ASTM D7191 Determination of Moisture in Plastics by Relative Humidity

The ASTM D7191 test method is used for the quantitative determination of water down to 20 ppm in plastics using a relati ve humidity sensor. The moisture content has a direct impact on the physical qualities and processability of various plastics.... Read More

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    Scope:

    The ASTM D7191 test technique includes the assessment of moisture in plastics by relative humidity. Moisture content has been identified as an important element in plastics, thus accurate measurements are taken during the production of plastic products and to ensure product quality.  Easy compounding and injection molding, as well as smooth surfaces and good mechanical qualities in injection-molded goods, are all made possible by polymer resins with the optimum moisture content. 

    This test method utilizes a sealed, airtight flow system that prevents contamination of the analyzer from water present in the atmosphere. Thus, this test method can be used as a control,  acceptance, or evaluation test. 

    Procedure:

    The specimen is loaded into a septum-capped glass vial that is moved into a heater to develop the volatiles from the sample into the headspace in accordance with ASTM D7191. As the vial enters the heater, a coaxial needle, or two-needle set pierces the septum. The generated volatiles in the headspace are then carried into the sensor manifold by a dry carrier gas that flows into the vial. The carrier gas is cooled in the sensor manifold, allowing high-boiling volatiles to condense on a hydrophobic filter. The hydrophobic qualities of the filter allow moisture in the carrier gas to pass through, which is subsequently noted as a potential elevation at the relative humidity sensor. The total mass of water in the sample is calculated using this sensor signal, which is integrated over time. The moisture content is calculated by dividing total moisture by sample mass.

    Video 01: How to select a moisture analyzer for the plastic industry

    Specimen Size :  

    For ASTM D7191, plastic is used in the form of liquid, powder, pellets, or ground material as a test specimen. 

    Data :  

    The ASTM D7191 test result is reported in total micrograms (µg) of water present, then the percent moisture content of the sample is calculated as follows: 

    Moisture content = R x 10-4/W 

    where: 

    R = total water content of the sample (µg)

    W = sample weight

    Conclusion:

    The ASTM D7191 test technique includes the assessment of moisture in plastics by relative humidity.

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