EFFECT OF THERMOPLASTIC MODIFIER VARIABLES ON TOUGHENING A BISMALEIMIDE MATRIX RESIN FOR HIGH-PERFORMANCE COMPOSITE-MATERIALS

A series of engineering thermoplastic toughness modifiers were used to modify the fracture toughness properties of a bismaleimide resin. The effects of thermoplastic loading, molecular weight and functionality were examined. Substantial improvements in K(Ic) stress intensity values were found when modifiers possessing reactive end-groups were used. Increases in modifier molecular weight and weight per cent loading produced steady increases in K(Ic) values and a limiting value at high concentrations and molecular weights. These observations were attributed to the thermoset network being the limiting material with respect to fracture toughness. By controlling molecular weight and adding reactive end-groups to the toughener, the fracture toughness properties of the commercial bismaleimide resin were successfully improved without sacrificing its desirable hot-melt processing characteristics. A unidirectional carbon-fibre prepreg was prepared and mode I and II fracture toughness tests were performed using double cantilever beam and end notch flexure specimens. Thermoplastic loadings of 15 and 20% of maleimide-terminated poly(ether sulphone) (M(n) = 12800 g mol-1) yielded composite G(Ic) values of 489 +/- 25 and 734 +/- J m-2, respectively-a substantial improvement over the unmodified composite value of 359 +/- 17 J m-2.