Environmental Engineering Reference
In-Depth Information
Picture 12.1
Aluminum cans in a recycling facility.
Courtesy Environmental Protection Agency.
97.2-98.2 percent of aluminum, 0.8-1.3 percent magnesium, and 1.0-1.5 manganese. Lids,
on the other hand, are made with a harder alloy, 5182, which contains between 94.5 and 95.8
aluminum, 4.0 and 5.0 percent magnesium, and 0.2 and 0.5 percent manganese (Boustead
and Hancock, 1981).
Steel
Steel is an alloy that contains iron and less than 1 percent carbon. Iron is made by heating iron
ore in a blast furnace with energy from coal and oil and the addition of limestone as a flux. The
resulting iron (pig iron) has a high carbon content and needs further refinement before use.
This is done by two processes: basic oxygen steelmaking or the electric arch steelmaking. The
basic oxygen is the most common method that consists of blowing oxygen through the molten
pig iron, resulting in lowering its carbon content and to become steel. Steel then goes through
a series of processes until it is converted into a thin sheet that is wound into a coil.
Low carbon steel used for packaging material needs a coating of a corrosion resistant
material, such as tin, chromium, or chromium oxide. Tin is applied by hot-dipping the
steel sheet into a molten tin bath or by electro-deposition. To improve corrosion resistance,
tin-plated steel is covered with an epoxy layer on the side in contact with the food product.
Tin-plated steel is mainly used for the fabrication of cans used in processed foods but has other
applications as well (Marsh and Bugusu, 2007).
Tin-free steel is made by coating steel sheets with chromium or chromium oxide to protect
steel from corrosion. It makes the steel inappropriate for welding but improves the adhesion
of lacquers, paints, and inks. Tin-free steel is used for cans, lids, closures, and drums (Marsh
and Bugusu, 2007).
Plastics
In the context of packaging, plastics is a generic name used to designate synthetic polymers
made by condensation or addition of monomers into long organic polymeric chains. There are
two categories of plastics: thermosets and thermoplastics. Thermosets are polymers that
solidify irreversibly when heated and are generally used for the manufacture of durable things.
