin coating protects the underlying steel from rusting and corrosion.
To prevent the product formulation from reacting with the metal shell, the interior of both aluminum and steel cans is lined with a protective organic coating. These internal lacquers or epoxy resins are tailored to the specific chemical properties of the contents. This barrier ensures product integrity and can longevity, as acidic or alkaline products could otherwise quickly corrode the metal.
Internal Mechanics and Valve System
The pressurized contents are managed by the valve system, which controls the product release. This system is primarily composed of non-metallic materials, including various polymers and synthetic rubbers. The actuator, the button pressed by the user, is typically plastic and dictates the final spray pattern and flow rate.
A plastic dip tube, often crafted from polyethylene or polypropylene, extends from the valve down into the liquid product. It draws the contents upward when the valve is opened. The valve assembly is securely crimped onto the can using a metal mounting cup, usually made of aluminum or treated steel.
Maintaining the high internal pressure requires specialized seals provided by gaskets made from materials like Buna or Neoprene. These rubber or synthetic elastomers ensure a perfect seal around the valve stem when the can is not in use. This prevents both leakage and the escape of the propellant gas.
The Role of Propellants
Propellants are the driving force within the can, responsible for pushing the product out and creating the spray effect. These substances are categorized into two main groups: liquefied gases and compressed gases.
Liquefied Gases
Liquefied gas propellants, such as hydrocarbons like propane, butane, and isobutane, are the most common type used today. These propellants remain in liquid form under pressure, constantly vaporizing to form a gas layer above the product. As the product is dispensed, the liquid immediately converts to gas to fill the available space, maintaining consistent pressure and spray performance. Historically, chlorofluorocarbons (CFCs) were used but were phased out globally due to their harmful effect on the ozone layer.
Compressed Gases
Compressed gas propellants, including nitrogen or carbon dioxide, are non-flammable and inert, making them safer for applications like food products. These gases sit entirely above the liquid product and provide pressure by expansion alone. The primary drawback is that as the product is used, the pressure inside the can drops steadily, resulting in a weakening spray toward the end of the can’s life.
Safe Handling and Material Recovery
Because aerosol cans contain pressurized propellants and sometimes hazardous chemicals, their disposal requires specific care. A can is considered empty only when the pressure has dissipated and no product is released when the valve is depressed. Improper disposal of cans containing residual product or propellant poses a significant risk of fire or explosion for waste and recycling workers.
For large-scale industrial waste, specialized aerosol puncturing systems manage cans that are not completely empty. These airtight devices safely puncture the can, allowing residual liquid product to drain for proper hazardous waste management, while the propellant is often captured or filtered.
Once fully emptied and depressurized, the metal shell is no longer classified as hazardous waste and can be treated as scrap metal. Both the steel and aluminum components are highly recyclable. The recovered metal is sent to facilities where it is shredded and melted down for reuse, which conserves energy compared to using virgin materials.