Donald L. Hunston, H Jianmei, R Raghavan, D Hoffman
The addition of a toughener to a thermoset resin, usually an epoxy, can increase the fracture energy by an order of magnitude or more. These materials are two phase systems with small toughener particles in a thermoset matrix. The fracture behavior depends on the constituents, the toughener concentration, and the morphology (particle size distribution). Unfortunately, the inability to alter just one microstructural feature at a time makes it difficult to establish structure-property relationships. The work here uses a new material system that starts with preformed particles so the toughener concentration can be varied without changing the morphology or constituents. The fracture results for this material show that the toughness increases to a maximum at about 15 phr (parts toughener per hundred parts of epoxy by mass) and then decreases gradually as the concentration is increased up to 67 phr. Micrographs of the initiation region on the fracture surface show much less deformation at high toughener concentrations than at 15 phr. This suggests that there is an optimum toughener concentration for achieving maximum energy.