ISO 1183 Plastics - Methods for Determining the Density of Non-Cellular Plastics

The “ISO 1183 Plastics - Methods for determining the density of non-cellular plastics” standard, developed by the International Organization for Standardization (ISO), consists of a number of subsections published under the same title. Namely,

• The standard “ISO 1183-1 Part 1: Immersion method, liquid pycnometer method and titration method” describes three methods for determining the density of non-cellular plastics in the form of void-free molded or extruded objects, powders, flakes and granules:
  • Method A: Immersion method for solid plastics (excluding powders) in void-free form.
  • Method B: Liquid pycnometer method for particles, powders, flakes, granules or small pieces of finished parts.
  • Method C: Titration method for plastics in any void-free form.
• The standard "ISO 1183-2 Part 2: Density gradient column method" describes a gradient column method for determining the density of non-cellular molded or extruded plastics or pellets in void form. Density gradient columns are columns containing a mixture of two liquids, the density in the column increasing smoothly from top to bottom.
• “ISO 1183-3 Part 3: The standard “Gas pycnometer method” describes a method for determining the density or specific volume of solid non-cellular plastics of any shape not containing closed pores.

Density is often used to monitor changes in the physical structure or composition of plastic materials. Density can also be useful in assessing the uniformity of samples or specimens. Often, the density of plastic materials depends on the choice of sample preparation method. In this case, it is intended that the precise details of the sample preparation method be included in the appropriate material specification. These descriptions apply to all three methods mentioned above.

For the purposes of this standard, the following terms and definitions apply:

• Mass refers to the amount of matter contained in an object. It is expressed in kilograms (kg) or grams (g).
• Apparent mass is the mass of an object obtained by measuring its weight using a suitably calibrated balance. It is generally expressed in kg/m3 or g/cm3.
• Density expresses the ratio of the mass of a sample to its volume (at a given temperature).

The volume of a sample of known apparent mass is determined by measuring the change in volume of gas in a pycnometer upon introduction of the sample. The volume change may be obtained directly by means of a moving piston or indirectly by measuring the change in pressure in the pycnometer and calculating the volume using the pressure-volume relationship for ideal gases. The volume obtained by this procedure relates only to the solid without porosity. Density is calculated using the equation given in the standard.

The smaller the molecules of the gas, the narrower the pores that can be penetrated. Preferably, gases with low adsorption affinity to the test material should be used. Especially for pycnometers measuring pressure, the accuracy of the method depends on the applicability of the ideal gas law (Boyle-Mariott law).

For high accuracy measurements, helium should preferably be used as it behaves like an ideal gas. It can penetrate pores of at least 1 µm in diameter and has a low tendency to adhere to the surface of the test material.

The test atmosphere shall be in accordance with the standard “ISO 291 Plastics - Standard atmospheres for conditioning and testing” or the appropriate material standard. In general, it is not necessary to condition the samples to a constant temperature, since the determination itself does not bring the sample to the constant temperature of the test. Samples whose density changes during the test to such an extent that the change is greater than the required accuracy shall be conditioned in accordance with the applicable material specification before measurement. When density changes with time or atmospheric conditions are the primary purpose of the measurements, the samples shall be conditioned as described in the material specification and, in the absence of a material specification, as agreed by the parties concerned.

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