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There are a number of ways to reduce
the AA level. First of all, choose the right PET
resin. Resins are available with properties that are specifically tailored
to the needs of water containers. For example, Voridian Aqua PET resins
18696 and 20837 are considered leaders in low AA levels. Each has a low
intrinsic viscosity (IV) of 0.72 and 0.76 respectively. The low I.V. minimizes
AA generation during melt-processing. However as the I.V
increases, the chains become longer, the mechanical strength increase and
the resistance to stress cracking is improved. For example, carbonated soft
drinks require materials with an I.V. of 0.78 or higher because the internal
gas pressure makes it susceptible to stress cracking. That is to say it
is important to choose a PET
solution with the proper balance between low AA levels and high performance.
Optimization of molding condition by minimizing
melt temperature and residence time can also reduce the AA generation.
However it cannot be entirely eliminated. Since AA boils at below room temperature,
it eventually migrates out of the PET into the atmosphere or into the packaged
contents, affecting taste. The amount of AA that migrates is directly related
to storage time and temperature. |
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