INVESTIGATION OF READILY PROCESSABLE THERMOPLASTIC-TOUGHENED THERMOSETS - II - EPOXY TOUGHENED USING A REACTIVE SOLVENT APPROACH

The fracture toughness of epoxy thermosets was increased by up to 220% using very low-molecular-weight (similar to 1000 g/mol) imide thermop lastic. The objective was to produce a low-viscosity prepolymer that c ould be easily autoclave-processed to give a tough thermoset. Here, an homogenous epoxy prepolymer was prepared by first synthesizing very l ow-molecular-weight linear aromatic imide (similar to 1000 g/mol) dire ctly in a liquid allyl phenol reactive solvent, followed by dissolutio n of the epoxy (Epon((R)) 825) and the cure agent (DDS) directly in th e thermoplastic solution. The allyl phenol both cures into the epoxy n etwork, through phenol functional groups, and accelerates the cure. Th e viscosity of the pure epoxy was 1.4 Pa . S at 30 degrees C. The prep olymer formulations ranged from similar to 5-33 Pa . S at 30 degrees C , but all reduced to less than I Pa S at 90 degrees C. The onset of cu re is well above 90 degrees C so the prepolymer viscosity is within th e range for autoclave processing. The cured resin plaques were not tra nsparent, but phase-separated domains were not found by scanning elect ron microscopy, indicating that the domain size is below the detection limit of the instrument. The reactive solvent causes a decrease in bo th the T-g and the high temperature modulus of the thermoset. Introduc tion of the thermoplastic results in partial recovery of the T-g and m odulus. (C) 1998 John Wiley & Sons, Inc.