While the word density has been attached to an assortment of meanings, including closely compacted substances, a measure of information on a storage medium (areal density), various ratios (population density, nutrient density), and even a jab at one’s intelligence, in physics, density, by default, refers to the mass per unit volume of a material. The more items that can fit into a fixed space, the denser that space becomes. For plastics, knowing this physical property is important for more than just comprehension, as plastic is sold on a cost per pound basis. The lower the density, the more material per pound.
Users depend on the ISO 1183 series of international standards for standard methods to determine the density of non-cellular plastics. Part one, ISO 1183-1:2019, specifies the following methods:
Method A: Immersion method
This method is intended for solid plastics (except for powders) in void-free form.
Method B: Liquid pycnometer method
This method is intended for particles, powders, flakes, granules, or small pieces of finished parts.
Method C: Titration method
This method is intended for plastics in any void-free form.
These methods are applicable to non-cellular plastics in the form of void-free molded or extruded objects, as well as powders, flakes, and granules.
When conducting these methods in air, ISO 1183-1:2019 calls for the correction of “apparent masses” to accommodate for the effect of the air buoyancy of the specimen and the balance weights used. To meet this purpose, the standard features formulas for calculating the true mass, the density of the air, and the approximation of saturated water vapor pressure.
ISO 1183: Other Methods
Please note that two other methods for determining the density of non-cellular plastics are addressed in two other international standards:
ISO 1183-2:2019 specifies a gradient column method.
ISO 1183-3:1999 specifies a gas pyknometer method for solid non-cellular plastics that do not contain closed pores.