Co-authors​ :

 Hans WITTICH (GERMANY), Louisa CHRISTENSEN , Julian M. KARSTEN (GERMANY), Bodo FIEDLER (GERMANY) 

Epoxy resins behave brittle due to their highly cross-linked macromolecule network and therefore exhibit low fracture toughnesses KIc. The goal of this work is to modify glassfiber-reinforced plastics (GFRP) with a procedure which has already increased the fracture toughness in tests with pure resin specimens, while the glass transition temperature Tg was not decreased. To increase the fracture toughness, the flexibilizer JD-4000 is used, but instead of simply adding it to the resin EPON162 and the hardener DETDA80 to gain a statistical distribution of it, a procedure with a two-step synthesis is used. A prereaction of resin and flexibilizer leads to the formation of partially reacted substructures (PRS), which are fully crosslinked in the next step by adding resin and hardener, so that the PRS form as domains in the composite. The first set of experiments vary curing temperature (160°C, 180°C, 200°C), curing degree (75%, 80%, 85%, 90%, 95%) and weight portions of the PRS in the composite (10 wt%, 20 wt%, 30 wt%, 40 wt%, 50 wt%) using pure resin so that the best possible combination can be chosen for high values of KIc, energy release rate GIc and Tg. In addition to the samples with PRS, reference samples with the same amount of flexibilizer using the usual one step synthesis are also prepared for comparisons. Using differential thermal analysis (DSC), scanning electron microscopy (SEM), and three-point bending tests (SENB), the samples are analyzed. The experiments show that curing temperature and time have no significant effect on the PRS size or Tg. By varying the weight portion, both KIc or GIc and Tg can be positively influenced, so this variation remains in the main set of experiments. The combinations of 10 wt%, 30 wt% and 50 wt% at 180 °C and 80% are investigated using GFRPs. Dynamic mechanical thermal analysis (DMTA), SEM, and Double Cantilever Beam (DCB) analyses provide information on Tg, PRS and interlaminar toughness GIc. The combination with a weight portion of 30 wt% PRS achieves the best results. The interlaminar toughness reaches a value of (490,05 ± 40,91) J/m2, which is an increase of 38% compared to the reference of statistical distributed flexibilizer and 118% compared to specimens without any flexibilizer. Tg can be increased by 7% compared to the reference of statistical distributed flexibilizer.