The incorporation of a rubbery phase into the matrix of a plastic is a common approachfor toughening thermoplastics (Bucknall, 1977). Therefore, as early as the 1970’s,McGarry and his co-workers (1970, 1971, 1973) began to study the addition of certainliquid rubbers into thermoset resins. Their study showed a significant improvement to the fracture toughness of the modified epoxy resins. Since then, much research has beencarried out on toughening epoxy resins by utilizing suitable liquid rubber copolymers of butadiene-acrylonitrile with various functional end groups, including the following end groups: carboxyl-terminated group (CTBN) (Kalfoglou and Williams, 1973, Kunz-Douglass et al., 1980, Monzione et al., 1981, Kunz and Beaumont, 1981, Kinloch etal.,1983a, 1983b, Bartlet and Pascault, 1985, Nae, 1986, Bucknall and Partridge, 1986,Pearson and Yee, 1991), amine-terminated group (ATBN) (Kunz et al., 1982, Levita, etal., 1985, Butta, et al, 1986, Hwang, et al, 1989, Shaw, 1993), hydroxyl-terminated group (HTBN) (Sankaran, 1990), and epoxy-terminated group (ETBN) (Hwang, et al, 1989,Montarnal, et al., 1989, Verchere, et al., 1990, 1991).

Rubber-modified epoxy resins exhibit increased fracture toughness, but also show the deterioration of other important properties compared to the unmodified resin. For example, Verchere et al. (1990, 1991) studied an ETBN-modified epoxy and found that the fracture toughness (KIC) and fracture energy (GIC) increased slightly with an increasing volume fraction of the dispersed rubber phase, while the value of the glass transition temperature (Tg) decreases in a roughly linear way with the initial amount of rubber added to the formulation. Hwang et al. (1989) blended ATBN and ETBN into the epoxy matrix and found that the impart stress, plane-strain fracture toughness, and fracture energy were increased, while Young.s modulus and yield strength decreased slightly when rubber was incorporated. Especially the CTBN-modified thermosets developed low stability against oxidation, due to the high amount of unsaturated bonds in the CTBN backbone. This deteriorates the chemical and physical properties of the resin,especially at elevated temperature (Kirshenbaum, 1984). Thus the CTBN-modified epoxy resins show undesirable lowering of the stiffness and strength, and often lower Tg,compared to the unmodified resin (McGarry et al, 1970, Pearson et al., 1986, 1989, Iijima et al., 1991). As an option, different kinds of liquid rubbers based on functionalized acrylic oligomers were chosen as toughening modifiers. These rubber modifiers do not contain unsaturated bonds in the backbone and can result in good resistance against oxidation processes (Bell and Ochi, 1983, Ochi and Bell, 1984, Kirshenbaum and Bell,1985, Schröder, 1998). However the improved fracture toughness was accompanied by decreases in the modulus and Tg of the modified epoxy resins.