Recycled Polyethylene Terephthalate Moisture Testing Methods
Accurate moisture testing below 50 ppm via Karl Fischer oven titration prevents hydrolytic intrinsic viscosity degradation in recycled PET during melt processing.
Chilled Mirror Hygrometry
Meaning
Measurement of the moisture content in a gas by cooling a polished surface until condensation begins represents a high-accuracy method used to monitor the dryness of air in polymer desiccant dryers. Implementing chilled mirror hygrometry provides a precise determination of the dew point in the drying circuit. This technique is critical for hygroscopic resins like polyethylene terephthalate and polyamide, which require extremely low moisture levels before processing.
Dewpoint Detection
Precision sensors track the temperature of the reflective surface to identify the exact onset of moisture condensation. This temperature corresponds directly to the dew point of the air stream. The system uses a thermoelectric cooler to lower the mirror temperature and an optoelectronic sensor to detect the change in reflectivity.
Drying Performance
Monitoring the dryer output prevents processing defects such as hydrolytic degradation and cosmetic splay during injection molding. When the drying air has a dew point higher than minus forty degrees Celsius, the resin may not dry sufficiently, leading to molecular weight reduction and brittle parts.
Condensation Mechanism
Physical condensation of water vapor occurs on the mirror surface as it reaches the dew point, which is the temperature at which the gas cannot hold all its moisture. A light beam reflects off the mirror into a photodetector, and when dew forms, the light scatters. The electronics then adjust the cooling current to maintain a thin layer of condensation, allowing continuous tracking of the dew point during the production run without requiring manual recalibration.