The effect of annealing on the short and long term behavior of PEEK

Short and long-term tensile tests have been done on injection-moulded poly-ether-etherketone (PEEK) in order to assess the effect of annealing on its mechanical behavior. In addition to the behavior reported before, namely increases in yield and drawing stress, the annealed specimens exhibit a distinct work hardening behavior in the beginning of the drawing phase at temperatures between 80°C and 160°C. Subsequent creep tests show that the annealing treatment also influences greatly the long-term static properties of PEEK with creep rates after annealing being considerably below those recorded on the standard injection. DSC thermograms of the samples annealed for more than 20 minutes exhibit similar curves as those reported before, two endothermic peaks: a major peak at 340°C and a minor one at 260°C which is 10 K higher than the annealing temperature. The experimental results support that a type of secondary crystallites grow in the material by the annealing treatment. The existance of those secondary crystallites enhances the mechanical properties of PEEK.