Book excerpt: A new technique for mixing solid curing agents into liquid epoxy resins using ultrasonic energy has been developed. This procedure allows standard curing agents such as 4,4'-diaminodiphenyl sulfone (4,4'-DDS) and its 3,3'-isomer, (3,3'-DDS) to be mixed without prior melting of the curing agent. It also allows curing agents with very high melt temperatures such as 4,4'-diaminobenzophenone (4,4'-DABP) (242 C) to be mixed without premature curing. Four aromatic diamines were ultrasonically blended into MY-720 epoxy resin. These were 4,4'-DDS; 3,3'-DDS; 4,4'-DABP and 3,3'-DABP. Unfilled moldings were cast and cured for each system and their physical and mechanical properties compared.