BS ISO 17744:2004

This International Standard describes procedures for determining the specific volume of plastics as a function of temperature and pressure in both the molten and solid states.

The standard specifies the use of a piston-equipped apparatus in which the test sample, held in a measurement cell, is pressurized by means of the piston. Measurements under conditions of constant pressure or constant temperature can be made. In the constant-pressure mode, the maximum heating and cooling rates permissible are restricted to 5 °C/min.

NOTE Higher heating and cooling rates can be used, but data will then have to be corrected for thermal gradients^[13].

For the acquisition of data needed for processing design, it is recommended that the isobaric cooling method be used (see ISO 17282). The result of this measurement cannot be used directly for injection-moulding simulation.

By using these procedures, it is possible to obtain:

• pvT diagrams that represent the relationship which exists between pressure, specific volume and temperature for a given material;

• compressibility and volumetric thermal-expansion coefficients;

• information on first-order and glass transitions as a function of temperature and pressure.

Although thermoplastic polymers are currently tested down to room temperature using these procedures, it is emphasized that, at temperatures lower than T_g, the difficulty in achieving a true hydrostatic state is a source of uncertainty on the specific volume measurement.