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PETes in drying process

AA is a colorless volatile with a fruity smell which arises as a by-product of PET thermal degradation by oxidation of the vinyl end group of the polymer chain. In PET bottles, AA is usually present at a few ppm concentration, which is slowly desorbed to the bottle content, altering its taste and aroma. This may not be a relevant issue for fruit juices or strong-tasting soft drinks.
But for bottled water, especially of the still one, low Acetaldehyde content is imperative: even AA concentrations in the water as little as 10-15 ppm can convey an undesirable off-taste.

While raw resin normally has concentration of 1 ppm acetaldehyde before drying, at the end of the moulding process its content increases up to 3-4 ppm. AA is normally generated in the plastic processing phase for two main reasons: shearing/friction in IMM screw and thermal degradation of the resin due to high temperature. It is possible to control final AA content in preforms at each step of the production process:

• limiting the barrel residence time of raw material
• using the correct processing parameters during the drying phase
• reducing the screw back pressure effective melt-out, shortening the cycle time and limiting the injection pressure at the IMM
• adding AA blockers (anti-oxidant), erucamide or similar to the resin.

Measuring AA content in PET preforms provides an index of the actual quality of the moulding process. To keep the process at its best and to avoid the subsequent phases of blowing, filling, distributing and putting on shelves bottles with a content exceeding the allowed AA level in the market, AA is best measured before the blowing and filling stage: in the PET preforms. Piovan optimises the PET drying process in order to reduce the original PET resin AA content.

To learn more, download the white paper.