Moisture content determination of PET

Determining moisture content in PET granulate. An explanation of which analysis technique is most suitable, but also why.

Polyethylene terephthalate (PET) is widely known from the PET bottle. It is a thermoplastic material with many other applications besides the PET bottle. Although it may sound contradictory, PET is sensitive to water, especially during processing at high temperatures and pressures. When the material contains water (> 0.02 %) and is subsequently heated, the water can cause the molecular chains to break down via hydrolysis. This can have a drastic effect on the quality of the material.

For many companies that process PET, it is therefore extremely important to dry PET as much as possible before processing. But how can one check whether this drying process has taken place correctly?
This can be done in two ways, following the weight loss of the material during drying or via Karl Fischer titration.

Monitoring the weight loss of the material during drying seems to be the most obvious method. However, in order to be able to indicate small amounts of water in ppm range (parts per million), one must have a very accurate balance, such as a thermogravimetric analysis (TGA). With this analysis technique, it must be taken into account that not only the decrease in weight of water is monitored, but the decrease of all volatile components (such as used solvents or gaseous degradation products). This can lead to incorrect assumptions about the volatile component present when a small weight decrease is registered.

PerkinElmer TGA 4000 at PTG/e
PerkinElmer TGA 4000 at PTG/e

The most suitable technique for determining the amount of water is by Karl Fischer titration. The PET granulate is heated in a glass vial, so that water is released and blown into a titration liquid by means of a dry nitrogen flow. The liquid only reacts with water and the amount of water can be accurately determined on a ppm scale. An additional advantage is that the granulate can be sampled at various locations or after various time intervals in hermetically sealed bottles in order to determine the course of the drying process.

Metrohm 831 KF Coulometer in combination with a Thermoprep 832 oven at PTG/e

PTG /e has a wide range of analysis techniques and can therefore choose the most suitable technique or choose to combine techniques.

Interested in the various techniques and what they can be used for?

Download our overview of analysis equipment and techniques.