Relationships between Chemical Structure and Fracture Behavior of Several Epoxy Mixtures

An investigation was performed to study the fracture behavior-chemical structure relationships of several epoxy-aromatic amine mixtures including those obtained from both tetra- and bifunctional epoxide oligomers as well as those containing a low amount of aliphatic amine in the curing agent and also those modified with rubber. For this purpose, dynamic-mechanical, mechanical, and fracture toughness (as measured using notched samples by both flexural and impact tests), analyses were carried out. Whereas flexural properties showed an evident dependence upon chemical structure, degree of crosslinking and free volume or ω relaxation, the fracture behavior of the epoxy mixtures studied was quite similar to the unmodified brittle bifunctional mixtures, lower for the tetrafunctional mixture, and only the modification with rubber seemed to produce significant increase in fracture toughness. Differences in fracture toughness for the brittle epoxy mixtures studied to each other obtained from flexural tests are more remarkable than from impact tests in the conditions used.