The standard “ISO 8130-2 Coating powders - Part 2: Determination of density by gas comparison pycnometry (arbitrator method)” developed by the International Organization for Standardization (ISO) describes a test method for determining the density of all types of coating powders using a gas comparison pycnometer.
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In principle, density is calculated from the mass and volume of the test piece. The volume of a weighed test piece is determined by measuring the volume of gas displaced in a container when the test piece is inserted. This is obtained by measuring the pressure difference created by the displacement of the gas.
Helium or nitrogen gas is used as the material during the test in a steel cylinder. However, other high purity gases can also be used, provided that the product being tested is not affected and any deviation from this method is stated in the test report.
When performing the test, ordinary laboratory devices are used as apparatus, together with a gas comparison pycnometer for automatic or manual determination of density.
The basic design of a typical gas comparison pycnometer uses a displacement gas as the medium outlined in the standard. The pycnometer consists of two chambers of equal size, the sample and the reference. The chambers are connected by a valve and a differential pressure meter. The test section is placed in the sample chamber. The displacement gas is allowed to flow from one chamber to the other and the resulting pressure difference is measured. Automatic gas pycnometers can help to minimize errors compared to manual pycnometers. Manual pycnometers can be used and due care should be taken during testing and measurement.
When performing the test, a representative sample of the product to be tested is taken as described in the standard “ISO 15528 Paints, varnishes and raw materials for paints and varnishes - Sampling”. The characterization is carried out in duplicate at a temperature of 23 degrees plus/minus 1 degree and, if applicable, at 50 percent plus/minus 5 percent relative humidity. The procedure depends on the type of pycnometer and the instructions for the instrument are followed. Calibration of the pycnometer can be done with a known volume standard or with previously measured similar test samples. The instrument is adjusted to allow for the volume of the test sample container.
The measured pressure difference should not exceed 1%. For manual pycnometers, the swept volumes of the measured cylinders should be equal to or within 0,5% (relative). The sample container is weighed to the nearest 1 mg and filled with the product under test. The outer walls of the container should be free of dust. The mass of the test portion is determined by weighing the container and its contents to the nearest 1 mg. The container and its contents are placed in the device, and the volume of the test portion is determined. To ensure consistency of the reading, the volume measurement is repeated, and the average of the two volumes is calculated. A change in density may be observed due to drying of the test portion depending on the gas temperature. At 23 degrees plus/minus 1, the density of the coating powder is calculated in grams per cubic centimeter. If the two results differ by more than 2%, the procedure described above is repeated. The average of the two valid determinations is calculated, and the result is reported as the nearest 0,01 g/cm³.
The test report must include at least the following information:
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