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.
Thermal Oven Vaporization
Meaning
Analytical extraction methods that heat a polymer sample in a sealed oven and transfer the released moisture to a titration cell provide accurate measurement of water content. Utilizing thermal oven vaporization avoids the issues associated with direct sample dissolution, such as side reactions with additives or poor solubility. This method is particularly useful for checking the moisture content of engineering resins before injection moulding to ensure they are dry.
The analysis must be carried out carefully to avoid ambient moisture contaminating the sample.
Extraction Process
Solid polymer pellets are weighed and placed into a small glass vial which is then heated to a specific temperature in the oven. During thermal oven vaporization, a dry carrier gas like nitrogen flows through the vial, carrying the evaporated water into the titration vessel. This temperature must be set high enough to release the bound moisture but below the degradation point of the polymer.
This process separates the water from the polymer matrix, preventing interference from additives or pigments. This separation is key to the accuracy of the test.
Measurement Accuracy
The carrier gas delivers the extracted water to a Karl Fischer titration cell where it is measured electrochemically. This method can detect very low levels of moisture, often down to ten parts per million, which is well below the limits of standard loss-on-drying balances. This high sensitivity is necessary for resins like polycarbonate, where even tiny amounts of water can cause hydrolysis and reduce the strength of the part.
This analysis is the gold standard for resin moisture testing. It provides a reliable basis for quality control.
Polymer Application
Hygroscopic resins like polyamide and polyester are the main materials tested with this technique. This test ensures that the resin is dry enough to prevent splay or polymer degradation during moulding. This measurement is particularly useful when troubleshooting defects on the production line.
It helps to isolate whether the dryer or the mould is causing the issue.