Epoxy toughening through high pressure and shear rate preprocessing

We have successfully conceived and demonstrated a simple, scalable process for improving the fracture energy of epoxy resins. The process is based on the combined application of high pressures (in the order of GPa) and shear rates (in the order of 10(6) s(−1)) in the pre-cured polymer, obtaining mechanical forces sufficiently high to increase the reactivity of the monomers due to the scission of the epoxy groups. To achieve these high values of pressure and shear rate, we take advantage of the elastohydrodynamic phenomena occurring at lubricated higher kinematic pairs as, for example, the rolling element – track pair in ball bearings. Experimental results show that, under certain combinations of pressure and shear rate, a substantial improvement in fracture toughness is obtained. SEM observations, Raman spectroscopies, nanoindentation and GPC and NMR measurements show that the process is able to increase the polymer chain length before curing, reducing the number of potential anchor points during the subsequent curing and hence reducing the crosslinking density. The chain lengths obtained are big enough to guarantee adequate stiffness and strength due to increased chain tangling, hence overcoming the drawbacks associated with other toughness promotion methods, such as stiffness and strength reduction.