Stretch-Blow Molding of PET Bottles
Chris E. Scott
The primay performance characteristic of interest for
bottles which package carbonated beverages is the
the ability to contain the carbon dioxide, which is under
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.
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
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
The part is quickly quenched in the mold in order to
keep the preform amorphous.
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.