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 (∼ 1000 g/mol) imide thermoplastic. The objective was to produce a low-viscosity prepolymer that could be easily autoclave-processed to give a tough thermoset. Here, an homogenous epoxy prepolymer was prepared by first synthesizing very low-molecular-weight linear aromatic imide (∼ 1000 g/mol) directly in a liquid allyl phenol reactive solvent, followed by dissolution of the epoxy (Epon® 825) and the cure agent (DDS) directly in the thermoplastic solution. The allyl phenol both cures into the epoxy network, through phenol functional groups, and accelerates the cure. The viscosity of the pure epoxy was 1.4 Pa · S at 30°C. The prepolymer formulations ranged from ∼ 5-33 Pa · S at 30°C, but all reduced to less than 1 Pa · S at 90°C. The onset of cure is well above 90°C so the prepolymer viscosity is within the range for autoclave processing. The cured resin plaques were not transparent, but phase-separated domains were not found by scanning electron microscopy, indicating that the domain size is below the detection limit of the instrument. The reactive solvent causes a decrease in both the Tg and the high temperature modulus of the thermoset. Introduction of the thermoplastic results in partial recovery of the Tg and modulus