The ISO 15512 specifies six methods by which moisture content in plastics can be measured. Samples from raw resins or finished articles in powder or granular can be tested, with lower detectable limits ranging between 0.001% to 0.1%. Moisture content is relevant for design and quality control.
Many polymers of commercial importance are polar in nature and hence absorb moisture from the atmosphere. Even non-polar plastics may have additives that absorb moisture. Excessive water content in raw powder and granular plastic resins may cause problems in the storage and processing of the material. These include silo and piping blockages due to agglomeration, bacterial growth, chemical reactions initiated by moisture, and boil-off effects in extrusion processes. Some plastic products with excessive moisture will show degraded properties. Hence, accurate information on the moisture content of resins and granules is necessary for the proper design and operation of storage and processing facilities. Testing of residual moisture in some finished plastic products is necessary for quality control purposes.
ISO 15512 describes 6 methods labeled A, B1, B2, C, D, and E to test the water content of plastics in powder, granular and finished article forms. The methods are recommended based on detectable moisture range and compatibility of the polymers.
The ISO 15512 lays emphasis on proper sample packaging, handling, and choice of temperature setting (for heating or cooling tests). Each of the six tests is briefly described below (note that moisture content is in mass percent):
Method A is applicable to all plastics in fine granular form (≤ 4 mm) or even prepolymer powders that are insoluble in anhydrous methanol. The method involves extracting the water in the sample by anhydrous methanol after which the methanol-water solution is subjected to Karl Fisher titration. The lower limit of moisture detection by method A is 0,1 % and accuracy is 0,1 %.
Method B1 can be used for all plastics in fine granular form (≤ 4 mm). The method involves the vaporization of moisture from the sample in a tube oven. Dry air or dry nitrogen conveys the moisture into a titration cell where either a Karl Fischer titration or a coulometric determination of collected water is conducted. Note that ISO 15512 does not specify temperature settings for vaporization methods, since optimization of this parameter involves many considerations including prevention of plastic degradation. The lower limit of moisture detection by this test is 0,01 % and accuracy is 0,01 %.
Method B2 is also applicable to all plastics in fine granular form (≤ 4 mm). It involves vaporization of moisture, using a heated sample vial. The evaporated water is conveyed by dry air or Nitrogen to a titration cell, where the collected water quantity is determined by a Karl Fischer titration. The temperature needs to be optimized to ensure complete vaporization while avoiding plastic degradation. The lower limit of moisture detection by this test is 0,01 % and accuracy is 0,01 %.
Method C is a manometric test and involves evaporating water in the sample under a vacuum and measuring the resulting pressure increase. Plastics that contain organic volatile compounds cannot be tested. If necessary, the absence of volatiles can be independently checked by gas chromatography. The lower limit of moisture detection by method C is 0,01 % and accuracy is 0,01 %.
Method D is a thermo-coulometric method. Vaporized water from the sample is conveyed by dry air or dry Nitrogen through a diphosphorus pentoxide (P2O5) moisture sensor where it is electrolyzed. The measured electrolytic current correlates with the incoming moisture flow rate and total moisture is determined. Plastics with volatiles other than water or volatile compounds that can react with P2O5 cannot be tested by this method. The lower limit of moisture detection by method D is 0,01 %. and accuracy is 0,01 %.
Method E involves evaporating water from the sample by a combination of vacuum and heating. The vaporized water is reacted with Calcium Hydride yielding Hydrogen and Calcium Hydroxide as products. The evolution of Hydrogen gas increases pressure, as per stoichiometry. The temperature needs to be optimized to ensure complete vaporization while avoiding plastic degradation. Volatiles do not participate in the reaction. The lower limit of moisture detection by method D is 0,001 %. and accuracy is 0,001 %.
Sample sizes should be smaller than 4 mm × 4 mm × 3 mm.
The moisture content is reported as mass percentage.