Karl Fischer Moisture Testing of Recycled Polyethylene Terephthalate
Coulometric Karl Fischer testing with oven desorption at 170 °C isolates true water content in rPET below 50 ppm, preventing costly IV degradation during melt processing.
Water Standard
Meaning
Moisture content thresholds dictate the maximum permissible H2O weight fraction allowed within a polymer resin batch during high-temperature injection moulding. A water standard defines these drying requirements to prevent hydrolytic degradation of sensitive molecular chains in polyesters and polyamides. Failure to observe this metric results in brittle mechanical parts and visible surface voids.
Processing Baseline
This threshold serves as the reference point for calibrating desiccant dryers or vacuum drying equipment prior to material feeding. Technicians set the drying temperature and dew point based on the specific resin grade requirements. An incorrect drying profile leads to molecular weight reduction during melt processing.
Consistent adherence to these values guarantees that the polymer viscosity remains stable throughout the barrel residence time. Excessive moisture absorption during storage necessitates longer cycle times for drying cycles before production commences.
Material Tolerance
Resin manufacturers provide a datasheet value that represents the laboratory limit for optimal part performance. Moulders maintain a tighter operating window than the supplier limit to account for ambient humidity variations on the shop floor. Achieving this stricter control ensures that the finished product withstands the intended structural load without premature failure.
Regular verification of resin moisture through titration methods confirms the success of the drying protocol.
Quality Impact
Surface defects often occur when the water standard remains unobserved during the melt process. Rapid expansion of steam within the molten plastic leaves characteristic streaks and bubbles across the geometry of the finished part. Mechanical integrity drops sharply as the polymer chains undergo hydrolysis during the transformation phase.
Strict monitoring of these moisture levels prevents scrap production and preserves the physical properties of the injected components.