Stretch-Blow Molding of PET Bottles

Stretch-Blow Molding of PET Bottles

Chris E. Scott
Managing Engineer
Exponent

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.

The performance of such bottles is quantified through standard tests that specify numerous criteria for determination after ageing under typically anticipated conditions such as 23^oC 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 injection molded. 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 95^o followed by stretching in the axial direction and blowing. This temperature is slightly above the glass transition temperature of 76^oC. 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.