Significance and Use

1 This test method is intended to be used in specifications where porosity of cellular plastics has a direct bearing on their end use. For example, for thermal insulation applications, a high percentage of closed cells is necessary to prevent escape of gases and to promote low thermal conductivity. In flotation applications, high closed-cell content generally reduces water absorption.

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

Scope

1 This test method covers cellular plastics, which are composed of membranes or walls of polymer separating small cavities or cells. These cells may be interconnecting (open cell), non-connecting (closed cell), or any combination of these types. This test method determines numerical values for open cells. It is a porosity determination, measuring the accessible cellular volume of a material. The remaining volume is that occupied by closed cells and cell walls. Since any conveniently sized specimen is typically obtained by some cutting operation, a fraction of the closed cells will be opened by specimen preparation and will be included as open cells, (see Note 2).

2 This test method provides good accuracy on predominantly highly open-celled materials. By not accounting for closed cells that were opened during specimen preparation, the accuracy decreases as the closed cell content increases and as the cell size increases.

Summary of Test Method

This test method is based on a determination of porosity in which the accessible cellular volume of a cellular plastic is determined by application of Boyle’s Law, which states that the increase in volume of a confined gas results in a proportionate decrease in pressure. The apparatus consists of two chambers of known volume connected by a valve. One of the chambers, the calibrated sample chamber, is accessible for insertion of the test specimen and is connected to a source of high purity (at least 99.99 %) dry gas, such as nitrogen or helium. The pressure in the sample chamber is increased to a predetermined pressure and this value, P1, is noted. The valve between the two chambers is then opened and the second, lower Pressure, P2, is again noted. The ratio of the pressure change P1/P2, is directly related to the volume of the sample chamber displaced by the specimen. The difference between this volume and the geometric volume of the specimen is a measure of the open-cell volume.

NOTE 3—The criteria for selecting a dry gas are that the gas shall not exhibit significant non-ideal behavior, dissolve the matrix, or readily diffuse into the matrix.

Apparatus

1 Gas Pycnometer—A schematic diagram of the pycnometer apparatus is shown in Fig. 1.

2 Cutting Device, for specimen preparation, such as a bandsaw or hobby jigsaw, the blade of which must be capable of producing a smooth cut. This will require a blade with at least 4 teeth/cm (10 teeth/in.). A cellular hole cutter is also acceptable.

3 Vernier Calipers, or micrometer measuring device, capable of measuring specimens to the nearest 0.003 cm (0.001 in.).

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