Effect of molecular weight between crosslinks on the fracture behavior of rubber-toughened epoxy adhesives

The effect of molecular weight between crosslinks, M-c, on the fracture beh avior of rubber-toughened epoxy adhesives was investigated and compared wit h the behavior of the bulk resins. In the liquid rubber-toughened bulk syst em, fracture energy increased with increasing M-c. However, in the liquid r ubber-toughened adhesive system, with increasing M-c, the locus of joint fr acture had a transition from cohesive failure, break in the bond layer, to interfacial failure, rupture of the bond layer from the surface of the subs trate. Specimens fractured by cohesive failure exhibited larger fracture en ergies than those by interfacial failure. The occurrence of transition from cohesive to interfacial failure seemed to be caused by the increase in the ductility of matrix, the mismatch of elastic constant, and the agglomerati on of rubber particles at the metal/epoxy interface. When core-shell rubber , which did not agglomerate at the interface, was used as a toughening agen t, fracture energy increased with M-c. (C) 2000 John Wiley & Sons, Inc.