The effects of SiO2 nanoparticles in toughened epoxy resins and fiber-reinforced composites made thereof

The properties of cured epoxy resins can be improved significantly by the addition of surface-modified SiO2 nanoparticles. A linear relationship between the increase of modulus respectively the fracture toughness and the addition level of nanosilica exists for most epoxy curing agents. Compressive strength and fatigue performance can be improved as well. Combining this modification with the classic toughening concept using reactive liquid rubbers or core-shell elastomers leads to so-called hybrid systems, which are characterized by both high toughness and high stiffness. The fatigue performance is improved further. Laminates manufactured by using these modified resins exhibit improved performance as well. Regardless if glass or carbon fibers are used as reinforcing material, the relative property improvements of the laminates are much smaller. A linear relationship between the percentual increase in fracture toughness (GIc) of the cured bulk resin systems and the percentual increase of the GIc of the laminates made thereof seems to exist; with a conversion factor of 0.18. If the fracture toughness of a fiber-reinforced part shall be increased by 100 %, then instead of the resin used a hybrid resin with a 555 % higher bulk GIc needs to be used. For the fatigue performance of laminates made from hybrid resins a tenfold increase in cyclic loadings upon failure can be achieved. In own trials a fast curing epoxy resins system based on DGEBA/IPD/TMD was employed to manufacture carbon fiber reinforced laminates; whose properties were investigated. The cured bulk resin systems exhibit the expected property improvements for the hybrid systems. However, the laminates based on hybrid resin systems, modified with both reactive liquid rubber and SiO2 nanoparticles, show no further improvements compared to the rubber-only modification but rather slightly lower values for GIc and GIIc. The ILSS is comparable; the residual strength after impact reduced. The agglomerates of silica nanoparticles, which were discovered, might be a potential cause. They form during the fast cure in presence of reactive liquid rubber; probably caused by the forced rapid phase separation of the rubber upon cure. Future research will provide further insights. In summary it can be stated that laminates made from hybrid epoxy resins are tough and stiff and exhibit improved compressive strength as well as excellent fatigue performance. This makes them especially suitable for highly stressed composites parts like in automotive applications.