5.1 The purpose of this test method is to provide a means for comparing relative water absorption tendencies between different cellular plastics. It is intended for use in specifications, product evaluation, and quality control. It is applicable to specific end-use design requirements only to the extent that the end-use conditions are similar to the immersion period (normally 96 h) and 5.1-cm [2-in.] head requirements of the test method.

Note 2-Studies by ASTM Subcommittee D20.22 show that some cellular plastics, particularly those with open cells or natural interstices, continue to absorb additional significant amounts of water beyond the 96-h immersion period. It was also found that water absorption of some cellular plastics is significantly higher when exposed to a greater pressure head, as might be encountered in certain underwater installations.

5.2 This test method provides a means for measuring absorption as a result of direct contact exposure to free water. Results by this test method cannot be used to compare the resistance of cellular plastics to water vapor transmission and subsequent condensation within the cells. To determine resistance to water vapor transmission, see Test Methods E96.

5.3 Water absorption testing is subject to several important variables, which if not considered, prohibit sufficient agreement among testing laboratories. Development of this test method has taken into account the most serious of the possible sources of error.

Note 3-In some methods, an error is encountered due to a rapid absorption of water before an accurate initial weight can be obtained. This test method accounts for that potential error by providing Procedure A for use with materials that behave in this manner. In this procedure the only submerged measurement required is a final weighing taken after the 96-h immersion period.Note 4-The increase in volume that occurs with some foams when immersed is accounted for in Procedure A. This procedure shall be used for materials that exhibit this type of behavior. This is accounted for by basing all buoyant force calculations on the volume of the wet specimen at the conclusion of the immersion period.Note 5-The problem of air bubbles clinging to the submerged specimen and affecting the end result is minimized by specifying deaerated distilled water.Note 6-Surface cells opened during specimen preparation result in an error when calculating the apparent volume of the test specimen. The degree of this error is a function of cell size. This test method accounts for this error in that all calculations are based on the true specimen volume. The true specimen volume is determined in Procedure A as the measured volume minus the volume of surface cells opened by cutting. This correction is not required in Procedure B since the true specimen volume is determined by direct measurement.

5.4 The volume error associated with surface cells opened during specimen preparation decreases as the cell size decreases. This test method provides the option to ignore this variable with cellular plastics that have an average cell diameter of 0.03 cm or less. For cellular plastics having greater than 0.03-cm average cell diameter and in all cases of dispute, measurement of cell size shall be mandatory in determining the specimen volume.

5.5 For most materials the size of the test specimens is small compared with the size of the products actually installed in the field. If the surface-to-volume ratios for the test specimens and the corresponding products are different, it is possible that the test results are misleading.

5.6 In most cases water retention is a secondary performance characteristic that has an influence on a primary characteristic, such as thermal performance, surface accumulation of moisture, localized collection of electrolytes, dimensional stability, etc.

5.7 Before proceeding with this test method, reference shall be made to the specification of the material being tested. Any test specimen preparation, conditioning, dimensions, or testing parameters covered in the materials specification, or both, shall take precedence over those mentioned in this test method. If there are no material specifications, then the default conditions in this standard shall apply.