When using mixed sulphuric and nitric acids to modify carbon fibres
(CFs), the time of treatment is of considerable importance. Based on
current studies, the suggested time of surface treatment varies from a
few minutes to several hours for different applications. Han et al. [3]
pointed out that the tensile strength of CF decreased after 1 h of surface
treatment and reduced more than 50% after 2 h. Langston et al. [15] stated
that the best functionalization efficiency and highest oxygen concentration
was obtained with the surface treatment of 80 min. Wang et al.
[16] found that 15 min was preferred to pursue best electrical
conductivity of CFs. In this study, carbon fibres were treated by mixed
acids, and the surface treatment time was optimized by balancing the
levels of tensile strength of fibre, adhesive strength and fracture toughness
of CF/epoxy interface.