Page 356 - Biosystems Engineering

Food Package Engineering 333

is perfectively transparent to microwave, leading to low-energy dis-
sipation. Because glass is an amorphous material, it does not show a
real melting temperature. It progressively softens, achieving a liquid-
like state. This change of state occurs over a temperature range known
as glass transition temperature range. Glass can be easily recycled.
The main disadvantages of glass packaging are its fragility, higher
density, and high energy requirement during manufacturing. Glass
containers do not withstand a large temperature differential during
heating (up to 60°C) or cooling phase (up to 40°C). Recent advances
in glass-manufacturing technology have resulted in improved strength,
reduced and density, and reduced energy in manufacturing. 1,2

Manufacturing of Glass Containers
This process involves mixing of silica, the largest constituent (68 to
73 percent) being cullet (scrap or recycled glass). Using cullet is econom-
ically desirable because much less energy is required to melt cullet
than raw silica. Cullet also reduces the amount of dust and other par-
ticulate matter that is often associated with manufacturing process
that uses raw silica only. In most cases, boron (B O ) and aluminum
2 3
oxide (Al O ) are also mixed to the thermal and mechanical perfor-
2 3
mance of the final container. To reduce the viscosity of melts and
melting temperature of silica, sodium carbonate and calcium carbon-
ate are also added. A large quantity of carbon dioxide gas is released
in glass manufacturing, together with a small amount of sulfur diox-
ide and water vapor. Ingredients such as, sulfate, nitrate, or sulfite of
alkaline ions is added to remove gases from the mass of molten glass.
Finally, other ingredients are added to give glass certain physical
properties. For example, lead gives clarity and brilliance, although at
the expense of softness of the glass; alumina increase hardness and
durability. The mixture of all ingredients in a preweighted quantity is
then transferred to a glass-melting furnace maintained at a tempera-
ture of approximately 1500°C. At this temperature, molten glass is
chemically homogeneous and gas free. Then the molten glass is cooled
to about 1150°C to form containers.
Glass containers are made using commonly known forming pro-
cesses: (1) blow and blow and (2) press and blow. In both processes,
two different molds are used in two subsequent stages. In the first
mold, the lump of molten glass, also known as a gob, is transformed
into a preform (or a parison) by pressurized air (in blow and blow
process) or by a plunger in the gob (in press and blow process). The
final shape of the container is achieved in the second mold by pres-
surized (around 160 to 250 kPa) air. Most bottles or narrow-neck con-
tainers are produced using blow and blow process, whereas jars and
wide-mouth containers are produced by the press and blow process.
However, it is currently possible to produce narrow-neck glass pack-
ages using the press and blow process, which produces containers