Answer to the October 15, 2001 Processing Puzzle

The Heat Deflection Temperature

The heat deflection temperature is a widely used indicator of a polymer's thermal performance. Measurement of heat deflection temperature is specified by ASTM (American Society for Testing and Materials) Standard D 648 entitled "Deflection Temperature of Plastics Under Flexural Load in the Edgewise Position."

For this test a bar of material is placed in a heated bath and stressed in a three-point bending mode. The temperature of the bath is slowly increased until the sample bar is sufficiently softened that a specified deflection is achieved. This temperature is then the heat deflection temperature.

For amorphous polymers, the heat deflection closely tracks the glass transition temperature. For example, the graph below provides the HDT at 1.82 MPa for several common amorphous polymers.