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.
Capacitive Moisture Sensors
Meaning
Electronic instruments that measure the change in dielectric constant of a polymer or a surrounding gas stream determine the moisture content of polymer resins. In many drying and analytical systems, capacitive moisture sensors utilize a porous dielectric layer sandwiched between two electrodes to detect water molecules. These sensors are limited by their susceptibility to contamination from outgassing volatile additives and polymer dust.
Operational Principle
The sensor operates by absorbing water molecules from the surrounding air or carrier gas into its porous dielectric layer, which increases the total capacitance of the electrical circuit. Because water has a very high dielectric constant compared to dry air or polymer resins, even tiny amounts of moisture generate a measurable change in the sensor signal. This signal is converted into a relative humidity or dew point reading to monitor resin dryers.
Continuous feedback ensures that the drying system maintains the required dew point throughout the moulding run.
Dryer Integration
In injection moulding operations, maintaining a dew point of minus forty degrees is necessary for drying hygroscopic polymers such as polycarbonate and nylon. Integrating these sensors into the dryer hopper allows for real-time monitoring of the air stream, preventing damp air from recirculating through the resin. If the sensor detects a rise in moisture, the system can trigger a regeneration cycle on the desiccant bed.
This automation protects the resin from incomplete drying, which would otherwise lead to polymer degradation.
Sensor Calibration
Exposure to volatile organic compounds and high temperatures can cause the sensor to drift over time. Regular recalibration against known moisture standards is required to ensure that the sensor remains accurate. Protecting the sensor with a fine particulate filter blocks polymer dust and heavier volatile compounds, extending the operational life of the dielectric layer.