Environmental Engineering Reference
In-Depth Information
Glass
Glass is one of the oldest packaging materials. Container glass, also knows as soda-lime glass,
is made by heating 72 percent quartz sand (silica, SiO), 11 percent of limestone (CaCO
3
),
13 percent soda ash (sodium carbonate, NaCO
3
), and 4 percent of minor components to around
1,500°C. When using recycled glass, pieces of broken glass (cullet) are also added to the
mixture.
Glass is desirable for packing food products, including:
Chemical inertness and impermeability to gases, which maintains the organoleptic attrib-
utes unaltered for long periods of time.
●
Easy to clean.
●
Reusability and recyclability.
●
Transparent nature makes contents visible.
●
Capability of accepting colors to protect light-sensitive products.
●
Temperature resistance makes glass an appropriate materials for sterilized products.
●
The main disadvantages of glass are high weight, which increases transportation energy, and
propensity to break on impact or thermal shock.
Metals
For the most part, two types of metal are used in the manufacture of food packaging: alu-
minum and steel.
Aluminum
Primary aluminum is produced from bauxite ore in a multiple-step process. The ore is surface
mined and treated with sodium hydroxide under pressure at temperatures around 150°C to
dissolve the aluminum and produce a sodium aluminate solution. The solution is then
transferred to settling tanks where impurities precipitate and then are separated with disk
filters. Clear liquor coming out of the filters is transferred to precipitators where it is left to
cool after adding small crystals of alumina to act as seeds. As the liquor cools, alumina crystals
grow and precipitate to the bottom, where they are removed and transferred to calcination
kilns. In the kilns, hydration water attached to the alumina is removed by heating at high
temperatures (1000°C) to produce “anhydrous alumina.” Anhydrous alumina is converted into
aluminum by the smelting process. This takes place in a reactor where anhydrous alumina is
mixed with cryolite (Na
3
AlF
6
) and melted. Then a direct electric current is passed through the
mixture and the alumina reduced into aluminum, which precipitates to the bottom of the
reactor in liquid form and is removed periodically and poured into molds to form ingots
(Boustead and Hancock, 1981).
Aluminum is lightweight, malleable, resistant to corrosion (because of the development of
a coating of aluminum oxide), flexible, impermeable to oxygen and water vapor, a good
barrier for light and microorganisms, and recyclable. All these characteristics make aluminum
a good packaging material.
For food packaging, aluminum is mixed with metals, such as magnesium, manganese, and
chromium, to improve its mechanical properties. The most common food-packaging applica-
tions of aluminum include cans, bottles, trays, foils, bottle and jar closures, and metallized
films. The bodies of aluminum cans are made with the soft aluminum alloy 3004 that contains
