Composition-dependent mechanical properties of epoxy composites embedded with helical multiwalled carbon nanotubes

Epoxy-based carbon nanotube–reinforced epoxy composites have been studied for many years as a way to improve the mechanical behavior of polymer materials. In the present work, helical multiwalled carbon nanotubes (HMWCNTs) were mixed into an epoxy resin at concentrations of 0, 0.5, 1.25, 2, and 2.75 wt%, followed by curing with a conventional hardener. After fabrication, the samples were tested under tensile loading, impact loading, and Shore D hardness conditions. An increase in tensile strength was observed as the amount of HMWCNT increased. The neat epoxy showed a tensile strength of 11.085 MPa, while the sample containing 2.75 wt% HMWCNT reached 20.86 MPa. The highest impact value, 11.845 J/m, was obtained at 1.25 wt% HMWCNT, which corresponds to a 36.42% increase compared with the unfilled epoxy. Beyond this concentration, the impact strength decreased. Hardness values increased gradually with nanotube content and ranged from 79 for the neat epoxy to 83 at the highest loading. The tensile specimens’ fracture surfaces were analyzed by scanning electron microscopy. It is found that fracture properties were changed considerably with increasing HMWCNT ratio. These changes may be ascribed to crack growth variations and stronger interaction between the epoxy matrix and the nanotubes. The results show that the mechanical response of epoxy composites is strongly dependent on HMWCNT content and confirm their suitability for structural use where improved strength and hardness are required.