Material Drying
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PET over-drying oxidation
Problem Phenomenon
Over-dried preforms turn yellow, the AA value rises instead, and IV further decreases
Cause Analysis
Excessively long drying time or excessively high temperature triggers PET oxidative degradation, which converges with hydrolytic degradation on the same outcome: AA increases, IV decreases, and products turn yellow; over-drying is as harmful as insufficient drying
Solution
①Release the over-dried material and re-draw material to dry according to the standard; ②the drying temperature must not exceed the upper limit and the time is controlled at 4-6 hours; ③downgrade or scrap the oxidized yellowed material
Key Takeaway
PET drying is range control, not the longer the better; both hydrolytic degradation and oxidative degradation must be guarded against, and yellowing is the alarm of over-drying; after confirming effectiveness, solidify the parameters and practices into the work instruction to prevent recurrence
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