Desiccant Dryer Dew Point Calibration and Karl Fischer Titration Verification
Dew point monitoring alone fails to guarantee dry resin; coulometric Karl Fischer titration per ISO 15512 Method B is mandatory to prevent hydrolytic scission.
Polyethylene Terephthalate Moisture
Meaning
Polymer moisture levels determine the maximum allowable concentration of dissolved water in a resin before melt processing. In the packaging industry, polyethylene terephthalate moisture must be kept extremely low, typically below fifty parts per million, to avoid degradation during bottle or film extrusion. When this value is exceeded, the polymer undergoes rapid hydrolytic degradation, resulting in a severe drop in molecular weight.
This parameter is monitored carefully at the dryer discharge before the material is fed to the extruder.
Drying Requirements
Hot air desiccant dryers are utilized to lower the moisture content of the polymer to safe levels. Polyethylene terephthalate moisture is highly bound within the crystalline structure of the resin, requiring a temperature of around one hundred and seventy degrees Celsius for several hours to extract it. If the dryer air flow is insufficient, the core of the pellets will remain wet and cause issues in the melt.
Processing Defect
Extrusion of resin with excessive moisture leads to sheet brittle behavior and bubbles. The melt becomes very runny, making it difficult to control the thickness of the extruded sheet. This defect causes significant material waste as the degraded sheet cannot be recycled easily without further processing.
Quality Assessment
Online sensors or Karl Fischer titration are used to measure the water content. If a batch fails the moisture check, the production line must be stopped until the material has been re dried to the specified limit.