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The primay performance characteristic of interest for bottles
which package carbonated beverages is the the ability to contain
the carbon dioxide, which is under pressure. Poly(ethylene
terephthalate) (PET) has long been used for these applications,
largely due to the availability of processing techniques which
generate a polymer microstructure with low permeability to
gases.
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The performance of such bottles is quantified
through standard tests that specify numerous criteria
for determination after ageing under typically
anticipated conditions such as 23oC for 120 days:
* loss of CO2, less than 15%
* no change in bottle shape
* no off-taste
* drop test requirements, such as survival of
a drop from 2 m
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In addition, packages are often exposed
to severe conditions such as a hot car interior which can
result in a rise of CO2 pressure to as much as 5 atmospheres
pressure.
Achievement of these performance characteristics requires
careful control of the microstructure through processing.
The bottles are manufactured using the stretch-blow molding
process. A preform with wall thickness of about 4 mm is first
injection molded. The part is quickly quenched in the mold
in order to keep the preform amorphous.
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The preform is next heated by
infrared heaters to approximately 95o followed
by stretching in the axial direction and blowing.
This temperature is slightly above the glass transition
temperature of 76oC. The process provides for
generation of stress-induced lamellar crystals.
The oriented crystallites result in lower gas
permeability as well as enhanced mechanical properties.
The size of the crystallites is sufficiently small
that the polymer remains optically clear, critical
for this packaging application.
In contrast, fast cooling of PET would result
in a totally amorphous material which is optically
clear but highly permeable to gases. Alternatively,
slow cooling would result in a a highly crystalline
material with large spherulitic crystals. This
product would not only be optically opaque, but
would also be quite brittle.
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Technical
Papers 
