Acetaldehyde A.A. in poly(ethylene terephthalate) processing

>> Wednesday, 1 April 2009

Acetaldehyde A.A. is an organic chemical compound with the formula CH3CHO. Is a colorless gas at room temperature and has a strong fruity smell. It occurs naturally in many fruits and other foodstuffs is used as a flavor enhancer for certain products and is produced by plants as part of their normal metabolism.

Acetaldehyde is also generated during the production and injection processing of poly(ethylene terephthalate), PET material. It can cause an off-taste in bottled water.
Generally, most acetaldehyde problems originate in the barrel during the injection process. All PET resins have some residual acetaldehyde after being manufactured, the quantity will vary according to the grade. Acetaldehyde is only generated while the PET resin is in its melt condition, therefore it can only be controlled by adjustments in the barrel (90%) and hot runner (10%) of the machine. The generation of acetaldehyde is not linked to moisture content of the material, although in the process of being dried, acetaldehyde can also be driven off. Therefore, correct drying is also important for acetaldehyde control.Possible causes listed in order of likelihood and / or ease of correction:

1. Barrel / hot runner heater overridingAcetaldehyde A.A. in poly(ethylene terephthalate) processing

Confirming that all heating zones are operating correctly will reduce problems such as: increased A.A. and lowered I.V. black specks, flashing, silver streaks etc. Check that:

  • Thermocouples are seated correctly and are free of any damage.
  • All heaters are drawing correct amperage using a clip-on ammeter. The actual temperature of each heating zone using an electronic or infra red thermometer.
  • All SSR's are switching correctly when commanded by the heat controller.
  • Wire connections are tight.

    2. Melt temperature settings too high
    Running the machine with the lowest practical melt temperature will reduce the risk of hydrolysis causing lowered I.V., and will limit degradation of the material that can lead to increased A.A. levels and other faults.
    Using a melt temperature that is too low can cause problems such as:

  • Damage to screw tip.
  • Broken low shear bolts.
  • Very long screw charge time.
  • Increased crystallization - crystals do not get melted properly so they become "seed crystals".
  • Poor mixing of the material.

    3. Injection velocity too high
    Keeping the injection velocity low will reduce the shear that occurs in the material. Shear is a major factor affecting overheating of the material and I.V. reduction, therefore reducing velocity will protect the PET resin from excessive damage.
    When working with hot preform method, the injection velocity will also make a significant difference to the material distribution in the finished container. Filling slower means that the preform will be hotter when the mold opens and its temperature balance will also have changed. Typically, the shoulder area will become relatively hotter than the base area giving more stretch at the top of the preform.
    Excessive injection velocity can also disturb the alignment of the injection core, especially if the design is long and thin.
    Reducing the injection velocity will also have the effect of making the holding time shorter since the V/P time will increase.

    4. Screw rpm too high
    The speed of screw rotation will be reduced. This will prevent excessive shear and associated frictional heating from ocurring. The end result will be better control of acetaldehyde and protection of the material I.V.

    5. Insufficient temperature / airflow in dryer
    A fault with the dryer will lead to hydrolysis of the material in the barrel of the machine. This will cause lowered I.V. of the material. Lowered I.V. is the major cause of bad quality in preforms and bottles. More than 60% of all PET processing related faults can be traced back to the dryer.
    Maintaining the dryer in optimum condition will allow the molding machine to perform at maximum efficiency and quality.

    Airflow - The most important parameter, there should be nothing causing a restriction in the process and regeneration air flow.
    Temperature - Process Temperature should be in the region of 145~170ºC Depending on the resin supplier and the drying time. Also check the Regeneration Temperature which should be around 200~230ºC depending on the maker.
    Time - Calculate (or measure) the time the material is in the hopper, this should be at least 3½ - 4 hours.
    Dewpoint - Correct dewpoint may vary according to the manufacturer of the dryer, consult the maker's manual before assuming an error exists.

    6. Incorrect choice of raw material
    One way to alleviate this is to use a copolymer. Comonomers such as CHDM or isophthalic acid lower the melting temperature and reduces the degree of crystallinity of PET. Thus the resin can be formed at lower temperatures and/or with lower force.

    7. Worn / damaged injection screw
    If the injection screw is worn, damaged or of an old design, it may not be plasticizing the material correctly and may be creating excessive shear.
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