EFFECTS OF THERMAL HISTORY ON MECHANICAL-BEHAVIOR OF PEEK AND ITS SHORT-FIBER COMPOSITES

The effect of crystallinity differences induced by mold wall temperatu re and annealing on mechanical behavior Is evaluated for poly(ethereth erketone) (PEEK) resin and its composites. The systems investigated we re neat PEEK, glass fiber (GF) reinforced PEEK, and carbon fiber (CF) reinforced PEEK. Both composite systems were reinforced with 10, 20, a nd 30 wt% fiber. The degree of crystallinity (X(c)) of PEEK was found to increase by processing at higher mold temperatures, by annealing, a nd by fiber length reductions, which appears to indicate the ability o f short fibers to nucleate the crystallization of PEEK under favorable thermal conditions. Improvements in Young's modulus and strength toge ther with ductility reductions are generally obtained as crystallinity increases in both neat PEEK and its composites. The contribution of c rystallinity to mechanical behavior is significant only for neat PEEK and PEEK reinforced by 10% fiber. SEM micrographs reveal that this is due to a change in failure mode. When PEEK is reinforced by carbon fib ers or by 20-30% glass fibers, a macroscopic brittle mode of failure i s observed irrespective of matrix crystallinity, and mechanical behavi or is principally determined by the nature and content of the reinforc ing fibers.