Meaning
Standardized test methods for determining moisture in plastics by relative humidity sensor provide a non-chemical alternative to Karl Fischer titration. The protocol specified in ASTM D7191 involves heating a polymer sample in a sealed chamber and measuring the liberated water vapor using a calibrated sensor. This standard applies to most thermoplastic resins, though it is constrained by the maximum operating temperature of the heating system.
Analytical Protocol
The procedure requires placing a measured weight of polymer pellets into a vial, which is then heated to a specified temperature to volatilize the moisture. Dry carrier gas sweeps the moisture from the sample chamber to the relative humidity sensor, which records the partial pressure of the water vapor. This method avoids the use of hazardous chemical reagents and provides rapid feedback on moisture levels before moulding begins.
Because it is highly automated, operators can perform the test on the production floor to verify resin dryness.
Resin Suitability
Certain engineering thermoplastics like polyamide and polycarbonate require precise dryness to prevent polymer chain scission during melt processing. Utilizing this test method allows processors to confirm that the resin meets the moisture limits specified by the material supplier, usually below 0.02 percent. Accurate verification prevents splay and loss of mechanical properties in molded components.
Process Verification
Routine testing using this standard replaces subjective checks, providing a consistent metric for dryer performance. If the dryer fails to reach the target dew point, the test will reveal high moisture levels before the resin is fed into the molding machine. This preventative testing reduces scrap rates and mold damage caused by out-of-specification resin.
