Thermal expansion behaviour of thermoplastic composites

The thermal-expansion behaviours of a variety of thermoplastic composites based on the ICI Victrex polymers PEEK, ITA, HTA and ITX are examined. It is shown that when each of the composites is raised above the glass-transition temperature, T_g, of the polymer matrix considerable permanent distortion occurs in the small samples used, though the effect is not evident below T_g. This is attributed to relaxation of process-induced residual stresses generated in the larger plates from which the samples were prepared. A number of models are used to calculate the thermal-expansion behaviour of the fibres transverse to their long axes; the results are shown to be inconsistent, and in poor agreement with directly measured properties. Finally, the utility of the data for calculation of thermal-expansion behaviour parallel to the fibres in the composites is considered.