Epoxy-based structural foam having improved toughness

Thermally expandable and hardenable composition, comprising: a) at least one epoxy prepolymer, b) at least one heat-activatable curing agent for the prepolymer, c) at least one blowing agent, d) at least one thermoplastic polyurethane or isocyanate, e) at least one block copolymer. Using these thermally expandable and curable composition for stiffening or reinforcement of components.