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.
Sample Crimping
Meaning
Mechanical sealing of laboratory containers prevents atmospheric contamination of resin specimens during the time between sampling and analysis. Reliable sample crimping ensures that a gas tight seal is formed, which is necessary for resins that absorb water rapidly such as polycarbonate or nylon. The action compresses a metal cap onto a glass vial to isolate the polymer from the surrounding humidity.
A properly sealed vial ensures that the moisture measured by the instrument is exactly what was present in the material at the time of sampling.
Prevention Integrity
Gas exchange between the vial and the environment is stopped by the sealing process. High quality sample crimping utilizes a calibrated tool to apply even pressure around the rim of the vial. Any gap in the seal allows humid air to enter, which would result in an artificially high moisture reading.
Maintaining this integrity is important when samples are collected on the factory floor and transported to a central laboratory.
Volatile Retention
Components must remain inside the vial until the moment of analysis. Effective sample crimping ensures that any water vapor released from the resin during transport does not escape. This retention is necessary for the oven method of moisture determination where the entire vial is heated.
If the cap is loose, the measured moisture will be lower than the actual value, potentially allowing wet material into the production line. Consistent pressure from the tool creates a deformation in the metal cap that locks the seal in place.
Visual Quality
Inspection of the cap ensures that the container is ready for the analyzer. Consistent sample crimping produces a cap that cannot be turned by hand and shows a uniform indentation from the tool. Standardizing this step reduces the variability in moisture data across different shifts and operators.