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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 in the walls, which from it is slowly desorbs to the bottle content, altering its taste and aroma. This may not be a relevant issue for fruit juices or strong-tasting soft drinks drinks. But for bottle water, especially of the flat kind, low Acetaldehyde content is imperative: even AA concentrations as little as 10-15 ppb can impart an undesirable off-taste.

While raw resin normally have 1 ppm Acetaldehyde before drying, at the end of the moulding process its content increases up to 3-4 ppm. AA is normally generated after the process phase for two main reasons: shearing/friction in IMM screw and thermal degradation of the resin due to the high temperature. It is possible to control final AA content in final 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 tine and limiting the injection pressure at the IMM
• adding AA blockers (anti-oxidant), erucammide or similar to the resin.

Measuring AA content in PET preforms gives an index of the actual quality of the moulding process. To keep the processing at its best and also to prevent the damage resulting from 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 PET preforms. Piovan optimises the PET drying process in order to maintain the original PET resin AA content constant or even reducing it.