THE DEPENDENCE OF FRACTURE PROPERTIES ON CURE TEMPERATURE IN A DGEBA/DDS EPOXY SYSTEM

The glass transition temperature (T(g)), water- and solvent-absorption characteristics, fracture toughness, and tensile properties of a diglycidyl ether of bisphenol-A (DGEBA) -based epoxy resin (Epikote 8283) cured with an aromatic amine curing agent, 4,4'-diaminodiphenyl sulfone (DDS), were studied as a function of cure temperature and cure cycle (one-step or two-step cure). The glass transition temperature (which depends on the extent of cure) shows a linear increase with increasing cure temperature and levels off toward the cure temperature of 200-degrees-C. Water and solvent uptake tended to increase with increasing cure temperature and, hence, with cross-linking level. Fracture toughness, tensile strength, strain to break, and tensile fracture energy also increased significantly with increasing final cure temperature regardless of cure cycle. In contrast, the small strain properties such as modulus, yield stress, and yield strain were not influenced greatly by cure temperature and cure cycle. From these results, we infer that the large strain properties of the resin are not inversely proportional to the cross-link density of the network as reported in many publications,1 but, rather, that they are dependent on other structural parameters such as free volume and the flexibility of the cross-linked chains.