Halloysite nanotubes functionalized with epoxy and thiol organosilane groups to improve fracture toughness in nanocomposites

The surface functionalization of nanofillers imbibed within the matrix provides great interfacial adhesion with the epoxy resin and as such, in this study, the pure and functionalized halloysite nanotubes (HNTs) were used as fillers to improve the fracture toughness of an SC-15 epoxy resin. The HNTs were grafted with an organosilane group having terminal epoxy and thiol functional groups as two different crosslinkers and were thoroughly characterized with infrared spectroscopy (IR), scanning electron microscope (SEM), energy dispersive X-ray spectroscopy (EDS), powder X-ray diffraction (PXRD), and thermogravimetric analysis (TGA). The nanocomposites resulting from these fillers were used in compact tension (CT) testing with varying weight % from 0 to 2%, which were then fabricated and tested as per ASTM D5045 standard to evaluate the fracture toughness. The critical stress intensity factor (KIC) and the critical strain energy release rate (GIC) were measured, and a correlation was established between the test results and the mode of fractured surface images obtained by the SEM. Experimental results showed that the synergistic effect was observed with the addition of HNTs nanofillers into the SC-15 epoxy. The highest improved KIC and GIC were achieved using 2% HNT-thiol, followed by 2% pure HNT, and 1% HNT-epoxy nanofillers, respectively. SEM observation further confirmed that the presence of pure and functionalized HNT in the SC-15 epoxy matrix changed the toughness mechanics from smooth and low pseudo-ductile to several energy absorption mechanisms such as fibrils, river liner, crack bridging, crazing, microvoids, and hackles.