Optimizing Water Content Analyses In Polymer Granules
The polymer extrusion process is highly dependent on the water content of the polymer granules used. Polymer water content can, however, be effectively monitored by Karl Fischer titration in connection with an oven sample changer. The METTLER TOLEDO InMotion™ KF Oven Autosampler‘s Temperature Scan function helps define parameters for a reliable analysis.
Granules are easy to handle, and thus the generally preferred form in which to transport thermoplastic polymers to a processing site. For their further use, they are melted down, then molded by extrusion process is the granules‘ water content, as even a small amount of moisture (< 0.10%) can lead to surface defects on the finished product. In the most severe scenario, it can lead to a complete breakdown of the process due to high vapor pressures in molding equipment. Hygroscopic polymers may even hydrolyze at high temperatures, causing reduction of polymers‘ chain-length and fluctuation of their mechanical properties.
Analysis via Karl Fischer titration
To determine the water content of polymer granules prior to production a Karl Fischer (KF) can be combined with an oven: Here, a METTLER TOLEDO C30S titrator was connected to an Inmotion KF Oven Autosampler. A sample is first placed in a vial that is then heated in the oven to evaporate the volatile components. With constant gas flow, the volatiles are transferred into the titration cell, where the water is selectively titrated. For best results, the measurement temperature must be high enough to ensure fast and complete water release. However, if the temperature is too high the polymer decomposes, resulting in degradation, further water formation, and overestimation of water content. Existing processes can therefore be optimized with a preliminary temperature assessment to identify the ideal measurement temperature (Figure 1)
Titration optimization with Temperature Scan
The InMotion KF Oven Autosampler’s Temperature Scan function increases oven temperature at a constant rate while monitoring water release; Figure 1 displays water release from the polymers PET, PE-LD grey, PE-LD, ABS and PS. Around 2 g of polymer were weighed into a vial and scanned fully automatically from 50 °C to 280 °C, with a heating rate of 2 °C/min.
An increase in water content is visible in the ABS sample (blue curve) up to 180 °C; this represents the water released from the sample. The second increase in water content, at 220 °C, is a clear indication of sample decomposition (Figure 1). As a general rule, the optimal oven temperature for subsequent water analyses should be set to at least 20–30 °C below the decomposition temperature.
After the scans, every sample was measured five times at the chosen temperature to determine its water content (Table 1). The highly repeatable results emphasize the liminary temperature assessment for accurate polymer water-content determination.