Karl Fischer Determination of Trace Water in Polyethylene Terephthalate
Polyethylene terephthalate trace moisture determination relies on Karl Fischer coulometric oven extraction at 170 to 180 degrees Celsius to prevent chain scission.
Septum Vial
Meaning
Sample protection during thermal analysis involves the use of containers with pierceable membranes. Specialized containers like the septum vial allow for the transport and heating of polymer samples without exposure to the air. In moisture analysis, a needle penetrates the lid to carry the liberated water vapor from the heated resin directly into the titration cell.
This setup protects the sample from ambient humidity which would otherwise invalidate the trace level moisture measurements.
Entry Access
Movement into the sealed environment of the container is managed through the flexible membrane of the lid. A septum vial allows the analytical system to extract gas samples using a syringe or a hollow needle. The material of the lid must reseal after the needle is withdrawn to prevent any moisture from entering or leaving the vessel.
This feature is used in both headspace analysis and automated moisture measurement systems.
Barriers Prevention
Contamination from the outside environment is blocked by the thick layer of the seal. A high quality septum vial uses a PTFE faced liner to ensure that no chemical components from the rubber leach into the sample. This protection is necessary for maintaining the purity of the resin during the heating process.
Any contamination from the seal itself would cause false readings in the sensitive titration instrument.
Thermal Performance
Resistance and chemical inertness are the primary attributes of the glass used in these containers. A septum vial must withstand the high temperatures used to drive moisture out of the polymer without breaking or releasing gases. Borosilicate glass is typically chosen for its ability to handle rapid temperature changes in an automated oven sampler.
Reliable performance of the vessel ensures that the moisture analysis is both safe and accurate.