Coulometric Titration Baseline Drift Calibration Methods
Drift calibration isolates cell background current from true sample moisture, preventing hydrolytic degradation and avoiding false incoming resin lot rejections.
PET Moisture
Meaning
Absorbed water within polyethylene terephthalate pellets alters the material behavior and causes chain scission during melt processing. Controlling PET moisture is a primary concern for preform and bottle manufacturers who require high clarity and impact strength. If the water content exceeds the critical limit, the polymer undergoes rapid hydrolysis in the extruder.
This reaction reduces the intrinsic viscosity of the resin, making the final product brittle.
Polymer Degradation
Melt processing of damp polyester degrades the polymer chain length by breaking ester bonds. Excessive PET moisture leads to a drop in molecular weight, which shows up as a loss in melt strength and mechanical performance. This degradation cannot be reversed by adjusting the moulding parameters like pack pressure or cool time.
The only remedy is to dry the resin to the specified moisture limit before processing.
Processing Window
Drying systems must maintain the polymer at low dew points for several hours to achieve the required dryness. The target PET moisture level for injection moulding is typically below fifty parts per million. Operating within this narrow window prevents the formation of acetaldehyde, a volatile byproduct that taints packaged beverages.
Constant monitoring of the dry air flow is needed to sustain this dry state.
Analysis Protocol
Lab checks verify pellet dryness before the batch is released to the moulding machine.