HEAT DISTORTION TEMPERATURE

The "heat distortion temperature" (HDT) is often used in the product literature of commercial polymers as an indicator of the mechanical softening temperature.  The alternative names “heat deflection temperature” (also represented by the acronym “HDT”), and “distortion temperature under load” (DTUL) are also sometimes used for this property. 

The HDT is closely related to (but usually slightly lower than) the glass transition temperature (T_g) for amorphous polymers.  This is not particularly surprising.  T_g is essentially an indicator of the intrinsic softening behavior of the polymer, while the HDT is measured under an applied load.  The deflection of an amorphous polymer specimen under load depends on the magnitude of the load in addition to depending on T_g.  Most measurements of the HDT are made at the standard load levels of 0.46 MPa and 1.82 MPa.  For example, typical values for atactic polystyrene are T_g=100 °C, HDT=95 °C under a load of 0.46 MPa, and HDT=85 °C under a load of 1.82 MPa.  For evaluation of the suitability of specimens in some highly demanding applications, it is sometimes also useful to perform a customized version of the HDT measurement under a significantly higher load selected based on the requirements of the targeted application.

The glass transition also plays a major role in determining the physical and mechanical properties of semicrystalline polymers.  The effects of the glass transition on these properties decrease both with increasing crystallinity and with increasing “strength” of the crystalline domains.  For example, the HDT of a semicrystalline polymer generally falls between T_g and the melting temperature (T_m) and increases with increasing percent crystallinity.  The HDT value measured for a semicrystalline polymer under a load of 1.82 MPa can be much lower than the HDT value measured under a load of 0.46 MPa if the crystalline fraction is low and/or if the crystallites are “weak” and thus easily disrupted by the mechanical load.  For example, a typical unreinforced grade of Polyamide 11 (Nylon 11) has HDT=140 °C under a load of 0.46 MPa but HDT=49 °C under a load of 1.82 MPa.  On the other hand, the HDT values measured under these two standard loads are usually much closer to each other in semicrystalline polymers whose crystallites are held together strongly and thus not disrupted easily under the applied load.  Finally, it is also possible to vary the HDT (and hence the performance in some applications) of some semicrystalline polymers appreciably by using process steps such as annealing at a temperature between T_g and T_m to enhance the extent and/or quality of the crystallinity.

A more detailed tabulation of HDT values measured under 1.82 MPa can be found on the Omnexus website.